Basics of psychrometrics
Share
Humidity is the mass fraction of gaseous water vapor in the air. Liquid water suspended in the air (e.g., raindrops, fog droplets) or ice (e.g., snow crystals) are therefore not included in the humidity calculation. Humidity is an important parameter for numerous technical and meteorological processes, for many life processes in living organisms, and for human health and comfort.Julius F. von Hann: ''Handbook of Climatology.'' 1st edition. Salzwasser Verlag, 2012, ISBN 978-3-86444-581-1, S44–50.
Depending on temperature and pressure, a given volume of air can only contain a certain maximum amount of water vapor. The "relative humidity," which is the most common measure of humidity, is then 100%. Generally, relative humidity, expressed as percent (%), indicates the mass ratio of the current water vapor content to the maximum possible water vapor content for the current temperature and pressure. The absorption of water vapor reduces the air density, since, at a constant overall pressure, an added number of water vapor molecules (H₂O) can be absorbed.2O molecules the same number of heavier N2- and O2-molecules displaced.Jochen Harsch: ''Mold – Causes and Connections''. epubli, Berlin 2014, ISBN 978-3-7375-0741-7.
== General == [[File:Condensing water vapor01.jpg|mini|Condensing water vapor as indirect evidence for humidity]]
Air containing no water vapor is called dry air. Tables of air composition generally refer to dry air, as the water vapor content of humid air varies considerably, ranging from 0 to 4 percent by volume. Humidity is primarily influenced by the availability of water, the temperature, and the degree of atmospheric mixing. Higher air temperatures allow for a higher concentration of water vapor. At very low concentrations of water vapor in the air, the humidity is also referred to as trace humidity.Rainer Müller: ''Thermodynamics. From the dewdrop to the solar power plant.'' Walter de Gruyter, Berlin 2014, ISBN 978-3-11-030198-4.
== Physical Principles == === Evaporation and Condensation ===
At a free water surface separating liquid water from the overlying air, individual water molecules constantly pass from the water into the air and vice versa. In liquid water, the water molecules are relatively strongly bound to one another by molecular forces, primarily hydrogen bonds, which is what allows the continuous liquid to form. However, due to their thermal motion, the water molecules each possess a certain amount of kinetic energy, which is distributed around a temperature-dependent average value (Maxwell-Boltzmann distribution). A small proportion of water molecules therefore always has enough thermal energy to overcome the binding forces of the surrounding molecules, leave the water surface, and enter the air, thus evaporating. The evaporation rate, that is, the amount of water evaporating per unit of time, depends on the proportion of molecules whose kinetic energy exceeds the binding energy of the liquid and is determined, among other things, by the prevailing temperature. Conversely, evaporated water molecules from the air also collide with the water surface and, depending on their kinetic energy, can be captured by the molecular structure there with a certain probability, i.e., condense. The condensation rate depends only on the partial pressure of water vapor in the air, but not on the contribution of the other components of the air to the air pressure.Alfred Dengler: ''Silviculture on an ecological basis. A textbook and handbook.'' 3rd edition. Springer Verlag, Berlin/Heidelberg 1944.
Four factors influence the amount of this exchange of substances:
# the size of the surface (turbulence increases this value compared to still water), # the temperature of the water, # the temperature of the air, and # the saturation level of the air. === Saturation ===
Considering an evaporation process in initially dry air, the evaporation rate corresponds to the temperature, while the condensation rate is initially zero due to the lack of water molecules in the air. The evaporation rate is therefore greater than the condensation rate, and the number of water molecules in the air increases. Consequently, recondensation also increases, and the net evaporation (evaporation rate minus condensation rate) begins to decrease. The density of water molecules in the air continues to rise until the condensation rate reaches the level of the evaporation rate, meaning that the same number of water molecules pass from the water into the air as from the air into the water per unit of time. At this point, an equilibrium state is reached in which net evaporation is zero, while the continuous exchange of particles between air and water continues.
The concentration of water molecules in the air at equilibrium is called the saturation concentration. As the temperature rises, the saturation concentration will also increase, because the increased evaporation rate must be compensated for by a higher condensation rate to reach a new equilibrium, which requires a higher density of water molecules in the air. Therefore, the saturation concentration depends on the temperature.
The saturation concentration is determined almost solely by the properties of water molecules and their interaction with the water surface; there is no significant interaction with other atmospheric gases. If those gases were not present, practically the same saturation concentration would be established above the water. Therefore, knowing the partial pressure of water vapor is sufficient to determine saturation. The colloquial expression, widely used even in scientific circles for its simplicity, that "the air" can absorb a certain amount of water vapor at a given temperature is easily misleading. Air does not absorb moisture analogously to a sponge, and the concept of saturation here should not be understood analogously to the saturation of a chemical solution. At moderate pressure, air consists of independently interacting gas particles that primarily interact through collisions. Therefore, neither oxygen is "dissolved" in nitrogen, nor is water vapor "dissolved" in the other components of air. At high density, however, other interactions become relevant, leading to a noticeable deviation of the saturation concentration relative to the state at low density. (Imagine a sealed container half-filled with water, with a vacuum above the water's surface. If kinetic energy is supplied to the liquid molecules through heating, particles with sufficient energy can detach from the surface (evaporate).)
For the same reason, the saturation concentration is not determined by the temperature of the air, but by the temperature of the evaporating surface. Referencing the air temperature is often justified in everyday practice, since evaporating surfaces with low thermal inertia usually approximate air temperature (for example, laundry drying in the air). However, if the evaporating surface is significantly warmer than the air, the water molecules evaporate into the cooler air (e.g., a hot stovetop) according to the high surface temperature, even if the saturation concentration of the air is exceeded. Some of the moisture then condenses in the air on the cooler aerosols, which are at the temperature of the air, and becomes visible as vapor or mist (for example, wisps of mist in cool air above a warmer autumn lake). However, if the surface is significantly cooler than the air, the moisture content of partially saturated air can also lead to condensation (for example, fogged-up windows in the kitchen or bathroom, or the increase in water level in a dew pond). More precisely, the water vapor condenses into water (dew if the surface temperature is below the dew point, or frost if it is below the frost point; see also Dew Point below).
=== Supersaturation === If the concentration of water molecules is increased above the saturation concentration ([[supersaturation]]) by supplying them, the condensation rate temporarily rises above the evaporation rate due to the greater density of water molecules in the air, and the concentration of water molecules therefore drops back to the equilibrium value.
It is important to note that this is not a case of the air being unable to hold the excess water vapor. Rather, under these conditions, the water vapor utilizes an available condensation surface to reduce its concentration to saturation through heterogeneous condensation. If such condensation surfaces or condensation nuclei are absent, the air can continuously absorb significant amounts of water vapor until water droplets spontaneously form (homogeneous condensation); see also the section "Surface Curvature of Water." This occurs, for example, in large volumes of relatively pure air, i.e., with a low aerosol concentration, and at a great distance from any surrounding surfaces (see fog chamber). Spontaneous condensation of water vapor into water droplets without condensation nuclei only occurs at extreme supersaturation of several hundred percent relative humidity. In practice, however, a sufficiently large quantity of aerosols is almost always present in the air, so supersaturations of several percentage points are rare in the atmosphere. === Partial saturation ===
The rate of water evaporation cannot exceed certain maximum values. Therefore, it takes a considerable amount of time for equilibrium to be restored after a disturbance. For example, if some of the moisture content condenses due to nighttime cooling, the air is initially unsaturated after warming and can only slowly reach saturation again. This partial saturation is the norm for our atmosphere due to frequent temperature fluctuations. How far the air is from saturation is of great importance for numerous processes. Various measures of humidity serve to quantitatively describe this state. == Dependence of saturation concentration on environmental influences == === Temperature === [[File:Humidity.png|mini|Water vapor concentration as a function of a larger and a smaller temperature range]]
As the temperature increases, the proportion of water molecules possessing sufficient kinetic energy to leave the water surface also increases. This results in a higher evaporation rate, which must be compensated for by a higher condensation rate to restore equilibrium. However, this requires a higher concentration of water molecules in the air.
The saturation concentration of water vapor therefore increases exponentially with rising temperature, as shown in the figure on the right. Water vapor has a uniquely determined saturation concentration for every temperature (and almost independent of ambient pressure). At atmospheric pressure of 1013.25 hPa, one cubic meter of air at 10°C can hold a maximum of 9.41 grams of water. The same volume of air at 30°C holds 30.38 grams of water, and at 60°C, it holds over 100 grams. This saturation concentration is called maximum humidity, which is tabulated in the article Saturation (Physics). In this context, [[Mollier diagram]]e according to [[Richard Mollier]] (1923) are also widely used to represent humidity.
Another way to represent the relationship between humidity, temperature, and altitude is the emagram (energy-mass diagram). === Pressure === As mentioned above, the saturation concentration of water vapor at a given temperature is practically independent of the presence of other atmospheric gases and therefore also almost independent of the ambient pressure. However, a slight dependence on the ambient pressure arises for three reasons:SA. Bell, S. J. Boyes: ''An Assessment of Experimental Data that Underpin Formulas for Water Vapor Enhancement Factor''. National Physical Laboratory, UK, 2001. ([http://www.npl.co.uk/content/ConPublication/2822 Online access]).
Water vapor and other gases are not perfectly ideal gases. There are weak interactions (van der Waals forces) between their molecules, the influence of which increases with increasing particle density.
* The mutual distance between molecules in liquid water, and thus their bonding forces, are slightly altered by the superimposed atmospheric pressure (the "Poynting effect"). This, in turn, affects the evaporation rate. * Atmospheric gases dissolved in the water also influence the bonding forces and thus the evaporation rate. The amount of dissolved gases depends on their partial pressure (Raoult's law) and thus ultimately on the total pressure. This weak pressure dependence can be accounted for by a correction factor if necessary. It depends on temperature and pressure and is in the range of 0.5% under atmospheric conditions (see the article on saturation vapor pressure for details). === States of matter of water ===
If we consider an ice surface instead of a liquid water surface, the same considerations apply to the sublimation and resublimation of water molecules. The ice cools the air layer directly above it significantly, resulting in a lower saturation concentration for water molecules. Sublimated water particles and the ambient humidity therefore lead to condensation or fog formation in the immediate vicinity of ice surfaces. However, within the ice crystal structure, the water molecules are subject to stronger binding forces than in liquid water, so the saturation concentration above an ice surface is lower than above a surface of liquid (supercooled) water at the same temperature. This fact plays an important role in the formation of raindrops in clouds (Bergeron-Findeisen process). === Purity of Water === {| class="wikitable float-right"
|+ Relative humidity of the air above saturated salt solutions |- class="backgroundcolor6" ! Substance ! relative humidity ! Source |- |[[Ammonium dihydrogen phosphate]] (NH4H2PO4) at 23 °C |93 % |DIN 52615: Determination of the water vapor permeability of building and insulating materials. Berlin 1987.
|- |[[Potassium nitrate]] (KNO₃3) at 38 °C | 88.5 % |
|- |[[potassium chloride]] (KCl) at 23 °C |85% |
|- |[[Sodium chloride]] (NaCl) at 20 °C |75.5% |
|- |[[Sodium dichromate]] (Na2Cr2O7•2 H2O) at 23 °C |52 % |
|- |[[Magnesium chloride]] (MgCl2) at 20 °C |33.1 % |
|- |[[Lithium chloride]] (LiCl) at 20 °C |11.3 %
|L. Greenspan: ''Humidity Fixed Points of Binary Saturated Aqueous Solutions.'' In: ''Journal of Research of the National Bureau of Standards - A. Physics and Chemistry.'' Vol. 81 A, No. 1, January-February 1977, S. 89–96. ([http://nvlpubs.nist.gov/nistpubs/jres/081/1/V81.N01.A06.pdf PDF]; 320 kB).
If other substances are dissolved in the water, they make it more difficult for the water molecules to leave the water's surface, thus reducing the evaporation rate and resulting in a lower saturation concentration (so-called dissolution effect). For example, the maximum relative humidity levels listed in the table occur in the air above saturated salt solutions.
Although the air above the solutions is saturated with moisture, the corresponding relative humidity levels are not 100%, since relative humidity is always referenced to the saturation concentration above a flat and "pure" water surface (see below). If the air above the salt solution falls below the relevant saturation humidity, water evaporates from the solution to restore the saturation state. If the air exceeds the saturation humidity, some of the atmospheric moisture condenses on the salt solution. This dilutes the solution; if it is to remain saturated with salt to maintain defined conditions, it must contain a sufficient residue of undissolved salt. Deliquescence, or deliquescence humidity, describes the specific ability of a substance (usually salts) to influence the relative humidity of the surrounding air. This effect is used for dehumidification in absorption dehumidifiers. The solution effect further illustrates that the saturation concentration in the air is not determined by the air itself, but by the evaporating surface. === Surface curvature of water ===
If the water surface is convex (curved outwards), as in a droplet, the water molecules are less strongly bound to the surface and can leave it more easily. This curvature effect therefore causes the evaporation rate to increase. When saturated air is in equilibrium with small fog droplets, its relative humidity is therefore slightly above 100%. The same effect also means that, without condensation nuclei, strong supersaturation is possible without homogeneous condensation occurring; depending on the degree of supersaturation, there is a certain minimum radius of the droplets below which they are not stable, because with a smaller radius the evaporation rate increases, but the radius decreases due to evaporation (see the section "critical radius" under "Kelvin equation").
If the water surface is curved inwards (as, for example, in the meniscus of a partially water-filled capillary), the water molecules are more strongly bound to the surface and cannot escape as easily – the evaporation rate decreases. When saturated air in a water-containing porous material is in equilibrium with the menisci, its relative humidity is less than 100%. == Humidity Measures == The water content of the air can be expressed using various so-called humidity measures. Synonymous terms are indicated by a slash, and related humidity measures are listed on the same line. * Vapor pressure (see also saturation vapor pressure) and saturation deficit/vapor hunger (Pascal, hPa, kPa, bar) * Relative humidity (percent/%, respectively, dimensionless)
* [[#vapor density|vapor density]]/less commonly: absolute humidity ([[grams|g]]/[[cubic meters|m³]], kg/m³) * [[#humidity level|humidity level]]/mixing ratio/absolute humidity according to DIN EN 12792 (g/kg dry matter) L., kg/kg tL) * [[#Specific humidity|specific humidity]]/Water vapor content (g/[[kilogram|kg]], kg/kg fL) * [[Dew point]] or frost point/icing point/frost point and dew point difference ([[degrees Celsius|°C]], [[Kelvin|K]]) * [[#Humidity temperature|humidity temperature]] ([[degrees Celsius|°C]]) * [[#Saturation degree|saturation degree]]/Saturation ratio ([[percent|%]] or dimensionless) === Relative humidity ===
The '''relative humidity''', abbreviated r. F., in English abbreviated RH (symbol: ''φ'', ''U''; Relative humidity (not definitively defined) is the percentage ratio between the instantaneous vapor pressure of water and its saturation vapor pressure (at air temperature) above a clean, flat water surface. Relative humidity directly indicates the degree to which the air is saturated with water vapor: * At 50% relative humidity, the air contains only half the maximum amount of water vapor it could hold at the corresponding temperature. * At 100% relative humidity, the air is completely saturated with water vapor. This is also referred to as reaching the "water vapor capacity." * If saturation exceeds 100%, the excess moisture can condense or fog.
Relative humidity therefore makes it easy to estimate how quickly evaporation processes will occur or how likely condensation is. Since the evaporation of moisture through the skin is strongly influenced by the relative humidity of the surrounding air, relative humidity is an important parameter for perceived comfort. [[File:FspFkt 100dpi de.png|mini|Moisture storage functions for some building materials]]
A second reason for the importance of relative humidity is that it determines the equilibrium moisture content of hygroscopic materials. Hygroscopic materials, especially porous ones like wood, brick, plaster, textiles, etc., absorb moisture upon contact with air and bind the water molecules to their pore walls through adsorption. The number of bound molecules is determined by the absolute humidity on the one hand (a higher water vapor concentration leads to a higher adsorption rate due to the greater frequency of contact with the pore walls) and the temperature on the other (a higher temperature leads to a greater desorption rate). The combination of these two opposing factors means that the resulting equilibrium moisture content is essentially determined by the relative humidity of the air. The moisture storage capacity of a material indicates the water content the material assumes at a given relative humidity; it is only slightly dependent on temperature. Materials whose physical properties used for measurement depend on their water content are most commonly used to measure the humidity of the air (change in length due to swelling and shrinkage, change in capacitance of a hygroscopic dielectric, etc.). Since this water content is in turn determined by the relative humidity of the ambient air, such instruments ultimately measure this relative humidity, which is therefore a particularly easy-to-measure and frequently used measure of humidity.
As the temperature rises, the amount of water vapor required for saturation increases. Consequently, the relative humidity of a given volume of air decreases as it heats up. Therefore, specifying the temperature is essential for comparing values. For example, if one cubic meter of air (approx. 1200 g) contains 7.6 g of water vapor, this corresponds to dry desert air with a relative humidity of 20% at 34.4 °C, whereas at an air temperature of 6.8 °C, this would correspond to a relative humidity of 100% and would thus lead to condensation upon further cooling (see also [[dew point]]). Therefore, phenomena such as haze or fog are indicators of high relative humidity and low temperatures. For comparison: The recommended relative humidity of 40–50% for living and office spaces corresponds to approximately 7.3–9.7 g of water vapor at 21–22 °C. The subjective perception of air as dry or humid is therefore more due to the temperature than to the actual amount of water it contains.René Du Bois-Reymond: ''Physiology of Man and Mammals.'' 4th edition. Springer Verlag, Berlin / Heidelberg 1920, S80–82.
Relative humidity can be calculated using the following formulas:
The individual symbols represent the following quantities: * ''e'' – [[vapor pressure#water vapor pressure in meteorology|vapor pressure]] (Note: according to the meteorological definition, see [[vapor pressure#definitions|definition of vapor pressure]]) * ''E'' – [[saturation vapor pressure]] * ''ρw'' – [[#Absolute humidity|absolute humidity]] * ''ρ''w,max – [[#Vapor density|Vapor density]] === {{Anchor|Water vapor density}} Vapor density ===
The '''water vapor density''', '''density of moist air''' or simply '''vapor density''' ([[symbol]]: ''ρ'', ''dd'' or ''f''; (Not definitively established), is the mass of water vapor in a given volume of air, i.e., its density or mass concentration. It is usually expressed in grams of water per cubic meter of (moist) air. It is limited at the upper end by the maximum humidity ρ.w, max, which prevails during a [[Saturation (Physics)|Saturation]] (see there for related formulas and values). Sometimes the term '''absolute humidity''' is also used for this, However, this creates a risk of confusion with the degree of humidity, which, at least according to EN 12792, is also referred to as absolute humidity.
Vapor density is a direct measure of the amount of water vapor contained in a given volume of air. It immediately indicates the maximum amount of condensation that can occur or how much water must evaporate to achieve a desired humidity level.
The vapor density changes when the volume of the air parcel under consideration changes, even without water vapor being added to or removed from the air. When the air parcel is compressed, the water molecules it contains are concentrated in a smaller space, their number per cubic meter increases, and the absolute humidity rises; the opposite occurs when the air parcel expands. The volume change of the air parcel can be caused by a change in its temperature or pressure. When comparing the moisture content of two air parcels, their temperature and pressure differences must therefore be taken into account. An air parcel rising in the atmosphere due to thermals decreases its absolute humidity as it ascends, even if it does not lose any water vapor, because its volume increases due to the decrease in air pressure with altitude. The density of the air parcel therefore changes solely through upward and downward movements. This is also referred to as displacement variance or [[unsteadiness]]. {{Anchor|Absolute humidity}}The absolute humidity ''ρw'' can be calculated using the following formulas, where the first term is obtained by rearranging the equation of state [[Ideal Gas|ideal Gases]]: :
The individual [[formula symbols]] stand for the following [[physical quantity|quantities]]: * ''Rw'' – [[Universal gas constant#Specific gas constant|individual gas constant]] of water = 461.52 [[Joule|J]]/([[Kilogram|kg]] [[Kelvin|K]]) * ''T'' – [[absolute temperature]] * ''m''water vapor – [[Mass (Physics)|Mass]] of the water vapor within the air parcel * ''V''in total – [[Volume|Total volume]] of the moist air
For precise table values, see ''[[Saturation (Physics)#Saturation of gases using water vapor as an example|Saturation]]'', typical values are slightly above 1 kg/m³. === Humidity level ===
The '''humidity level''' (Formula symbol: ''x'', ''Y''), also '''water loading''', '''Water content''', '''Moisture load relative to dry air''', '''Mixing ratio''' or '''moisture content''' This term indicates the mass of water contained in a specific mass of dry air. The numerical range extends from , where for dry air is and for air-free steam or liquid water The humidity level is given in g/kg and thus appears to be dimensionless. However, the numerator contains "mass of water" and the denominator "mass of dry air"; "water" versus "dry air" cannot be canceled out. By spelling out the unit, confusion with specific humidity is also avoided. Expressing the value in kg/kg is possible, but leads to impractical figures. Typical values range between 1 and 20 g/kg tL. Humidity level and specific humidity are identical in their properties. Below saturation, the numerical value does not differ significantly. According to DIN EN 12792:2004, which is used in the ventilation industry, the term '''absolute humidity''' denotes the humidity level. Because the same term is often used for vapor density, the units must always be carefully considered. The humidity level can be calculated using the following formulas, where it is defined by the first term, and all subsequent terms represent equivalents or approximations thereof (fL – humid air; tL – dry air; W – water vapor or water):
:
The individual symbols represent the following quantities: * ''mx'' – [[Mass (Physics)|Masses]] * ''MW'' – [[molar mass]] of pure [[water]] = 18.01528 [[grams|g]]/[[mol]] * ''MtL'' – [[molar mass]] of dry air = 28.9644 [[grams|g]]/[[mol]] (value of the [[standard atmosphere]]) * ''p'' – [[air pressure]] The conversion from humidity level to specific humidity and vice versa is done via the relationships:
=== Specific Humidity ===
The '''specific humidity''' (symbol: ''q'')This indicates the proportion of the mass of (gaseous) water contained in a given mass of moist air. The numerical range extends from , where for dry air is and for air-free steam or liquid water Specific humidity is given in g/kg and thus appears to be dimensionless. However, the numerator contains "mass of water" and the denominator "mass of moist air"; "water" versus "moist air" cannot be canceled. By spelling out the unit, confusion with the degree of moisture is avoided. While expressing it in kg/kg is possible, it leads to impractical figures, as typical values range between 1 and 20 g/kg fL.
Unlike the previous measures of humidity, this quantity remains unchanged when the volume of the air parcel under consideration changes, as long as no moisture is added or removed. For example, if the volume of the air parcel increases, both the (unchanged) mass of the moist air and the (unchanged) mass of the water vapor are distributed over a larger volume, but the ratio of the two masses within the air parcel remains the same. Specific humidity, for instance, retains its value when air flows through a heat exchanger, even if the temperature and density change significantly. Similarly, an air parcel rising in the atmosphere retains the numerical value of its specific humidity as long as no moisture is added (e.g., through the evaporation of raindrops) or removed (through the condensation of water vapor). This advantage is offset, however, by the difficulty of measuring specific humidity, which is generally reserved for a laboratory. Specific humidity "q" can be calculated using the following formulas, where the respective quantity is defined by the first term and all subsequent terms represent equivalents or approximations thereof (fL – moist air; tL – dry air; W – water vapor or water). Only the last terms mentioned are of practical importance; all others serve for derivation and traceability.
:
with it: :
where:
The individual symbols represent the following quantities: * ''mx'' – [[Mass (Physics)|Masses]] * ''ρfL'' – [[Air density|Density of moist air]] * ''VG'' – [[Volume|Total volume]] of moist air * ''RW'' – [[Universal gas constant|individual gas constant]] of water
* ''RtL'' – [[Universal gas constant|individual gas constant]] of [[Dry air|dry air]] * ''MW'' – [[molar mass]] of pure [[water]] = 18.01528 [[grams|g]]/[[mol]] * ''MtL'' – [[molar mass]] of dry air = 28.9644 [[grams|g]]/[[mol]] (value of the [[standard atmosphere]]) === Dew point === {{Main article|Dew point}}
The dew point, or dew point temperature, is the temperature at which an equilibrium of condensing and evaporating water is established on an object (given the presence of moisture); in other words, the temperature below which condensation just begins. It is measured with a dew point hygrometer. The dew point of a sample depends solely on pressure, whereas relative humidity is a quantity that depends on both pressure and temperature. The dew point curve indicates, for a given atmospheric pressure and temperature, the maximum amount of moisture that air can hold (= 100% relative humidity). Cooling the air below the dew point temperature leads to condensation; warming increases its capacity to hold water vapor. === Wet-bulb temperature ===
The quantity known as '''wet sphere, wet sphere or [[cooling limit temperature]]''' is the temperature that an [[air parcel]] would have if it [[adiabatic process|adiabatic]] were cooled [[isobaric process|constant pressure]] by evaporating water in the parcel until saturation, thereby removing the required [[enthalpy of vaporization]] from the parcel.RE Huschke: ''Glossary of Meteorology.'' American Meteorological Society, Boston 1959. It is measured using a psychrometer (for example, an Assmann aspiration psychrometer). Knowing the temperature and humidity, the wet-bulb temperature can be read from a psychrometer table. However, there is no exact formula for directly calculating the wet-bulb temperature.
In practical applications, numerous approximation formulas have been developed, but these usually only work well within a specific temperature and pressure range. In applied meteorology, it is often used to distinguish between precipitation types (snow/rain) at unmanned weather stations. As a guideline, precipitation is considered rain at a wet-bulb temperature greater than or equal to 1.2 °C, and snow at a wet-bulb temperature greater than 1.2 °C.f less than or equal to 1.2 °C when snow falls. However, this only allows for rough estimates.
Recent studies for the Vienna Hohe Warte station (WMO: 11035) have shown that precipitation occurs at ''T''f Below 1.1 °C and above 1.4 °C, the substance occurs in solid or liquid form in two-thirds of the cases. Essentially, the guideline value of 1.2 °C wet-bulb temperature could therefore be confirmed.J. Rohregger: ''Methods for determining the snow line.'' Diploma thesis. Institute of Meteorology and Geophysics, University of Vienna, 2008.
=== Degree of Saturation === The degree of saturation or saturation ratio is defined as follows: When expressed as a non-percentage value, i.e., in the range of 0 to 1, this is also referred to as the saturation ratio. In the extreme cases ψ=0 and ψ=100%, the degree of saturation is identical to the relative humidity. Between these values, the degree of saturation is always slightly lower than the relative humidity; the difference increases with rising temperature and is less than one percent. == Measurement ==
[[File:Hair-Hygrometer.jpg|mini|Hair Hygrometer]] [[File:Humindic.jpg|mini|Humidity indicator for inclusion with moisture-sensitive goods; this example is included with electronic components that, after excessively humid storage, must undergo drying (baking) before further processing to prevent damage during the soldering process; details under [[Moisture Sensitivity Level]]]] Devices for measuring humidity are called [[hygrometers]]. Types include, for example, [[absorption hygrometers]] ([[hair hygrometers]]), [[psychrometers]], and [[dew point mirror hygrometers]].
[[Hygrometer#Other Methods|Humidity Sensors]] deliver an electrical signal, while absorption sensors rely on the changing electrical properties of certain materials and material structures depending on their water absorption. Examples of electrical sensors include impedance sensors, where it is the electrical conductivity that changes. In capacitive sensors, humidity acts on the dielectric, thus changing the sensor's capacitance. In resonant circuit-based humidity sensors, humidity alters the resonant frequency of the resonant circuit. Various measuring devices are used to measure humidity in official weather stations worldwide. One method is an aspiration psychrometer mounted in the climate chamber, which consists of a dry and a wet thermometer.
Using a table, the current relative humidity in percent and the dew point can be determined from the readings of both thermometers. Furthermore, there are separate sensors for the dew point, which consist of a sensor above a lithium chloride solution.Herbert Maria Ulrich: ''Handbook of the chemical investigation of textile fibers.'' Volume One, Springer Verlag, Vienna 1954.
Humidity indicators, for example, consist of silica gel (blue gel) mixed with cobalt chloride and change color at specific humidity levels. They are used to accompany moisture-sensitive goods, particularly in tropical regions and areas with significant temperature fluctuations, to monitor their transport conditions with regard to relative humidity. Blue gel (or the cobalt-free orange gel) is also used in hermetically sealed assemblies behind viewing windows to monitor the internal humidity. == Variability == === Diurnal Cycle ===
Humidity exhibits a typical daily cycle, which, although it can vary considerably depending on environmental conditions and doesn't always follow a specific pattern, generally does. For example, in Berlin during the summer, the average absolute humidity is approximately 10.6 g/m³ at 7 a.m. local time, 10.0 g/m³ at 2 p.m., and finally 10.6 g/m³ again at 9 p.m. In winter, the values are 4.5 g/m³ in the morning, 4.6 g/m³ at midday, and 4.5 g/m³ again in the evening. Thus, in winter, humidity rises after sunrise and falls after sunset in line with the daily temperature fluctuations and as expected due to increased evaporation. In summer, the influence of [[convection]] is added, as rising air parcels cause the penetration of drier air masses from higher altitudes and therefore lead to a midday to afternoon minimum. In the evening, the absolute humidity rises again as convection decreases. In summer, this results in two vapor pressure maxima, one around 8 a.m. and one around 11 p.m. At night (especially when there is no cloud cover), the relative humidity often reaches 100% near the ground, as the temperature of the air layers near the ground falls below the dew point due to contact with the ground, which cools down through heat radiation into space. On windless days, the dew point is reached on insulated horizontal surfaces (car roofs, flat roofs) shortly after sunset (from about 20 minutes). It takes somewhat longer for vertical surfaces (car windows, traffic signs). The result is dew or frost. === Annual Cycle ===
Over the course of the year, based on either daily or monthly averages as long-term averages, relative humidity reaches its maximum in late autumn and early winter, i.e., during the period of greatest fog formation. Conversely, minimum values occur in spring and early summer. Vapor pressure is lowest in winter and highest in summer. The determining factors are evaporation and advection of water vapor, which exhibit very strong regional and local variations. === Dependence on Altitude ===
Water vapor pressure decreases very rapidly with increasing altitude and thus decreasing air temperature, and then only slowly above three kilometers. At ten kilometers altitude, it is only about one percent of the value at ground level. Relative humidity does not show such a clear trend, but is usually very low in the tropopause, which in Central Europe is at an altitude of about 11 kilometers. Here, it is normally about 20% and continues to decrease with increasing altitude, which is also the reason why cloud formation is almost exclusively limited to the troposphere. == Significance and Applications ==
Humidity is important in a multitude of applications, with meteorology and climatology forming its theoretical, but not its application-oriented, center. The role of water vapor, its properties, and especially its technical applications outside of atmospheric conditions are explained there. The general properties of water and its natural distribution on Earth can be found separately. === Everyday Life === Numerous phenomena in everyday life can be attributed to humidity, some of which will be presented here as examples.
If you observe wet objects or open water surfaces for an extended period without any additional water being added, their wetness will decrease or the water surface will dry out. Laundry will dry over time, puddles will disappear, and food will become hard and inedible. This is called evaporation. However, this is only possible as long as the air is unsaturated, meaning the relative humidity is below 100%. [[File:Frost on window.jpg|mini|Ice flowers]]
When entering a heated room from a cooler environment, you often notice that your glasses fog up. The same applies to windows. If the windows are colder than the room, they fog up. This also restricts your field of vision, for example, in vehicles. The same effect occurs in bathrooms and saunas, where mirrors and other colder objects often fog up as well. The reason for all these effects is the cold surfaces that cool the air in their immediate vicinity: the higher the relative humidity of the air, the faster it reaches the dew point and water condenses. The greater the temperature difference between the surfaces and the surrounding air, the stronger the tendency for dew to form or fog up. For this reason, the described phenomena occur primarily in winter, in damp rooms, on exterior walls, and outdoors at night under clear skies (cooling of the Earth's surface through radiation into space). If surface temperatures drop below 0 °C, frost or dew forms. Countermeasures against dew and frost include: * Blowing warm air onto the windows * Placing radiators on exterior walls and under windows in living spaces * Heating objects (e.g., car rear windows, aircraft components). This effect also leads to the icing of freezer compartments or the evaporator in refrigerators and chest freezers, while unpackaged refrigerated goods simultaneously dry out. The water in these goods first evaporates or sublimates, then condenses on cold surfaces or resublimates into ice. This effect is used in [[freeze-drying]].
Icing of carburetors in gasoline engines (for example, in motor vehicles or small aircraft) leads to engine failure. It is primarily due to the cooling of the air caused by the evaporative cooling of the gasoline, and partly due to the low pressure, which further cools the air. [[File:FA-18C vapor LEX and wingtip 1.jpg|mini|Misty formation in wingtip vortices]]
The phenomenon of temperatures dropping below the dew point can also be observed in airplanes or fast racing cars. The wingtip vortices at the ends of the wings or spoilers lead to a local drop in air pressure and, according to the thermal equation of state for ideal gases (Amontons' law, Gay-Lussac's second law), to local cooling of the air. The dew point is locally reached, and fog forms there. If the humidity is particularly high at sub-zero temperatures, airplanes experience the dreaded wing icing – then the low pressure above and behind the wings and tail surfaces is sufficient to trigger the formation of icing.
The exhaled air of humans and other warm-blooded animals is significantly warmer and more humid than the inhaled air. This is evident in the visible condensation of water vapor from exhaled air, forming wisps of fog in winter or at low temperatures and high humidity. The warm, moist exhaled air cools below the dew point, resulting in the formation of water droplets. The same applies to the exhaust fumes from vehicles, aircraft, and power plants, whose cloud formation or contrails are often mistaken for their pollutant emissions. === Meteorology, Climatology, and Hydrology === [[File:Hail in Finland.jpg|mini|[[Hail]]shower in [[Finland]]]]
When air saturated with water vapor is cooled below the dew point, liquid water condenses from the air, provided the necessary condensation nuclei (aerosols) are present. However, under natural conditions, these are almost always present in sufficient concentration, so significant supersaturation of several percentage points occurs only in exceptional cases. Condensation, and below 0 °C also resublimation of water vapor, leads to, among other things, cloud formation, hail, snow formation, fog formation, dew formation, and frost formation. Water vapor is therefore not a permanent atmospheric gas and exhibits high mobility, with a statistical residence time of approximately ten days.
Although water vapor is present in the atmosphere at relatively low concentrations, its high mobility and the associated metabolic processes mean it contributes significantly to the global water cycle and therefore plays an important role in the water balance. Humidity is also a crucial input for precipitation formation and its calculation, as well as for determining evaporation, transpiration, and interception. This, in turn, plays a vital role in various climate classifications within the framework of the climatic water balance.
Important meteorological quantities, such as the condensation level and the virtual temperature, can also be derived from humidity. Humidity, or rather water vapor, also plays a significant role in the atmosphere's radiation balance – water vapor is the most important greenhouse gas. Water vapor, and especially clouds, strongly prevent the Earth's surface from cooling down at night, as they balance the Earth's surface radiation (heat radiation) through absorption and re-emission. The enthalpy of condensation stored in the liquid state of water accounts for the difference between the wet and dry adiabatic lapse rates – one of the prerequisites for the formation of foehn winds.
Air with low relative humidity is a commonly used drying agent in everyday life, for example, when drying textiles on a clothesline. When drying materials by evaporation, it is crucial that the humidity is sufficiently low. At 100% relative humidity, the material being dried cannot dry any further; an equilibrium is reached. Therefore, in drying processes, such as in dryers (including clothes dryers), attempts are made to reduce the relative humidity of the environment. This can be achieved by increasing the temperature, air exchange (hair dryer, vented dryer), adsorption of water (adsorption dryer), or condensation of water (condenser dryer).
In other cases, however, the effect of the wind is usually relied upon, which constantly blows in fresh air with low relative humidity and thus draws the water out of, for example, hay, freshly cut wood, mortar, hanging laundry, tobacco leaves, coffee or cocoa beans. [[File:Tomato leaf stomate 1.jpg|mini|[[Stoma (Botany)|Stoma opening on a leaf]]]] === Biology ===
In biology, and especially in ecology, humidity is of great importance. It not only determines the occurrence of climate zones or specific ecosystems, but also plays a major role in transpiration through the stomata of leaves and in their intercellular spaces (water vapor partial pressure). Humidity is therefore an important parameter for the water balance of plants, animals, and humans (sweating, respiration, fungal growth). Humidity also plays a special role for those animals that primarily breathe through their skin. These include many snails and other mollusks, which consequently also have a low ecological tolerance to desiccation. === Health ===
For living and office spaces, a relative humidity of 40 to 50% is recommended, with room temperatures between 21 and 22 °C.{{Literature |Hrsg=Fachverband Gebäude Klima e. V. |Titel=Questions and Answers on Indoor Air Humidity |TitelErg=FGK Status Report 8 |Ort=Bietigheim-Bissingen |Datum=2020-05 |Seiten=4-5 |Online=https://downloads.fgk.de/downloader.php?FILENAME=139_SR8_Fragen_u_Antworten_Raumluftfeuchte_V9_200429.pdf |Format=PDF |KBytes=5200 |Accessed=2021-10-24 |OCLC=699878249}}{{Literature |Author=Felix Nienaber, Kai Rewitz, Paul Seiwert, Dirk Müller |Title=Influence of humidity on humans and their health |TitleAdd=White Paper (RWTH-EBC 2021-001) |Publisher=RWTH Aachen, Institute for Energy Efficient Buildings and Indoor Climate (EBC) |Location=Aachen |Date=2021 |DOI=10.18154/RWTH-2021-01238}} In cooler areas, higher humidity is more tolerable than in particularly warm areas (below 20°C, even levels above 70% can still be perceived as comfortable). Generally, humidity levels above 95% and below 23% are uncomfortable. Under normal conditions, the air in heated rooms (in winter, especially at low outside temperatures) can become too dry without active humidification. On the other hand, the humidity in the bedroom should generally be somewhat lower with the windows closed, as exhaled breath further increases the humidity, and a starting humidity of 60% can exceed the threshold for mold growth. It is advisable to place a hygrometer in the living spaces to measure the current humidity and, if necessary, counteract it by regularly ventilating the room or using a dehumidifier.{{Internet source |url=https://dgk.de/gesundheit/umwelt-gesundheit/informationen/wohnen/gesunde-luftfeuchtigkeit.html |title=Healthy humidity |work=Information portal for health: Environment and health |publisher=German Green Cross e. V. |date=2002 |retrieved=2021-10-24 |retrieved-hidden=0}}{{Internet source |author=Regine Rundnagel, Ulla Wittig-Goetz |url=https://web.archive.org/web/20220413010200/https://www.ergo-online.de/ergonomie-und-gesundheit/arbeitsplatzgestaltung/umgebungseinfluesse/klima-im-buero/ |title=Climate in the Office |work=Ergo Online |publisher=Hessian Ministry for Social Affairs and Integration; Advisory Center for Technology Impact Assessment and Qualification at the Education Center of the United Services Union (ver.di) in the State of Hesse (BTQ Kassel) |date=2018-08-14 |accessed=2021-10-24 |accessed-hidden=0 |comment=archived version on ''archive.org''}}
==== Causes and health risks of low humidity ====
Especially in enclosed, well-ventilated, and well-heated rooms, the recommended humidity levels are often not met, which can lead to reduced respiratory function and impairment of the skin and mucous membranes. This is particularly true in winter, as the cold outside air then has a low absolute humidity, and the relative humidity drops significantly when it warms up to room temperature. If the humidity drops too drastically, the unwanted air exchange can be reduced by minimizing leaks. However, the humidity should not exceed 80%, even in the coldest areas of the room (exterior walls behind furniture), as mold growth cannot be ruled out at higher levels. Depending on the use and thermal insulation of the rooms, humidity levels that are significantly below the medically recommended values are often necessary to prevent mold growth.
In very cold regions, during cold seasons, or at night, the human body often exhibits increased fluid consumption, even though the lack of fluid loss through sweating would suggest the opposite. This is due to the humidification of the dry inhaled air and the associated water loss. When the cold outside air is warmed during inhalation, its water vapor capacity increases, thus lowering the relative humidity. Conversely, the saturation deficit increases, and the tendency of the liquid lung tissue water to transition into a gaseous state also increases. In summer, or in warm ambient air, the inhaled air is hardly warmed further and therefore retains its usually high relative humidity. If the additional water losses through sweating are not too significant, the body's water requirement is therefore higher in cold ambient conditions.
Too low humidity is detrimental to respiration, as oxygen is then less efficiently absorbed from the lungs (alveoli) into the bloodstream (cardiovascular system). Skin requires high humidity to prevent dehydration, as this is closely linked to skin moisture. Mucous membranes are particularly susceptible to drying out because they have limited protection against evaporation and rely on high moisture levels to maintain their function. For example, low moisture in the nasal mucosa can lead to an increased incidence of nosebleeds (epistaxis). Generally, this also weakens the skin's immune defenses (increasing the risk of catching a cold) and reduces its ability to exchange substances, particularly affecting the oral mucosa. Low humidity also increases susceptibility to skin irritation, redness, or even skin inflammation. If these inflammations occur only in certain rooms or buildings, this is usually due to additional pollution of the indoor air with pollutants (e.g., particulate matter, solvents, formaldehyde, etc.). When administering inhalation anesthesia, humidifying the inhaled gas mixture is very important, as the medical gases used are stored anhydrous; otherwise, the resulting evaporation effects in the patient's lungs would cause cooling (evaporative cooling) and a degree of dehydration.W. Petro (ed.): ''Pneumological Prevention and Rehabilitation.'' 2nd edition. Springer Verlag, Berlin/Heidelberg 2000, ISBN 3-642-64112-1.
==== Health risks associated with excessive humidity ====
High relative humidity, on the other hand, hinders the regulation of body temperature through perspiration and is therefore quickly perceived as muggy. Despite higher temperatures, very hot deserts are often much easier for the body to tolerate (provided it does not suffer from dehydration) than rainforests with high humidity and comparatively moderate temperatures. The effect of humidity on perceived temperature is described by the humidity index (Humidex), whereby the basic relationship between increasing humidity and increasing perceived temperature also applies to low humidity levels and can thus be used, for example, to reduce room temperature and therefore heating requirements.Wolfgang Oczenski (ed.): ''Breathing – Respiratory Aids. Respiratory Physiology and Ventilation Techniques.'' 8th edition., revised edition. Georg Thieme Verlag, Stuttgart 2008, ISBN 978-3-13-137698-5.
=== Agriculture and Forestry === [[File:Nebelwald.jpg|mini|vertical|[[Sauerland|Sauerländer]] Forest in the Fog]]]
In agriculture, excessively low humidity poses a risk of fields and crops drying out, leading to crop failure. The increased vapor pressure gradient between the leaf surface and the atmosphere draws moisture from the plants (see section on biology), especially when their stomata are open during the day and they have limited protection against evaporation, as is the case with many native plants (C3 plants). While this increases soil desiccation, the plants also protect the soil from direct sunlight and overheating, and their roots draw water from deeper layers to the surface. Many bog and marsh plants possess a regulatory mechanism that reduces the rate of evaporation as desiccation begins.
The water balance in open-field cultivation is also significantly improved by nighttime dew – plants are more prone to dew than bare soil, as they cool down faster at night through heat radiation than bare soil with its higher heat capacity.Josias Braun-Blanquet: ''Plant Sociology. Fundamentals of Vegetation Science.'' Springer Verlag, Berlin/Heidelberg 1928.
However, humidity also plays a role in forestry and the wood processing industry. Freshly cut wood has a high inherent moisture content, which is lower in wood cut in winter. This wood moisture content decreases during the storage period and adjusts to the ambient humidity. If freshly cut wood is processed, it shrinks and warps. Changes in wood moisture content due to fluctuating humidity also lead to changes in the dimensions of seasoned wood across the grain and are of great importance to all wood processing trades and industries. When storing fresh wood in sawmills, sprinkler systems are often used to slow down the drying process and thus prevent shrinkage cracks.
Even seasoned wood (boards, squared timber, and beams) is stored in such a way that it is surrounded by air and held parallel by its own weight. This is intended to guarantee that the wood does not warp or even rot. When laying plank and parquet flooring, it must be taken into account that the wood, due to its hygroscopicity, adapts to the ambient humidity. Below the fiber saturation point, this leads to swelling or shrinkage of the wood. This is also why wooden barrels leak when not in use.Bernd Wittchen, Elmar Josten, Thomas Reiche: ''Wood Science.'' 4th edition. Teubner Verlag, Wiesbaden 2006, ISBN 3-519-35911-1.
=== Storage and Production === [[File:Humidor Preparation.jpg|mini|vertical|A prepared [[humidor]] with hygrometer]]
In food storage, humidity is crucial for controlling fruit ripening, especially for stored fruit. High humidity can also promote corrosion, particularly through the indirect effect of increased dew formation, and therefore must be considered during the storage and transport of moisture-sensitive goods. Examples of products requiring specific humidity levels include chemicals, cigars (humidifiers), wine (corks), salami, wood, artwork, books, and optical or electronic assemblies and components, such as integrated circuits. Humidity must be monitored or controlled to maintain certain [[room climate]]ta in [[storage room|storage rooms]], [[museum|museums]], [[archive]]s, [[library]]s, [[laboratory]]s, [[computer center|computer centers]] and industrial production facilities ([[microelectronics]] manufacturing).
During the transport of goods in weatherproof ISO containers or sealed plastic bags, condensation and dew can form if the air inside drops below the dew point, for example, during transport from tropical to colder regions. Therefore, bags containing silica gel or zeolites are placed in foil packaging for moisture-sensitive goods to buffer the moisture. Humidity indicators are used to monitor the humidity levels inside the packaging during transport.
Moisture-sensitive devices, such as those in electronics and optics, must be allowed to warm up after storage at low temperatures before their packaging is opened. Otherwise, condensation will form on and inside the devices, which can lead to failure, especially if the condensed devices are operated immediately.''Ecotrophology 2.'' 1st edition. Verlag Neuer Merkur, Munich 2005, ISBN 3-937346-03-1.Johann Hamdorf, Heribert Keweloh: ''Management systems for food safety. DIN EN ISO 22000 in practice.'' 1st edition. Beuth Verlag, Berlin 2009, ISBN 978-3-410-16826-3, S. 16–17.
=== Exterior walls of buildings === [[File:Humidity meter.jpg|mini|vertical|Device for measuring humidity]]
In building physics, the dew point, in the form of the dew point plane, plays an important role. This refers to the set of all local dew points within a building (masonry or thermal insulation) at which vapor condenses into water. The underlying principle is that warm air can hold more moisture than cold air. If warm, moisture-laden air moves within the exterior wall or insulation layer through diffusion or convection, liquid water forms as soon as the dew point is reached. In the case of "cold bridges" (actually thermal bridges), the dew point plane lies at the surface of the interior wall.
Any type of thermal insulation in buildings shifts the dew point plane, where vaporous humidity diffuses into liquid water, from the cold side towards the heat source. With interior insulation, the dew point plane lies further inwards than with exterior insulation. Where moisture condenses, the building components become damp, their moisture content increases, their thermal conductivity rises accordingly, and their thermal insulation effect decreases. Mold and algae can grow in damp building components if organic materials (from paints, wallpaper, insulation materials, or wood) and air are present. This leads to the risk of health-endangering mold growth, failure of the thermal insulation due to water absorption (increased heat conductivity), and damage to building materials due to frost weathering.
Countermeasures therefore consist of reducing moisture diffusion or shifting the inevitable dew point drop to locations (outdoors) where the moisture can dry out or be dissipated (through ventilation or vapor permeability) by using suitable building materials or other measures. If this is not possible (for example, with interior insulation), the thermal insulation layer must be fitted on the inside with a vapor barrier (closed membrane, no water diffusion possible) or a vapor retarder (water diffusion is limited) to prevent humid room air from penetrating the thermal insulation layer. This is particularly important if the masonry has low diffusion capacity, for example, due to an exterior coating.Horst Bieberstein: ''Mold in living spaces – what to do.'' 3rd edition. Bieberstein Alpha and Omega Publishing House, Stuttgart 1995, ISBN 3-927656-06-2.
In addition, an insulation layer can also become wet "from the outside". Dew or other precipitation can be drawn in (for example, through the joints of brick veneer) due to stress or shrinkage cracks via capillary action. If the interface between the thermal insulation and the outside air is then liquid- or vapor-tight and there is no ventilation (e.g., a ventilated curtain wall), the moisture that has penetrated cannot dry out and the insulation material becomes saturated across its entire surface and irreversibly (see also Moisture#Moisture in building components).
The effectiveness of the ventilation system for drying depends on the moisture content of the incoming supply air. High humidity and low surface temperatures of the building components can cause condensation to form in the ventilation layer, thus triggering further damp penetration.G. Kain, F. Idam, F. Federspiel, R. Réh, L. Krišťák: ''Suitability of Wooden Shingles for Ventilated Roofs: An Evaluation of Ventilation Efficiency.'' In: ''Applied Sciences.'' 2020. [https://www.researchgate.net/publication/344286589_Suitability_of_Wooden_Shingles_for_Ventilated_Roofs_An_Evaluation_of_Ventilation_Efficiency (researchgate.net)]
During the winter period – often referred to as the dew period in this context – the temperature and water vapor pressure are higher inside than outside. The exterior wall therefore exhibits a gradient towards the outside for both values. However, this gradient is not uniform even in a homogeneous exterior wall, as its time-dependent storage capacity for heat and water vapor varies, and the temperatures and vapor pressures also change differently over time. In the case of inhomogeneous walls, the gradient also differs between the individual materials. For example, a vapor barrier has a large vapor pressure gradient but hardly any temperature gradient. With insulation materials, it is often the other way around: here, the water vapor pressure gradient is small, but the temperature gradient is high. Condensation always occurs when the relative humidity locally exceeds 100%, either temporarily or (for example, in winter) permanently.
Condensation can also be prevented by using building materials with high water vapor permeability and/or high water absorption capacity (buffering) combined with low thermal conductivity. Examples include straw/clay or wood. In these cases, vapor barriers can often be omitted. Proper ventilation of living spaces (especially during renovations involving exterior painting, improperly installed vapor barriers, and sealed windows) has a significant impact on preventing mold growth.Michael Köneke: ''Recognizing, preventing, and combating mold in the home.'' 3rd, revised edition. Fraunhofer IRB Verlag, Stuttgart 2008, ISBN 978-3-8167-7295-8 S. 17–18.
{{See also|Low-energy house}} === Aerospace ===
In aviation, there is a risk of icing on wings and tail surfaces due to the resublimation of water vapor in the air. This effect can severely impair airworthiness within a very short time and is responsible for numerous accidents. This process is countered by de-icing systems that heat critical areas (for example, the leading edge of the wings) to prevent ice buildup.Niels Klußmann, Arnim Malik: ''Lexicon of Aviation''. Springer Verlag, Berlin/Heidelberg 2004, ISBN 3-540-20556-X.
A less expensive method involves covering the wing's leading edge with a rubber skin and intermittently pumping compressed air between the rubber skin and the wing. The skin bulges, and this deformation blasts off the rigid ice. However, this method carries a certain risk. If the resulting ice layer is still thin when the compressed air de-icing is activated, the rubber skin will only bulge it, not blast it away. Consequently, more ice will accumulate, and subsequent de-icing attempts will be ineffective. To mitigate this risk, pilots often wait to activate the de-icing system until they are confident that it will achieve the desired effect.
In space travel, rocket launches experience similar problems caused by low ambient temperatures. Launch windows are therefore also chosen according to meteorological considerations, and launches are aborted if necessary. Failure to observe this principle can lead to a crash. === Respiratory Protection === Humidity is an important parameter when filling compressed air cylinders, for example, for self-contained breathing apparatus (SCBA). For this purpose, humidity is specified according to DIN EN 12021 "Compressed air for respiratory protective devices" as the maximum water content of the air stored in compressed air cylinders and the air measured at the compressor outlet, i.e., the absolute humidity a, d, or f. According to DIN EN 12021 "Compressed air for respiratory protective devices," the maximum water content in compressed air cylinders is: * at 200 bar nominal pressure: 50 mg/m³3
* at 300 bar nominal pressure: 35 mg/m³3
The absolute humidity of the air supplied by the compressor for filling 200-bar or 300-bar compressed air cylinders should be 25 mg/m³.3 Do not exceed the specified limit. Humidity is measured using [[test tubes]] measuring devices while wearing respiratory protection. The unit of measurement refers to air at atmospheric pressure.[http://www.atemschutzlexikon.de/lexikon/d/druckluft-fuer-atemschutzgeraete/ Compressed air for respiratory protection devices] atemschutzlexikon.de, accessed on March 16, 2017.
=== Heat exchange === Condensation of humidity can occur on [[heat exchangers]]n and cold pipes that are colder than the ambient air, and icing can also occur if the freezing point is undershot.
Condensation therefore forms inside a refrigerator, which is typically operated just above freezing. Formerly (around 1960/1975), the only cooling surface was a horizontal plane made of anodized aluminum, forming the bottom of the freezer compartment and thus somewhat shielded above the refrigerator compartment. This cooling surface iced up due to humidity from the room air and water-containing foods and therefore had to be defrosted approximately weekly. The ice then melted and dripped either into a tray, permanently inserted into the refrigerator, consisting of roof- and channel-shaped ribs, which had to be manually pulled out and emptied. Later models, better insulated not with fiberglass but with expanding foam, had a continuous plastic tray with a drain plug at the rear of the refrigerator compartment. To defrost, the plug was opened to drain the meltwater into a container placed underneath. Since around 1980, the seamless, blow-molded plastic back panel has formed the cooling surface of the refrigerator compartment. Condensed water – which may temporarily freeze during a cooling cycle – runs down into a molded groove and then through a permanently open outlet into a plastic tray on the outside of the warm cooling unit, where it evaporates. These refrigerators are self-defrosting. The freezer compartment, which is largely airtight and therefore almost vapor-tight thanks to plastic strips filled with magnets, is rarely opened and consequently builds up very little ice on its own cooling surface, which then needs to be defrosted manually. When the dew point of air rises in basements during the summer, humidity condenses on the pipe of a flowing drinking water line.
A range of gases (propane, butane, CO)2Nitrous oxide (laughing gas) is stored under pressure in liquefied form in pressurized cylinders, cartridges, or small metal cartridges. Sufficient quantities are withdrawn from the gaseous phase at a sufficiently high rate and replenished from the liquid phase by evaporation or boiling, causing it to cool. This leads to liquid condensation of atmospheric moisture on the outside of the upright cylinder and, at sufficiently low ambient temperatures, to the formation of frost, which visibly marks the level of the liquid phase of the contents.
If untreated compressed air is rapidly released from a boiler, the air in the jet cools down so much during expansion that the surrounding air entrained in can be cooled below its dew point, causing a small amount of fog to form temporarily and locally. A similar effect occurs when a beverage containing carbon dioxide under pressure is quickly opened upright. If the beverage doesn't foam out, a small wisp of fog is briefly visible above the opening of the bottle or can. When cold drinks are poured into drinking glasses, moisture from the air condenses on the outside. To protect tables, coasters are placed underneath. Stemware usually keeps the stem dry as long as the coating of fine droplets hasn't clumped together into larger ones that run off. Beer mats are often placed over the stems of pilsner glasses to absorb the foam and condensation that runs off.
Air conditioners mounted on exterior walls cause water to condense in the cooled airflow. Small amounts of liquid water are sometimes channeled through small pipes onto the sidewalk in front of a business. Dehumidifying and drying air and fabrics: Dehumidifiers, ranging in size from small units to travel bags, work by cooling the air blown through them below the dew point, allowing the water condensing on the cooling surfaces to drain into a collection container, and then reheating the air. Typically, the compressor cooling unit is driven by an electric motor.
The use of hygroscopic materials (solid, rarely liquid) is only recommended for small volumes of air. Small paper bags of dried silica gel are included with electronic devices and also with leather goods prone to mold to absorb moisture that diffuses through cardboard packaging during sea transport in containers and can condense upon cooling, up to a certain amount. Tissue paper or similar materials are often packed as an interlayer between water vapor-proof layers of glass or plastic film and the like to promote moisture exchange and prevent liquid condensation and the associated transport processes and capillary effects.
In chemistry labs, substances are often required in anhydrous form for weighing or processing without water content. Drying is achieved roughly by air drying, or more or less intensively by heating, sometimes even to incandescence. During cooling, atmospheric humidity causes substances to reabsorb water. Therefore, substances are stored in trays in a desiccator next to or above drying agents. At room temperature, the substance to be dried releases water vapor as atmospheric humidity, and substances such as silica gel, calcium chloride, or concentrated sulfuric acid absorb this water vapor due to their higher hygroscopicity. Air is usually extracted from the desiccator using a water jet pump, which facilitates the escape of water vapor (and other vapors) from the sample and the diffusion of the water vapor towards the drying agent. By creating a vacuum down to about 1/100 bar, the absolute humidity increases up to a hundredfold. If, for example, water at ambient temperature (e.g., 20 °C) is present in the desiccator as the water vapor source, the relative humidity does not change after equilibrium is reached. This is because the water vapor pressure at 20 °C (ideal gas law considered ideally) always results in saturation with water vapor, i.e., 100% relative humidity, regardless of any air molecules also present in the same volume. A water jet pump is advantageously operated with cold water, as it represents a water vapor source at the pump's temperature when operating towards a vacuum. In a desiccator, it is typically used only intermittently and not continuously for extracting organic vapors (e.g., from solvents).
Freeze-drying is a gentle process for drying frozen items, often food, in a vacuum without heating. Evaporating water vapor is drawn in under vacuum. Aromatic compounds that are less volatile than water or adhere more strongly to the material are retained. == References == * H. Häckel: ''Meteorology.'' (= ''UTB.'' Volume 1338). 4th edition. Ulmer Verlag, Stuttgart 1999, ISBN 3-8252-1338-2. * E. Zmarsly, W. Kuttler, H. Pethe: ''Basic Meteorological and Climatological Knowledge. An Introduction with Exercises, Tasks and Solutions.'' Ulmer Verlag, Stuttgart 2002, ISBN 3-8252-2281-0. * P. Hupfer, W. Kuttler: ''Weather and Climate.'' Teubner, Stuttgart/Leipzig 1998, ISBN 3-322-00255-1. * W. Weischet: ''Introduction to General Climatology.'' Borntraeger, Berlin 2002, ISBN 3-443-07123-6. == External links == {{Wiktionary}} * {{DNB-Portal|4125789-3}} == References ==
Dietmar Göhlich (ed.): ''Dubbel Handbook for Mechanical Engineering.'' 26th edition. Springer, 2020, {{DNB|1233062506}}.
Karl-Josef Albers (ed.): ''Pocketbook for Heating and Air Conditioning Technology.'' 80th edition. InnoTech Medien, 2021, ISBN 978-3-96143-090-1.
Manfred Zeller, Ulrich Busweiler: ''Humination and dehumidification of air.'' In: P. Stephan, D. Mewes, SKabelac, MKind, K., Schaber, T., Wetzel (eds.): ''VDI Heat Atlas.'' 12th edition. Springer, 2019, ISBN 978-3-662-52990-4.
{{Literature |Author=Martin Dehli |Title=Humid Air |Collection=Compendium of Technical Thermodynamics: For Study and Practice |Publisher=Springer Fachmedien |Place=Wiesbaden |Date=2021 |ISBN=978-3-658-34540-2 |Pages=217–232 |DOI=10.1007/978-3-658-34540-2_10}}
Testo Industrial Services (ed.): ''Moisture Guide: Measurement Technology and Calibration.'' 3rd edition. Self-published.