Soil thermometer

In meteorology, measuring the Earth's surface temperature is of crucial importance, as the ground acts as a thermal energy storage medium and significantly influences the energy exchange between the atmosphere and the lithosphere.

Here is a scientific overview of the types, placement, and physical background of these instruments.

1. Measuring principle and depth levels

Soil thermometers are deployed at standardized depths to obtain comparable data on vertical heat flow. According to the guidelines of the WMO (World Meteorological Organization) and the DWD (German Weather Service), these are:

  • Near-surface measurement: 2 cm, 5 cm and 10 cm depth.

  • Depth measurement: Depths of 20 cm, 50 cm and 100 cm.

In the upper layers (up to 20 cm) the daily fluctuations are massive, while at a depth of 100 cm often only the seasonal cycle is recognizable.

2. Designs of soil thermometers

Depending on the depth, we examine two different types of construction:

A. Soil angle thermometer (for shallow depths)

These thermometers are designed for depths of 5 cm to 30 cm used.

  • Construction: The glass tube is at an angle of usually 135° bent.

  • Reason: The thermometer bulb (the reservoir) lies horizontally at the desired depth, while the scale protrudes diagonally upwards from the ground. This allows for readings without having to pull the thermometer out of the ground, which would disturb the temperature profile.

B. Ground-level immersion thermometer (for great depths)

For depths from 50 cm Special station enclosures are used.

  • Construction: A protective tube made of plastic or metal is permanently buried in the ground. The actual thermometer is mounted in a wooden or plastic shaft and lowered into the tube on a chain.

  • Inertia principle: To prevent the reading from changing immediately due to the air temperature when the thermometer bulb is pulled out for reading, it is often enclosed in a thermal buffer mass (e.g., paraffin or a solid glass shell). This increases thermal inertia.

3. Physical background: Heat conduction

The temperature distribution in the soil follows the Heat conduction equationSince soil is a poor conductor of heat, two effects occur:

  1. Phase shift: The maximum temperature reaches deeper layers significantly later than the surface. At a depth of 100 cm, the summer maximum may not occur for weeks or even months.

  2. Amplitude damping: With increasing depth, the difference between day and night (or summer and winter) decreases exponentially.

$$T(z, t) = T_{mittel} + A_0 \cdot e^{-z/d} \cdot \sin(\omega t - z/d)$$

  • $from$: Depth

  • $A_0$: Amplitude at the surface

  • $d$Damping depth (depending on soil condition and moisture)

4. Important installation rules

Strict rules must be followed to ensure that the measurements are scientifically usable:

  • Soil conditions: The measurement is usually taken under a barren, level surface of earth (black field) or under a short grass cover.

  • No cavities: When inserting the thermometer, no air gap should be created between the floor and the thermometer, as air insulates and distorts the measurement.

  • Avoiding shading: The measuring point must not be shaded by buildings or trees.

Would you like to learn more about the electronic sensors (Resistance thermometers such as Pt100) have largely replaced glass thermometers in automatic weather stations today?

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