A pilot balloon is used by meteorologists to determine the height of the underside of the clouds above the ground during daytime hours. The balloon is filled in such a way that its ascent speed is known. From the measured time interval between the start of the balloon and its immersion in the cloud, one can thus infer the height of the cloud base. By tracing the trajectory, formerly with a theodolite, today by radar or GPS, the direction and speed of high winds can be determined.
