Radio soundings are the best observations for revealing the possibility of icing. A sounding shows wind, temperature T and dew point Td as functions of height. The existence of clouds can be seen from the difference of T and Td (dew point depression).

At temperatures above 0 °C, if T = Td, the relative humidity is 100 %, and the air is saturated with respect to water vapor. In the saturated stage there is liquid water, and an equal number of water molecules are evaporating from the water and condensing into it.

Below the freezing point there is ice in the saturated stage. The bonding between neighboring molecules is stronger in solid than in liquid phase. It is harder for the water molecules to evaporate from ice than from water, so it requires more energy, that is, higher temperature. The air is saturated when the temperature reaches frost point Tf.

The frost point is between air temperature and the dew point. Therefore, in cold air the air can be saturated and clouds formed even though the temperature has not reached the dew point. In practice the supercooled clouds can be detected from soundings between -1°C … -15°C as having T≈ Td, while for ice clouds there is a difference of one or two grades.

In the following example there's supercooled water between 980 and 810 hPa, and ice clouds between 420 and 280 hPa:

Fig. 4.1. Ice and supercooled water clouds in a sounding.

Note that a sounding is a point measurement, steered by the upper winds. Therefore, the sounding may not always fully represent the clouds above the sounding station, and the sounding may miss the most freezing conditions.