Page 270 - PPL-engelsk 2025

Meteorology

3.5.3.3 Humidity indication in aviation

For aviation, the air's humidity is given by the current temperature and dew

point temperature. The difference between the two temperatures is called the

spread.

The larger the spread, the drier the air. If the two temperatures are the same,

the air is saturated with water.

In saturated air, the relative humidity is always 100%, whereas the absolute
water content in saturated air depends on the temperature.

For any air mass, there will be a temperature at which the

relative humidity / humidity level is 100%.

This temperature is called the dew point temperature.

In the example below, the current temperature is 16°C, and the air must be
cooled to 12°C before it is saturated with water vapor. So, 12°C is the dew point

temperature in this case.

Spread x 400 feet gives the approximate height up to the base of convective

clouds (cumulus clouds).

o
The temperature change in saturated air is approximately 0.5 C
per 100 meters of altitude change.

Back to table of contents

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 270