Atmospheric Moisture

AVIATION WEATHER

CHAPTER TWO

At night, the air in contact with the mountain slope is cooled by outgoing terrestrial radiation and

becomes more dense than the surrounding air. As the denser air flows downhill, from the top of

the mountain, it is called the mountain wind, and a circulation opposite to the daytime pattern

forms.

These winds are of particular importance for light aircraft, helicopter, and low-level operations.

In mountainous areas where the performance of some fixed-wing aircraft or helicopters is

marginal, the location of mountain and valley winds can be critical.

Atmospheric Moisture

Moisture is water in any of its three states: solid, liquid, or gas. As water changes from one state

to another, it releases (or absorbs) heat to (or from) the atmosphere. For example, when water in

the atmosphere freezes, it releases heat into the air, and the air becomes warmer. Air can hold

only a certain amount of water depending on the air temperature. The higher the temperature,

the more water vapor the air can hold (Figure 2-13). The air reaches saturation when it contains

the maximum amount of water vapor for that temperature.

Figure 2-13 Dots Illustrate the Increased Water Vapor Capacity of Warmer Air

The dew point temperature (TD) is the temperature at which saturation occurs. The dew point is

a direct indication of the amount of moisture present in the air. The higher the dew point, the

greater chances for clouds, fog, or precipitation.

If there is a difference between the air temperature and the dew point temperature, this is known

as the dew point depression or dew point spread, and the dew point will always be the lower of

the two. The dew point can never be higher than the air temperature, only equal to or less than.

This spread provides a good indication of how close the atmosphere is to the point of saturation.

When the dew point depression reaches about 4F, the air is holding close to the maximum

amount of water vapor possible. If this spread continues to decrease, moisture will begin to

condense from the vapor state to the liquid or solid state and become visible. This visible

moisture can form dew or frost on exposed surfaces, fog near the ground, or clouds higher in the

Atmospheric Mechanics of Winds, Clouds and Moisture, and Atmospheric Stability

2-13