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Lesson 6.2 – Temperature, Moisture & Cloud Formation
Temperature and moisture are two of the most important ingredients in weather. Their relationship determines whether the air is stable or unstable and whether clouds and precipitation will form.
Reference:
FAA Pilot’s Handbook of Aeronautical Knowledge (PHAK), Chapter 12 – Weather Theory
Temperature
Temperature
Temperature is a measure of the average kinetic energy of air molecules. It is usually measured in degrees Celsius in aviation.
Lapse Rate
The rate at which temperature decreases with altitude is called the lapse rate.
Standard (average) lapse rate: approximately 2°C per 1,000 feet.
Actual lapse rates vary and strongly influence atmospheric stability.
Moisture in the Atmosphere
Moisture in the Atmosphere
Air always contains some water vapor. The amount of moisture the air can hold depends mainly on its temperature — warm air can hold more water vapor than cold air.
Important moisture terms:
Dew Point – The temperature to which air must be cooled (at constant pressure) to become saturated.
Relative Humidity – The amount of water vapor actually in the air compared to the maximum amount the air could hold at that temperature, expressed as a percentage.
Spread (Temperature–Dew Point Spread) – The difference between the temperature and the dew point. A small spread (3°C / 5°F or less) indicates high relative humidity and the possibility of fog, clouds, or precipitation.
Cloud Formation
Cloud Formation
Clouds form when moist air is cooled to its dew point and the water vapor condenses into visible water droplets or ice crystals.
The most common ways air is cooled to form clouds:
Lifting – Air is forced upward (orographic lifting over mountains, frontal lifting, convergence, or convective lifting from surface heating).
Cooling from below – Warm moist air moves over a colder surface.
Evaporation / mixing – Adding moisture or mixing air masses.
Stability
Stability
Atmospheric stability determines whether air that is forced upward will continue to rise or will resist vertical movement.
Stable air – Resists vertical movement. Produces smooth flying conditions, layered (stratus-type) clouds, and steady precipitation.
Unstable air – Allows or encourages vertical movement. Produces turbulence, cumulus-type clouds, showery precipitation, and possible thunderstorms.
Rule of thumb:
When the actual lapse rate is steeper than the standard rate (temperature drops quickly with altitude), the air is more likely to be unstable.
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Key Points to Remember
Key Points to Remember
Warm air can hold more moisture than cold air.
When the temperature–dew point spread is small, relative humidity is high and clouds or fog are more likely.
Clouds form when moist air is cooled to its dew point.
Stable air = smooth conditions and layered clouds.
Unstable air = turbulence, cumulus clouds, and showery weather.
Lifting of moist air is the most common cause of cloud formation.
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