8.22 Clouds

8.22.2 Describe the cloud formation process.

Cloud is made up of tiny water droplets, ice crystals, or a mixture of both, suspended in the atmosphere.

Cloud forms when moist air is cooled to its dew point, causing the air to become saturated. Once saturation is reached, water vapour condenses into visible water droplets or ice crystals.

The usual process is:

• Air is lifted
• As it rises, it expands because pressure decreases with height
• Expansion causes the air to cool adiabatically
• When the air cools to its dew point, it becomes saturated
• Water vapour condenses onto hygroscopic nuclei
• Cloud forms if enough condensation occurs

Hygroscopic nuclei are tiny particles in the air, such as dust, salt, smoke, or other aerosols, which provide a surface for water vapour to condense onto.

8.22.4 Describe the operational characteristics of the cloud sensor used in Automatic Weather Stations (AWS), and reported in METAR AUTO reports.

Automatic Weather Stations, or AWS, are used to automatically observe and report weather conditions.

They commonly measure:

• QNH
• Surface wind speed and direction
• Visibility
• Cloud base
• Air temperature
• Dew point