8.22 Clouds

8.22.2 Describe the cloud formation process.

Cloud can be described as suspended water in liquid or solid form in the air.

Cloud is formed by air being lifted and cooled adiabatically until the water vapour condenses out as water droplets.

This occurs when the temperature of the air reaches its dew point and the air becomes saturated and cannot take on any more water vapour.

Condensation forms on the hydroscopic nuclei. (small particles present in air).

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 (AWS) are used by the Meteorological Service of New Zealand to monitor a number of different weather phenomena.

• Atmospheric pressure QNH
• Surface wind speed and direction
• Visibility and cloud
• Air temperature
• Dew Point

AWS Cloud Sensor

Laser ceilometers are used to measure cloud base.

A photo electric cell is positioned at the base of a vertically scanning searchlight.

The light reaching the cloud is scattered and detected by the photo electric cell.

The cloud base can then be calculated.