Estimation of the height of the cloud base
The cloud base is the lowest zone in which the obscuration corresponding to a change from clear air or haze to water droplets or ice crystals causes a significant change in the profile of the backscatter extinction coefficient (WMO definition) – or more simply, the lowest level in the atmosphere where the air contains a perceptible quantity of cloud ‘particles’.
Cloud base height
The cloud base height is always expressed in terms of the height above the station level. This station is usually the observation point or the aerodrome reference point (ARP). For all reporting and broadcast mechanisms in Australia the unit used to express the height of cloud is feet.
When clouds are observed over distant hills, the height of cloud base still refers to the height above the observing point, and not the height above the hills. For example, the cloud base may be only 500 feet above the tops of the hills, but if the tops of the hills are 3000 feet above the station level, then the cloud base is reported at 3500 feet.
With regular practice the height may be determined by visual estimation with a high degree of accuracy, especially for low clouds. The following pages outline various methods to assist with determining cloud height.