Estimation of the height of the cloud base
Local aids - Hills or high objects
Where clouds are seen over hills or high objects, such as radio masts or high buildings, the height of the cloud base can be compared with the known height of the tops of the hills or other objects. Similarly, when there are several layers of clouds and the height of one layer is estimated or known, the heights of other layers can be estimated.
It is important that all heights are referenced back to that above the observation station level.
In the example below, a topographical chart indicates the height of a nearby hill top to be 2000 ft above mean sea level. If the aerodrome elevation is 500 ft then the hill top is 1500 feet above the aerodrome. Further, the foot of the hill is at a similar elevation to the aerodrome. The observer estimates the distance between the base of the lowest cloud (Cumulus) and the hill top is approximately the same as the distance from the foot of the hill to its peak, 1500 ft. Another layer of cloud (Stratocumulus) is observed above the Cumulus. The base of this higher layer appears to be the same distant again from the Cumulus base, another 1500 ft. It can then be deduced that the Cumulus base is approximately 3000 ft (2 x 1500 ft) above the aerodrome, and the Stratocumulus 4500 ft (3 x 1500 ft) above the aerodrome.