Cloud cover (cloud amount)

The total amount of cloud should be estimated by considering how much of the apparent area of the sky is covered by cloud. In determining the amount of cloud of a specified form or type present, the observer should estimate, by taking into consideration the evolution of the sky, the cloud amounts of each layer or mass at the different levels as if no other clouds were present.

Care should be taken to avoid unconsidered guessing and the best safeguard against this is a knowledge of the evolution of the clouds under consideration. On occasions of fog which is so thick as to make it impossible to tell whether there is cloud above or not, the state of sky should be recorded as sky obscured. If the cloud can be seen through the fog, the cloud amount should be estimated as well as circumstances permit. If the sun, moon or stars can be seen through the fog and there is no evidence of cloud above the fog, the state of the sky should be recorded as clear.

At night the observation of total cloud amount is noted by observing which stars are showing and which are obscured. It is more difficult to differentiate between low, middle and high clouds and a reliable observation depends upon the degree of illumination and the experience of the observer.