Forecasting terms Clear: Free from cloud, fog, mist or dust haze. Sunny: Little chance of the sun being obscured by cloud. Note: High level cirrus clouds are often thin and wispy, allowing a considerable amount of sunlight to penetrate them, sufficient to produce shadows. In this case the day could be termed 'sunny' even though more than half the sky may be covered in cirrus cloud. Cloudy: Predominantly more cloud than clear sky for example, during the day the sun would be obscured by cloud for substantial periods of time. Overcast: Sky completely covered with cloud. Forecasts of cloud cover normally give an average, if no significant variations are expected. A clear day, for example, may at some times see a few cloud patches. Forecasters expecting significant variations in cloud amount may use such terms as sunny periods, sunny breaks, cloudy periods, cloudy at times, mostly/mainly sunny, mostly/mainly cloudy. If expecting a major change in cloud cover, they usually indicate a distinct trend, e.g. becoming sunny or cloud increasing. To top
The fraction of the sky dome covered by clouds. This fraction is typically expressed either as tenths (1/10, ..., 10/10) or eighths (1/8, ..., 8/8). Some researchers refer to this as cloud amount, to clarify the distinction from cloud type, which is the nature of the cloud cover.