Reading the sky

Weather observers report how much of the sky is covered in eighths, called oktas: 0 is a clear sky and 8 is overcast.

Three questions for any cloud

Faced with a sky full of clouds, ask three questions in order. First, how high is it? Use what you know of the levels: if it hides the top of a hill or a tall building, it is low; if it looks small and far away with crisp, fibrous edges, it is probably high. Second, is it a heap or a layer? Heaps grow on rising bubbles of air; layers form when a broad sheet of air is lifted or cooled gently. Third, is anything falling from it? Rain or snow reaching the ground points to nimbostratus or cumulonimbus. Streaks that hang below a cloud and fade before reaching the ground are called virga.

Clouds as forecasts

Some patterns are worth learning because they come up again and again. Small cumulus that stay small all day mean fair weather. Cumulus that grow tall by early afternoon warn of showers or thunderstorms later. A sequence of cirrus, then a cirrostratus veil with a halo, then thickening altostratus usually means a warm front is approaching, with steady rain from nimbostratus to follow. Stratus or fog on a calm morning often lifts and breaks up as the Sun warms the ground.

Temperature changes the picture

The model shows why warm days and cold days have different skies. Warmer sea and air put more water into the air: the NWS formula raises the vapour ceiling from about 8.7 hPa at 5 °C to about 31.7 hPa at 25 °C, more than three and a half times as much. Warm air also pushes the 0 °C level up. In the standard atmosphere it sits about 0.8 km up at 5 °C and about 3.9 km up at 25 °C, so the same cloud can drop snow on a winter hill and rain on a summer one.

Professional observers still look up. Alongside satellites such as NOAA's GOES, which photograph clouds from space every few minutes, weather stations report the cloud types and the amount of sky covered, in oktas. A sky with 4 oktas is half covered. You can practise the same way: note the time, the cloud types by level, the oktas and what the weather did over the next few hours. After a few weeks the sky starts to read like a forecast.

The WMO sets out how to judge cloud cover fairly. Make the estimate from an open place where the whole sky can be seen. If part of the sky is hidden by hills or haze, judge from the part you can see, and count sky veiled by falling rain as covered by that cloud. On a dark night, use the stars: the share of sky where they are dimmed or hidden is the cloud cover. When there are several layers, note the amount of each one; the WMO points out that the layer amounts can add up to more than the total, because one layer can hide behind another.