Low clouds: cumulus, stratus, stratocumulus

Low clouds have their bases below 2 km, close enough to hide the top of a tall building.

The low level

The WMO divides the troposphere into three levels by the heights at which each kind of cloud is most often seen. The low level runs from the ground to about 2 km, at every latitude. Four genera belong to it: stratus, stratocumulus, cumulus and cumulonimbus. The last two are special. Their bases are low, but they can grow so tall that their tops reach the middle and even the high level. This unit covers the three that stay low; cumulonimbus gets a unit of its own.

Cumulus: heaps on rising air

The WMO definition reads like a description of a summer sky: detached clouds, generally dense with sharp outlines, growing upwards in mounds, domes or towers, the top often looking like a cauliflower. Each cumulus marks a column of warm, rising air, a thermal. Glider pilots look for them to find rising air. The flat base is the height where that rising air reaches saturation. On a fine day cumulus form in late morning as the ground heats up, and often fade away in the evening as the heating stops.

Small, flat cumulus that are wider than they are tall mean fair weather. If they keep growing taller through the day, so that the towers are taller than they are wide, the air is unstable and showers are possible by afternoon. Watching how fast cumulus grow is one of the simplest forecasts you can make yourself.

Stratus and fog

Stratus is the opposite of cumulus. The WMO describes a generally grey layer with a fairly uniform base, which may give drizzle, snow or snow grains. It forms when a whole layer of moist air is cooled gently, not lifted in bubbles: damp sea air drifting over cold land, or air cooling slowly overnight. Stratus can be thin enough for the Sun's outline to show through, or thick enough to make a gloomy day. When the cloud forms right down at the ground we call it fog. The USGS table gives fog droplets a median size of only about 0.01 mm.

Stratocumulus: the most common layer

Stratocumulus is a layer broken into rounded masses, rolls or patches, grey or whitish, often with darker parts. It can look like a field of cotton balls pressed together, with blue gaps between them. It forms when a layer of cloud is stirred by weak rising and sinking motions, or when spreading cumulus flatten out under a stable layer of air in the evening. It rarely gives more than light drizzle, and from the ground it often makes a dull, grey day with the Sun breaking through now and then.

Telling stratocumulus from the similar middle-level altocumulus is a common puzzle. The WMO identification guide uses the apparent size of the separate cloud elements: altocumulus elements look 1° to 3° across, and stratocumulus elements look larger, because they are closer to you. The little finger held at arm's length covers roughly 1° of sky, so a quick check with your hand is often enough.