Middle and high clouds

At 9 km, where cirrus float, the standard atmosphere is about −43 °C, so these clouds are made of ice.

Levels move with latitude

The WMO's levels are not fixed numbers. The troposphere, where clouds live, is deeper in the warm tropics and shallower near the poles, so the levels stretch and shrink with it. In temperate regions such as Korea, Europe or most of the United States, the middle level is about 2 to 7 km and the high level about 5 to 13 km. In polar regions the high level is only 3 to 8 km; in the tropics it reaches 6 to 18 km. The levels overlap on purpose, because clouds do not respect neat boundaries.

The middle level

Altocumulus is a layer or patches of small rounded clouds, white or grey, often arranged in rows or waves, larger-looking than the tiny grains of cirrocumulus. Altostratus is a greyish or bluish sheet, often covering the whole sky, thin enough in places for the Sun to show as through ground glass but without the halo that a high ice cloud gives. The WMO notes that altostratus often extends higher than the middle level. Nimbostratus is the thick, dark grey rain layer that brings steady rain or snow for hours. It is almost always based in the middle level but usually reaches into the levels above and below.

The high level: clouds of ice

Cirrus are thin white filaments, patches or narrow bands with a fibrous, hair-like look or a silky sheen, in the WMO's words. Using the NASA standard atmosphere, the air at 9 km is about 15 − 6.49 × 9 ≈ −43 °C on an average day, so cirrus are made of ice crystals. As the crystals fall slowly through changing winds, they trail into the hooks and streaks that give the cloud its name. Cirrocumulus is a thin white sheet or patch of very small grains or ripples, with no shading. Cirrostratus is a transparent whitish veil that can cover the whole sky; its ice crystals bend sunlight and moonlight into a halo, a ring around the Sun or Moon.

High clouds alone rarely bring bad weather to the ground, but they are useful messengers. Cirrus spreading in from one side of the sky and thickening into cirrostratus, then altostratus, often means a warm front and a long spell of rain are on the way; UCAR notes that once altostratus forms, rain usually follows within about six hours.

The WMO's cloud identification guide gives two quick tests. The first is size: hold your arm out straight. The grains of cirrocumulus look less than 1° across, smaller than the width of your little finger, while the rounded masses of altocumulus look 1° to 3°. The second is shadows. Thin cirrostratus still lets the Sun cast shadows on the ground and often shows a halo; altostratus casts no shadows, even when the Sun can be made out as a blurred disc. With these two tests and the level table, most middle and high clouds can be named in a few seconds.