Reading the Clouds

How a cloud is born

In the last lesson you learned that air becomes saturated when it cools to its dew point. A cloud is what you see when that happens high above the ground: billions of tiny water droplets or ice crystals, each far too small to fall on its own, floating together like dust in a sunbeam.

Almost every cloud forms because air is rising. As air climbs, the pressure around it drops, so it expands, and expanding air cools. Lift it far enough and it cools to its dew point. There are four main ways the atmosphere lifts air:

• Convection: the sun heats the ground, and bubbles of warm air rise like hot-air balloons.
• Fronts: warm air slides up and over a wedge of colder, heavier air.
• Terrain: wind pushes air up a mountain range or a long slope.
• Convergence: air flowing together near the ground has nowhere to go but up, as happens around a low-pressure centre.

The shape of a cloud tells you how the air is rising. Gentle, widespread lifting makes flat, layered sheets. Vigorous, bubbling lift makes heaped, puffy towers.

The cloud families

Cloud names come from a handful of Latin words, and once you know them the names decode themselves: cirrus means a curl or wisp of hair, stratus a layer, cumulus a heap, and nimbus rain-bearing. Alto is used for middle-level clouds.

Clouds are grouped by the height of their base. The heights below are approximate for mid-latitudes like ours and are measured above the ground. They shift with the season: everything sits lower in winter and higher in summer.

High clouds: messengers from far away

High clouds live where it is always well below freezing, so they are made of ice crystals. That gives them a thin, silky, white look, and they often glow orange and pink at sunrise and sunset long after lower clouds have gone grey.

• Cirrus: delicate wisps and streaks, sometimes hooked at the end, nicknamed “mares’ tails.” A few scattered strands on their own mean little. Cirrus that thickens and spreads across the sky from the west or southwest is often the first sign of an approaching weather system.
• Cirrostratus: a thin, milky veil that whitens the blue sky. Its ice crystals bend sunlight or moonlight into a halo, a ring roughly the span of your outstretched hand at arm’s length from the sun. “Ring around the moon, rain or snow soon” has a real basis: a halo often means moisture is arriving aloft ahead of a system, though plenty of halos come and go with nothing following.
• Cirrocumulus: tiny white ripples or grains, like fish scales. Pretty, short-lived and fairly uncommon.

Middle and low layers: the grey stuff

• Altocumulus: rows or patches of grey-white puffs, each about the size of your thumbnail at arm’s length (bigger than cirrocumulus). Usually harmless. But altocumulus with little turrets on top, called castellanus, on a warm summer morning means the middle atmosphere is unstable, a classic clue that afternoon thunderstorms are possible.
• Altostratus: a grey or bluish sheet. The sun shows through as if through frosted glass, with no halo and no shadows on the ground. When cirrostratus thickens into altostratus, precipitation is often a matter of hours away.
• Nimbostratus: the thick, dark, featureless deck of a long, steady rain or snow. You rarely see its base clearly because precipitation hides it.
• Stratus: a low, flat, grey blanket, basically fog that isn’t touching the ground. It can bring drizzle or, in winter, freezing drizzle.
• Stratocumulus: low lumpy rolls or patches with blue gaps between them. The most common cloud on Earth, and usually a sign of dry or only showery weather.

Clouds that grow: from cumulus to cumulonimbus

On a sunny morning, bubbles of warm air rise from the ground. When they reach their dew point, fair-weather cumulus appear: cotton-ball clouds with flat bottoms (the level where the rising air reaches saturation) and rounded tops. If they stay small and well spaced, the afternoon is likely to stay fine.

If the air is unstable, the bubbles keep rising. The cumulus grow taller than they are wide, with hard, cauliflower-like tops: towering cumulus. When a tower reaches high enough for its top to freeze, the crisp edges turn soft and fibrous and spread out into a flat anvil. It is now a cumulonimbus, a thunderstorm cloud, capable of lightning, downpours, hail and damaging winds.

Watch the trend through the day. Cumulus that are noticeably taller by noon than they were at ten o’clock are telling you the atmosphere is “cooking.”

Mountain clouds and the chinook arch

When strong westerly winds blow across the Rockies, the air bounces up and down downwind of the mountains in a series of standing waves, like water flowing over a rock in a stream. Where the air rises in each wave, it can cool to saturation and form smooth, lens-shaped lenticular clouds (altocumulus lenticularis). They look like stacked plates or flying saucers, and they often hover in one spot for hours because the wave stays put while the wind blows through it.

The most famous wave cloud is the chinook arch: a long sheet of cloud with a sharp, straight western edge running parallel to the mountains, with a band of clear blue sky beneath it over the Rockies. From the ground, perspective makes that straight edge look like a great arch over the western horizon.

How much sky? Counting in oktas

Weather observers measure cloud cover in oktas: eighths of the sky. Picture the whole dome of sky divided into eight equal slices, mentally push all the clouds together, and count how many slices they would fill. Zero oktas is a perfectly clear sky; eight oktas is completely overcast.

A practical tip: clouds near the horizon look packed together because you are seeing them edge-on from a distance, so most people overestimate cover. Judge the overhead sky first, then adjust.