Cumulus

Every cumulus in the sky over a given place tends to have its base at the same height, and that height is not arbitrary. It is the altitude at which the air rising off the ground gets cold enough for its water vapour to condense. The flat undersides of a field of fair-weather cumulus are a physical measurement of the atmosphere, drawn at scale, visible from a lawn chair.

A heap, and not much more

The Latin is literal: cumulus, a heap. The World Meteorological Organization's definition is a good deal more careful — detached, generally dense clouds with sharp outlines, developing vertically as rising mounds, domes or towers, with bulging upper parts that often resemble a cauliflower. The sunlit parts are brilliant white. The bases are relatively dark and horizontal.

That contrast between top and bottom is worth understanding, because it is not a trick of the atmosphere so much as a trick of optics. Cloud droplets are close in size to the wavelengths of visible light, which means they scatter every colour equally — Mie scattering, as opposed to the Rayleigh scattering off air molecules that makes the sky blue. Equal scattering of all wavelengths reads to the eye as white. The base is dark because the cloud above it is thick enough to block the sunlight coming through, and because the eye is comparing it against a much brighter background. Contrast does most of the work.

Cumulus is classified as a low cloud, with bases typically within the first 6,500 feet of the surface. That classification describes the base, not the cloud. The same genus, grown large enough, becomes cumulonimbus, which extends well into the high-cloud range.

Where the flat base comes from

A parcel of air leaving a warm surface expands as it rises into lower pressure, and expansion cools it. This is the adiabatic process: the temperature change comes from the parcel's own expansion rather than from any heat exchanged with the air around it. The rate is fixed at 5.5°F for every 1,000 feet of ascent until the parcel reaches saturation — the dry lapse rate.

NOAA's textbook case: a parcel starting at the surface at 85°F with a dew point of 65°F reaches saturation at roughly 4,000 feet. That is the cloud base. Every parcel rising off the same air mass hits saturation at nearly the same altitude, which is why the bases line up as though ruled.

Condensation itself needs somewhere to happen. Water molecules cannot bond into droplets on their own; they need a surface at least a micrometre across. Smoke, volcanic ash, ocean spray and wind-blown soil supply it. These condensation nuclei are hygroscopic — they attract water — and are about a hundredth the size of the droplet that forms around them. Every cloud droplet in every cumulus has a speck of something at its centre.

The base is a useful diagnostic in itself. Drier air near the ground means the parcel must climb further before cooling to its dew point, so the base sits higher. Cumulus bases at 9,000 feet have been observed over North Central Texas, and thunderstorms with bases between 11,000 and 12,000 feet near San Angelo. That happens when the lower atmosphere is dry but the mid-levels are moist and unstable, and parcels have to rise two miles or more before they condense.

Why the edges keep changing

A cumulus is not a fixed object. Rising air continually entrains drier surrounding air, so condensation and evaporation are both happening at once, everywhere in the cloud. Where condensation outpaces evaporation, the cloud grows. Where evaporation wins, it thins and vanishes. This is why the crisp cauliflower shoulders of a growing turret look sharp while the ragged margins look like they are dissolving — they are.

Reading the afternoon

Over land, cumulus is a creature of the diurnal cycle. On a clear day it appears in the morning as the surface heats, grows through the afternoon, and more or less dissolves again toward evening. Individual cells are short-lived; the field they belong to is not.

The thing to watch is not whether cumulus is present but whether it is winning. If the tops stop climbing, flatten out and spread — the cloud has run into stable air aloft that it cannot penetrate, and the afternoon stays fair. If the tops keep hardening and rising, that lid is gone.

The next stage is towering cumulus, also called cumulus congestus: great vertical development, still the cauliflower appearance, still no anvil. This is the transitional stage, and the distinguishing feature is negative. No anvil yet.

Cumulonimbus is what happens when the tower reaches its ceiling. The upper portion loses the sharp outline of a water-droplet cloud and turns smooth, fibrous or striated, flattening into an anvil or a broad plume — a form of cirrus, and therefore made of ice. The base is often very dark, with low ragged clouds beneath it that may or may not merge with it. Cumulonimbus produces precipitation, sometimes as virga, streaks of rain that evaporate before reaching the ground. It also produces hail and tornadoes.

What that means for standing underneath one

Ordinary fair-weather cumulus is a benign forecast signal, and it is honest about what it is. The hazard lives entirely in the transition. A sky of small, well-separated heaps with flat bases and rounded tops is telling you the boundary layer is convecting and something up there is holding the lid on. A sky where one of those heaps is visibly taller than its neighbours, with a top that keeps building rather than flattening, is telling you the lid has failed at that spot.

The genus does not change when this happens. The cloud does. A cumulus that has become a cumulonimbus is the same convective column that started as a warm bubble over a parking lot, given a few hours and nothing in the way.