Altocumulus

Altocumulus is the most common mid-level cloud, and its name is a misnomer. Altus is Latin for high, but altocumulus sits in the middle of the atmosphere, roughly 8,000 to 20,000 feet up. The prefix was never a claim about absolute altitude — it exists to separate this cloud from its low-level relative, cumulus, which forms from the same physics a mile or two closer to the ground. Same heap, higher shelf.

That matters more than a naming quibble, because the middle atmosphere is the layer you cannot feel. Surface temperature and humidity are available to anyone with a thermometer and a porch. Conditions several thousand feet overhead are not. A field of altocumulus is a free readout from that layer, and a forecaster who sees one at nine in the morning already knows something about the afternoon.

What the cloud is made of

Altocumulus is composed primarily of water droplets, which is the fact that drives nearly everything else about its appearance. Droplets scatter all visible wavelengths roughly equally — Mie scattering, named for the particle size involved, where the droplets are close in size to the wavelengths of light passing through them. Equal scattering across the spectrum is what the eye reads as white.

At sufficiently low temperatures the cloud can contain ice crystals instead, which is why altocumulus occupies an awkward middle ground in cloud physics. Its cousins overhead, cirrus and cirrocumulus, are ice. The clouds below it are liquid. Altocumulus can be either or both, depending on where in the layer it forms and how cold that layer happens to be.

Where the patches thicken, they turn gray, for the straightforward reason that thicker cloud blocks more sunlight from reaching your side of it. A single altocumulus field can therefore show white and dark gray elements simultaneously — bright where the cloud is thin, shaded where it has built up. Part of that contrast is also a trick of perception: a moderately bright cloud seen against a much brighter background reads as darker than it is.

Convective rolls, two miles up

The formation mechanism is convection, but convection confined to a layer rather than rising from the ground.

Air within a mid-level layer that is unstable — cooling quickly enough with height that a displaced parcel keeps going — will overturn. Warmer air rises in one place, cooler air sinks nearby, and the circulation organizes itself into rolls or cells. As each rising parcel climbs it expands into lower pressure and cools, at about 5.5°F for every 1,000 feet, until it reaches its dew point. Water vapour then condenses onto the microscopic particles suspended in the air — smoke, soil, sea salt — that give every cloud droplet its solid core.

The result is cloud where the air rises and clear sky where it sinks. That is the whole explanation for the pattern. The regular grid of rounded masses, the rolls, the honeycomb spacing — you are looking at the plan view of a convective circulation, cloud marking the updraughts and gaps marking the downdraughts. The spacing between elements reflects the size of the cells, which reflects the depth of the unstable layer.

Terrain can force the same result without local instability. Air lifted over a mountain range cools by the same adiabatic process, and the wave pattern set up downwind produces its own regular series of cloud bars.

The three-finger test

The reliable way to separate altocumulus from the clouds above and below it is angular size, measured with your hand at arm's length.

Hold your arm out straight and look at your little finger. It covers roughly one degree of sky. Three fingers cover roughly five degrees.

Cirrocumulus elements are smaller than your little finger — under one degree apiece, fine grains and ripples, and always higher and thinner. Altocumulus elements fall between one and five degrees: bigger than the little finger, smaller than three fingers. Stratocumulus elements exceed five degrees, larger than three fingers, and belong to the low layer.

This works because apparent size is a proxy for distance. The same physical cloud element looks smaller the farther away it is, and cloud height is the dominant term. It is the closest thing in casual weather observation to a genuine measurement, and it takes about two seconds.

Two secondary cues help. Altocumulus elements are darker than cirrocumulus, because they are thicker and made of droplets rather than crystals. And altocumulus rarely appears in a single tidy sheet. Multiple layers at different heights at the same time are common, as is altocumulus mixed in with other cloud types entirely.

Corona, not halo

There is one optical test that settles composition outright.

When the thin edge of an altocumulus patch drifts across the sun or moon, a coloured ring appears within a few degrees of the disc — red on the outside, blue on the inside. That is a corona, and it is produced by diffraction around water droplets.

A halo is a different thing with a different cause. Halos come from ice crystals refracting light, and they are the signature of cirrostratus, the high milky veil that can otherwise look like thin fog. Same sky, same sun, different physics.

If you see a tight coloured ring hugging the sun, you are looking at liquid water in a mid-level cloud. If you see a wide bright circle standing well off from it, the layer is ice and it is higher than you think.

What the morning field is telling you

A sheet of altocumulus at breakfast is evidence that the middle atmosphere is already overturning on its own, without help from surface heating. That is the part worth noticing. It means the instability is not confined to a shallow layer near the ground that a few hours of sun will exhaust.

Add daytime heating from below, and any parcel that gets lifted off the surface enters a layer that is already willing to let it keep rising. The morning cloud does not cause the afternoon storm; it reports the condition that would allow one. Whether that condition is realized depends on moisture and on whether anything comes along to force the lift — a front, a sea breeze, a mountain slope.

The signal is probabilistic and it is easy to over-read. Plenty of altocumulus mornings end in nothing but a pleasant, slightly cluttered sky. But when the elements are growing turrets during the morning rather than flattening out, the layer is not merely unstable in principle. It is convecting, visibly, in front of you.