Reading the Sky by Eye: Clouds, Cover and Halos

You can read the sky by eye without any instrument. Start with the layer a cloud sits in, count how much of the sky it covers in eighths, then look for the light effects that a thin veil or a shower can produce. Three habits, in that order, turn a walk into an observation.

Why does height decide the name?

Meteorology sorts clouds by the altitude band where they form, because that band controls what they are made of and how long they last. High clouds, roughly above six kilometres in mid-latitudes, are cold and thin: cirrus, cirrostratus and cirrocumulus. Middle clouds, from about two to six kilometres, hold more liquid water: altocumulus, altostratus and nimbostratus. Low clouds, below two kilometres, are the ones you see against buildings and hills: cumulus, stratocumulus, stratus, and fog when the base reaches the ground. The cumulonimbus is the exception that crosses every band, and its name is reserved for the storm cloud with a flat, anvil-shaped top.

A French-language guide to this naming work is Nommer le ciel, which walks through the ten genera by storey and then through the optics of the sky. Its structure matches the order of a real observation: first place the cloud, then describe it, then look for the light.

The height rule is not a technicality. A cirrostratus veil at eight kilometres is ice, and it produces a halo. An altostratus at three kilometres is water, and it produces a dim, grey disc of sun with no halo. Same flat sheet, different altitude, different name, different effect.

What are the ten cloud genera by height?

High, above roughly six kilometres. Cirrus: detached, white, hair-like streaks, often hooked. Cirrostratus: a transparent veil that gives the sun or moon a halo. Cirrocumulus: thin, rippled patches, sometimes called a mackerel sky.

Middle, roughly two to six kilometres. Altocumulus: grey-white rolls or patches, larger than the cirrocumulus ripples. Altostratus: a uniform grey sheet that dims the sun without a halo. Nimbostratus: a thick, dark, featureless layer that gives steady rain or snow.

Low, below two kilometres. Cumulus: detached, dense, flat-based heaps with bright tops. Stratocumulus: low, lumpy sheets, often in rows. Stratus: a uniform grey layer close to the ground. Fog: the same stratus, but with its base at the surface.

The tenth genus is the cumulonimbus, the deep storm cloud that starts in the low band and reaches the top of the troposphere. Its anvil can be seen from tens of kilometres away, and its distance is read from the delay between lightning and thunder: about three seconds per kilometre.

How is cloud cover measured in eighths?

Observers describe how much of the sky dome is hidden by cloud using a scale of eighths, called octas. Zero octas is a clear sky. Eight octas is overcast. The intermediate values are not a percentage of cloud in the air but a fraction of the visible sky, judged from a single point with a full view of the horizon.

The practical method is to imagine the dome divided into eight equal wedges and to count how many contain cloud. One or two octas is a mostly clear sky. Three or four is a partly cloudy sky. Five to seven is a mostly cloudy sky. Eight octas means no blue is visible anywhere.

Two cautions. First, a single large cumulus can cover two or three octas on its own, so the count is coarse by design. Second, thin cirrus that does not hide the blue still counts: the question is whether cloud is present in the wedge, not whether it blocks the light. For a beginner, the honest answer is often a range, such as five to six octas, and that is acceptable.

Which optical phenomena can you name?

Optical effects are the reward for looking at thin cloud rather than thick cloud. They come from ice crystals or water droplets bending and reflecting light, and each has a name and a geometry.

The 22-degree halo is a ring around the sun or moon at a radius of about 22 degrees, roughly the span of an outstretched hand at arm's length. It appears when a cirrostratus veil contains hexagonal ice crystals, and it is the most common halo. Parhelia, also called sun dogs, are bright patches on the halo at the same height as the sun, often with a reddish edge facing the sun. A circumzenithal arc is a bright, upward-curving arc near the top of the sky, sometimes mistaken for a rainbow, but it faces the opposite way and sits much higher.

Rainbows belong to the water-droplet family. The primary bow has a radius of about 42 degrees, with red on the outside. The secondary bow is wider and fainter, at about 51 degrees, with the colours reversed. Both are centred on the point opposite the sun, which is why your shadow points at the bow's centre. Coronae and glories are smaller and tighter: coronae are coloured rings close around the sun or moon in thin cloud, and a glory is the same effect seen around your own shadow on a misty surface, often from a hilltop or an aircraft.

What should a beginner do first?

Pick one observation per walk and write it down. Note the date, the time, the octa count, the genus you think you see, and any light effect. A single line per day is enough. After a month, the notes show which genera you actually meet in your area and which ones you keep misreading.

Then add one constraint at a time. Spend a week on high clouds only, and check whether a veil produces a halo. Spend another week on the low band, and separate stratus from stratocumulus by looking for lumps. Add the optics last, because they depend on the cloud you have already identified.

When does the reading fail?

It fails in three common situations. The first is a featureless overcast sky, where nimbostratus and altostratus can look identical from below; the difference is the rain and the altitude, and without a weather map you may have to record it as an uncertain layer. The second is a sky full of overlapping bands, where a low cumulus sits in front of a high cirrus and the two are easy to merge into one name. The third is a bright sun near the horizon, where glare hides the thin veil that would have produced a halo.

None of these failures is a reason to stop. They are the normal limits of naked-eye observation, and naming them is part of the skill. A careful observer who writes down an uncertain reading is doing better than one who guesses a confident name. For the same sky seen from orbit, the public image archives are the place to start.