What clouds make the best sunsets?

Which clouds are best for a sunset?

Cirrus, cirrostratus, cirrocumulus and altocumulus top the list. They sit high enough to catch light from a sun that has already dipped below the ground-level horizon, and they are thin or broken enough to let color through. A sky with 30 to 50 percent of this kind of cloud, plus an open strip near the horizon, is the classic recipe for a sunset that lights up.

A clear sky works too, but it gives a smooth gradient and little drama, because there is nothing for the color to land on.

How high are the clouds that glow?

Wikipedia says cirrus form between about 6,000 and 20,000 meters, higher in the tropics and lower near the poles, and that they are made of ice crystals. The rising or setting sun can color them yellow or red (Wikipedia: Cirrus cloud). Altocumulus form around 2,000 to 6,100 meters and show up as patches or rolls, sometimes called a mackerel sky (Wikipedia: Altocumulus cloud).

Altitude matters because the Earth curves away. Low clouds go dark early, while high clouds still see the sun.

Why do low clouds in the west matter so much?

The sun sets behind whatever is on the western horizon. If a low bank of cloud sits between you and the sun, the light never reaches the clouds above it, however perfect they are. That is why a forecast that looks only at overhead cloud misses so much.

Afterhue looks at a second point about 150 km toward the setting sun, and lists a chip such as Low clouds to the west when that path is blocked. The Glow Score explains how that chip pulls the score down.

What does a cloud chip in Afterhue mean?

Reasons such as High clouds 32% mean a third of the sky at high altitude is cloud, which is a good canvas. Clear to the west means the path is open. Thick overcast means a ceiling is likely to hide the show. The app picks up to three reasons and orders them by how much each moved the score, so the first one is the main story.

What are crepuscular rays?

Crepuscular rays, sometimes called god rays, are visible shafts of sunlight that appear when the sun is just above or below a layer of clouds. They can look orange or red at twilight, because the light has crossed about 40 times more air than at midday (Wikipedia: Crepuscular rays). They need a cloud layer with gaps, which is another reason a partly cloudy sky beats a clear one.

What about cumulus and storm clouds?

Towering clouds can be spectacular when the sun is low, because their sides and anvils catch intense light against a dark background. They are also tied to unstable air, so a good sunset behind a storm may come with wind or rain. Wikipedia notes that towering altocumulus often signals thunderstorms later in the day (Wikipedia: Altocumulus cloud). Enjoy the view, and keep an eye on the weather.

Does the percentage of cloud cover matter?

Yes, and the sweet spot is partial cover. Very little cloud gives a soft gradient. Too much cloud means the sun is hidden. Between roughly a quarter and a half of the sky covered by high or mid-level cloud, with an open western horizon, is where the strongest displays tend to come from. This is a rule of thumb, not a law, and the best evenings still surprise people.

How do I use this when planning?

Look at a forecast that splits clouds by height, not just a single cloud percentage. A weather app's one number cannot tell thin cirrus from a low gray lid. A sunset forecast that shows layers will tell you more. See our explainer on how sunset quality is forecast for what such a forecast can and cannot see.

How can I read the sky in the last hour?

Your own eyes beat a forecast in the last hour. Walk out about 30 minutes before sunset and look west and up.

• Is the horizon open or capped by a dark bank?
• Are the high clouds thin and wispy, or a flat gray sheet?
• Do you see gaps where blue sky shows through?
• Is there a band of clear sky just above the horizon?
• Is the air hazy or crisp?

Can a cloudy day still give a great sunset?

It can, when the clouds break near the horizon. A gap on the western side lets a low sun shine under a ceiling, and the underside of the ceiling lights up in fire colors. This is a lucky setup, and it is hard to forecast. See our piece on why a cloudy evening can still be a dud and the guide to sunset after rain.

How do I tell high clouds from low clouds?

High clouds look small, fine and pale, and they seem to move slowly. Low clouds look large, dark and textured, and they move faster. If a cloud seems to have a clear shape and you can see shading on its underside, it is probably lower. Wispy streaks that look like brush strokes are probably cirrus. Keep a few evenings of notes and you will build a feel for it. A rating after each sunset helps the app tune its score to your sky too.