How to Predict a Great Sunset: The 3 Clues

The lesson

A great sunset needs three things: clouds high enough to catch the light, a clear path for that light from the horizon toward the setting sun, and clean air. Get all three and the sky lights up. Miss one and it fades to grey, even when the sky over your head looks perfect.

Why sunsets are red in the first place

Sunlight is a mix of every colour. When the sun is overhead, its light crosses a thin slice of atmosphere. At sunset it skims in sideways, through about 38 times more air than when the sun is straight overhead.

On that long trip, air molecules scatter short blue wavelengths out of the beam. It's the same scattering that makes the daytime sky blue. What survives the journey is orange and red.

Key idea

Think of a sunset as a projector: the setting sun is the lamp. But a lamp on its own only lights a thin strip of horizon. It needs a screen.

Clue one: the screen

The screen is cloud, and the height matters. Mid-level clouds such as rippled altocumulus (about 2–6 km up) and high, wispy cirrus (about 6–12 km) sit so far above you that they are still in sunlight after the sun has set where you're standing. They catch the red light from underneath and glow.

High wispy and rippled clouds glowing pink and orange from below while the forest beneath is already in shadow — High clouds stay in sunlight after the ground has gone dark, so they light up from underneath.

Low cloud doesn't work the same way. A solid grey deck near the ground blocks the light instead of catching it, and a completely clear sky gives you a glow on the horizon but nothing overhead to light up. Broken, scattered mid and high cloud makes the best screen.

Three versions of the same lake at sunset: a cloudless sky, a low grey overcast, and a sky full of broken clouds lit red and gold — No clouds: nothing to light. A low grey lid: the light can't get in. Broken high clouds: the screen.

Clue two: a clear path for the beam

This is the clue almost everyone misses. To light clouds from underneath, the sunlight has to come in low, almost level with the ground, from the direction of the setting sun. A thick bank of low cloud in that direction is like someone standing in front of the projector.

And because that low beam skims across the curve of the Earth for hundreds of kilometres before it reaches the clouds above you, the blocker can be 100 km or more away, over a town you can't see. Light that reaches a cloud 10 km up after sunset last grazed the surface roughly 350 km toward the sun.

Diagram: low sunlight skimming along the curved Earth is stopped by a low cloud bank more than 100 km away, while the clouds above the town stay grey — The beam: low sunlight has to travel 100 km or more to reach the clouds above you. A low cloud bank along the way blocks it.

Key idea

The sky over your head can look perfect and the sunset can still fail, because of weather beyond your horizon.

That's why a quick look outside isn't a forecast. You can judge the clouds above you. You can't see the gap, or the wall, on the far side of the horizon.

Clue three: the lens (clean air)

The cleaner the air, the purer the colour. Thick haze, humidity-swollen particles or wildfire smoke near the ground wash the sky into a flat, muddy orange, like a projector with a dirty lens. Smoke can turn the sun itself deep red, but it usually mutes the clouds.

Left: a dull brown-orange sunset through thick smoke haze. Right: a vivid sunset over wet fields after a storm, with blazing broken clouds — Thick haze washes colour out (left). After a storm clears, cleaner air and broken clouds can make a spectacular sky (right).

That's one reason the sky right after a storm clears can be spectacular: rain helps scrub the air, and the departing system often leaves broken clouds behind.

Bonus: don't leave when the sun disappears

High clouds can stay lit for ten to twenty minutes after sunset, longer at high latitudes in summer, because they are still in sunlight long after you are not. The colour often peaks after most people have already gone inside.

Deep blue twilight over a beach with high clouds still glowing rose and coral long after sunset — The afterglow: the beach is dark, but the high clouds are still catching the sun.

The three clues

A few hours before sunset, check:

The screen: is there broken mid- or high-level cloud overhead, not a solid low grey deck?
The beam: is the horizon toward the setting sun clear of thick low cloud, as far as 100 km out and beyond?
The lens: is the air clean, with no thick haze or smoke?

Transcript

Video transcript

Read the full transcript

0:00Same sun. Same spot. One evening, the sky catches fire. The next, it fades to grey. The difference is often a cloud a hundred kilometres away, over a town you can't even see. Here are three clues that tell you, hours ahead, whether tonight could light up.

0:20Quick physics. At sunset, light skims through about thirty-eight times more air than when the sun is overhead. Along the way, the air scatters the blue out of the beam. What's left is orange and red.

0:35Think of a sunset like a projector. The setting sun is the lamp. But a lamp needs a screen.

0:42Clue one: the screen. Mid-level and high clouds, rippled altocumulus and wispy cirrus, sit miles above you. They're so high, they're still in sunlight after the sun has set for you. They catch that red light from underneath, and glow. No clouds, nothing to light. A low grey lid, and the light can't get in. Broken clouds make a beautiful screen.

1:08Clue two: a clear path for the beam. That light comes in low, from the horizon. A thick bank of low cloud toward the setting sun is like someone standing in front of the projector. And because that beam skims across hundreds of kilometres to reach your clouds, the blocker can be a hundred kilometres away. The sky over your head can look perfect, and the sunset still dies.

1:33Clue three: clear air. The cleaner the air, the purer the colour. But thick haze or smoke washes everything into a dull, muddy orange, like a dirty lens. It's one reason clearing weather after a storm can be spectacular.

1:51And when the sun disappears? Stay. High clouds can stay lit for ten to twenty minutes after sunset. The colour can peak after most people have gone inside.

2:03The screen and the lens, you can judge from your porch. The beam, a hundred kilometres away, you can't. Forecast Bar can. Its Sunset Quality score reads clouds at three heights from national weather models, then scans the sky hundreds of kilometres toward the setting sun. Across the lower forty-eight, it factors in smoke, and near sunset, live satellite images. Turn on sunset alerts, and when an evening looks promising, you'll get a heads-up, with time to get outside.

2:36Forecast Bar. Catch more of the good ones.

Where Forecast Bar comes in

You can see the screen and the lens. Forecast Bar checks the beam.

Sunset Quality

A score for every sunrise and sunset, with the reasons.

Forecast Bar's Sunset Quality score reads cloud at three heights from national weather models, then scans the sky hundreds of kilometres toward the setting sun, the far horizon you can't see from your porch. In the contiguous US it also factors in smoke, and as sunset nears it blends in live satellite imagery.

A 0–10 score with a plain-language reason, like “color-bearing clouds have a clear path to the Sun”
The peak-colour time, often after the sun has set, and which way to look
Scores for sunrise too

Forecast Bar's sun path view for tonight's sunset

Beautiful Sunset alerts

A heads-up before the good ones in the contiguous US.

In the contiguous US, turn on sunset alerts and when an evening looks promising Forecast Bar sends a notification with time to get outside. In the contiguous US, live satellite confirmation keeps the alerts for evenings that are actually shaping up.

Choose how often: only confirmed great evenings, more of them, or a daily rating
Sunset Quality and alerts are part of Forecast Bar Gold on iPhone and iPad, and Pro on Mac

FAQ

Questions about sunsets

What makes a sunset colourful?

Three things: mid- or high-level clouds to catch the light (the screen), a clear path for low sunlight from the horizon toward the setting sun (the beam), and clean air (the lens). Sunlight is red at sunset because it crosses about 38 times more air than when the sun is overhead, which scatters the blue out of the beam.

Do clouds make a sunset better or worse?

Both. Broken mid- and high-level clouds such as altocumulus and cirrus make sunsets better, because they are high enough to stay sunlit after sunset and glow from below. A solid deck of low cloud makes it worse, especially toward the setting sun, because it blocks the light before it reaches the higher clouds.

Why was the sunset grey when the sky above me was clear?

Usually because of cloud you couldn't see. The light that colours the clouds above you travels in low across hundreds of kilometres. A bank of low cloud 100 km or more toward the setting sun can block it, leaving the sky overhead clear but colourless.

When is the best time to watch a sunset?

Arrive before sunset and stay for 10 to 20 minutes after it. High clouds stay in sunlight after the sun has set for you, so the most intense colour often comes after the sun has dropped below the horizon.

Does wildfire smoke make sunsets better?

It can make the sun look deep red, but thick smoke or haze near the ground usually mutes the clouds into a flat, muddy orange. The most vivid skies tend to come with clean air.

How much does Forecast Bar cost?

Forecast Bar is free to download, and live weather requires a subscription. On iPhone and iPad, every subscription starts with a 7-day free trial for new subscribers, so you can try everything your plan includes before you're charged. Mac plans do not include a trial. In the US App Store, iPhone and iPad are $11.49 a year for Silver and $19.99 a year for Gold, and Mac is $9.99 a year for Plus or $19.99 a year for Pro. Prices vary by region, so the App Store shows what you will actually pay. Mac and iOS plans are separate; Apple TV is included with Forecast Bar Gold on iOS. See Compare Plans for what each tier includes.

Catch more of the good ones.

Sunset Quality works worldwide; Beautiful Sunset alerts cover the contiguous US. Both are part of Forecast Bar Gold. On iPhone and iPad, every subscription starts with a 7-day free trial for new subscribers.

Download on theApp Store Download on theMac App Store

Free to download; live weather requires a subscription. On iPhone and iPad, every subscription includes a 7-day free trial for new subscribers; Mac plans do not. Mac and iOS plans are separate; Apple TV is included with Forecast Bar Gold on iOS.

Keep exploring

How to predict a great sunset.