How to tell if tonight's sunset will be colorful

Why sunsets turn red in the first place

When the sun is low, its light has to cross far more atmosphere to reach you than it does at midday. Stephen Corfidi of NOAA's Storm Prediction Center explains in The Colors of Twilight and Sunset that along that long path the violet and blue light is scattered out of the beam, leaving the oranges and reds. His short version: sunsets are red because the daytime sky is blue.

That reddened light only becomes a show if something catches it. On a cloudless evening you get a warm band along the horizon and not much else. Corfidi told National Geographic that clouds act as a sort of projection screen for the low sun colors.

So predicting a good sunset comes down to three questions. Are there clouds of the right kind at the right height? Can the sunlight actually reach them? And is the air between you and the sun clean enough to let the color through?

The clouds that catch the color

Height matters most. Corfidi writes that the most spectacular scarlet, orange and red most often appear on cirrus and altocumulus layers, because a cloud has to be high enough to intercept sunlight that has not lost its color passing through the dusty lowest layer of the atmosphere. NOAA's JetStream guide describes cirrus as detached white, delicate filaments high in the sky, and altocumulus as white or gray patchy sheets or rolls at middle levels.

Low clouds such as stratus and stratocumulus sit in that lower layer and usually light up only briefly. Sunsethue, which publishes sunset quality forecasts, says low clouds tend to glow for a couple of minutes while high clouds can hold color for up to 30 minutes after sunset. It defines low clouds as below 2,000 m, mid-level as 2,000 to 6,000 m and high as above 6,000 m.

How much cloud is ideal depends on who you ask. Sunsethue aims for around 40 to 60 percent cover. Landscape photographer Stephen Bay suggests 50 to 75 percent cloud cover with a clear horizon where the sun is setting. Both point the same way: a partly cloudy sky beats both an empty one and a full overcast.

Look west for the horizon gap

Clouds overhead only light up if sunlight can reach their undersides. Stephen Bay puts it as the sun needing a direct and unobstructed line of sight to the underside of the clouds. A bank of cloud sitting low on the western horizon can switch the whole display off even when the sky above you looks perfect.

The gap does not have to be close. Bay describes one evening when he judged that a dense cloud bank perhaps 20 to 30 miles offshore was far enough away for the sun to break through at the horizon. In practice, look for a strip of clear or bright sky low in the west in the half hour before sunset. If you can see one under a sheet of high cloud, the odds are good.

If the west is a solid wall of grey right down to the ground, the evening is unlikely to deliver, however promising the clouds above look.

Clean, dry air and the time of year

It is widely assumed that haze and pollution make sunsets more vivid. Corfidi says the opposite for the lower atmosphere: aerosols there do not enhance sky colors, they subdue them, and clean air is the main ingredient common to brightly colored sunrises and sunsets. He told National Geographic that large particles in the lower atmosphere tend to mute and muddy the colors. Sunsethue adds that humid air absorbs part of the light, so clean and dry is the combination to hope for.

That is why season matters. Corfidi writes that late fall and winter are the most favored times for sunrise and sunset viewing over most of the United States, and told National Geographic that you see bright ones in fall and winter especially in the East. Summer air tends to carry more moisture and haze near the ground.

There is one exception to the clean air rule. Corfidi notes that aerosols injected high into the stratosphere by large volcanic eruptions can produce colorful sunrises and sunsets, including afterglows about 15 minutes after sunset, when the lower atmosphere is already in shadow but the stratosphere is still lit.

Common mistakes when watching for a good sunset

Leaving the moment the sun disappears. On high clouds the strongest color often comes afterwards. Sunsethue says high clouds can stay lit for up to 30 minutes past sunset, and Corfidi describes afterglows about 15 minutes after it.

Watching the wrong part of the sky. The clouds that matter are the ones the low sun can reach, and the gap that matters is on the western horizon. A clear sky overhead tells you little.

Treating red sky at night as a color forecast. The Weather Channel explains that the saying comes from mid-latitude weather moving west to east: a red sunset suggests high pressure and clearer skies arriving from the west. It says the rule applies in regions with westerly winds, roughly 23 to 66 degrees from the equator, and that it will not always be correct. It tells you something about tomorrow's weather, not tonight's colors.

Checking too late. The signs above are easiest to read in the last half hour of daylight, which is also when it is easy to be indoors and miss it. Everysky sends a notification thirty minutes before sunset at your location, worked out on the phone, which leaves time to step outside, look west and decide whether it is an evening to stay for the afterglow.

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