THE FAN
It is perspective
The rays are nearly parallel from cloud to ground. They seem to meet at the sun for the same reason a long straight road seems to meet at the horizon.
See the numbersSunbeams through a gap in the clouds seem to spread like a fan.
Scientists call them crepuscular rays. They look as if they burst from one point, but physicists and NASA's astronaut photographs show they run almost parallel. The fan is perspective, and the beams only show up because something in the air scatters the light.
Updated
Will I see rays?THE SHORT ANSWER
Shadows, haze
and perspective.
A sunbeam is really a stripe of lit air
between stripes of shadow.
THE BEAMS
Something must scatter
Clouds or hills cast the shadows, but NASA notes the light must also be scattered by dust, aerosols, water droplets or air to show the contrast.
Will I see rays?
THE TWINS
Look behind you
Anticrepuscular rays stretch across the sky and seem to meet again at the point exactly opposite the sun.
How they formA QUICK SKY CHECK
Will I see
rays today?
Pick a scene to see how the conditions line up. The bar shows how likely rays are to be visible.
Based on NASA astronaut-photography captions, the University of Illinois Physics Van and the US National Weather Service glossary.
TWO NUMBERS WORTH KNOWING
Straight lines,
bent by distance.
One figure on where the twin rays meet,
one on how much the beams really spread.
180° from the sun
Where the twin rays appear to meet
The University of Illinois Physics Van explains that anticrepuscular rays appear to converge at a point exactly opposite the sun, which only works because the rays are parallel.
ANTICREPUSCULAR · ILLINOIS
≈ 0°
How far the rays actually fan out
NASA's astronaut photograph over India in 2011, taken from above the rays, gives a clear illustration of their parallel nature. The fan seen from the ground is distance and perspective.
REAL SPREAD · NASA
In a 2020 astronaut photograph over the Arabian Peninsula, NASA notes that dust in the air helped make the sunbeams more clearly visible.
FOUR WAYS TO SPOT SUNBEAMS
Where to look,
and when.
These follow from how the rays form: you need gaps, scattering and contrast.
Go out at dawn or dusk
The low sun gives the strongest contrast between bright beams and dark cloud or sky.
Look for broken cloud
Gaps in cloud, or a ridge or tall cloud on the horizon, give the shadows that separate the beams.
Hope for a little haze
Dust, aerosols, mist or water droplets scatter the light so the beams can be seen.
Turn round
At sunset, check the eastern sky, opposite the sun, for faint anticrepuscular rays converging there.
ANTICREPUSCULAR RAYS
The same beams,
seen from behind.
Crepuscular rays do not stop overhead. When the bands of light and shadow cross the whole sky, the far ends appear to come together again at the antisolar point, opposite the sun, just as railway lines appear to meet in both directions.
Why the beams look golden at sunset is a separate effect. At a low sun angle, sunlight passes through much more atmosphere, so Rayleigh scattering removes more blue and leaves the reds and oranges, as the Met Office explains.
Questions
Questions people ask
What are crepuscular rays?
The US National Weather Service defines them as the alternating bands of light and dark, rays and shadows, seen at the Earth's surface when the sun shines through clouds. Crepuscular refers to twilight, when they are most often seen.
Why do sunbeams seem to spread out from the sun?
It is perspective. The rays are nearly parallel all the way from the clouds to the ground, but like the sides of a long straight road they appear to converge in the distance, here towards the sun.
Why can I see the beams at all?
Clouds or mountain tops cast the shadows, but the light also has to be scattered by airborne dust, aerosols, water droplets or air molecules. That scattering lights up the sunlit bands against the shadowed ones.
What are anticrepuscular rays?
They are the same parallel rays seen across the sky, appearing to converge at the point exactly opposite the sun. With a setting sun they can extend from the west all the way across the sky to the eastern horizon.
Can rays happen in the middle of the day?
Yes. The University of Illinois Physics Van notes that crepuscular rays can be seen at any time of day; dawn and dusk simply give more contrast.
Sources
Where the facts come from
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