Do clouds block UV rays?

Only partly. Water droplets scatter UV more than they absorb it, so the direct beam becomes diffuse light from the whole sky but the energy still arrives. Thin high cloud passes most of it, a solid grey overcast passes roughly a third, and only a thick storm deck brings it near zero. You can burn on a day when you never see the sun.

Is the UV index a measurement of sunlight?

No. The UV index is a sunburn-weighted score: UV weighted by how well each wavelength reddens skin, which makes it a proxy for UVB and nearly blind to UVA. Cloud changes how much light arrives and from which direction, but the index still only tells you about burning. Two overcast afternoons with the same index can carry very different spectra.

How does Sunlight is Life calculate UV on a cloudy day?

It runs a solar physics engine that models the actual sunlight reaching your coordinates. The result is cloud-adjusted, so it falls under a grey deck and recovers in a gap, and the app shows the UVA and UVB reaching you separately, updating while it is open. It is a model, not a sensor reading of your sky.

Do clouds protect from UV rays?

Not reliably. A cloudy sky cools your skin, so you feel less warning and stay out longer, while the UVB dose keeps accumulating. Use your shadow, not the temperature, to judge exposure.

Can you get vitamin D on a cloudy day?

Yes, when the cloud-adjusted UV index is still around 3 or above. Under thin summer cloud that is usual. Under heavy overcast in spring or autumn at mid-latitudes it usually is not, and you should assume little to no production.

Does the UV index account for clouds?

Peter CowanSeptember 16, 20265 min read

Partly. The published UV index forecast is a clear-sky calculation, cut by a single cloud factor from the day's weather forecast. Many apps skip even that and show the clear-sky value. Real clouds vary minute to minute and pass far more UV than they look like they should, which is why overcast days still burn people.

The US forecast, produced by the National Weather Service and published by the EPA, first calculates UV for a cloudless sky, then reduces it for the forecast cloud. Sun angle for your latitude and date, satellite-measured ozone, and elevation give a clear-sky index for solar noon at a grid point. The model then multiplies by a transmission factor from the forecast cloud cover: close to 100 percent for clear sky, about 89 percent for scattered cloud, 73 percent for broken cloud, and 31 percent for overcast, in the EPA's published method. One number, one cloud category, one grid point, for the day.

"It's raining at my place with no sunlight. It is showing 9 UV index"

That review is describing a clear-sky number, or a daily maximum that never moves. Most UV apps buy data from a weather provider and display whatever field arrives: a clear-sky value, a noon forecast issued before dawn, or an hourly cloud adjustment made hours earlier for a grid cell tens of kilometers wide.

A forecast issued at 6 AM that assumed 40 percent cloud cannot know the storm cell parked over you at 1 PM. So the app says 9 in the rain. The reviewer who wrote "cloud cover doesn't affect UV details and suggestions" was right about that app. It does not.

Thin cloud passes almost all of it

High cirrus is ice crystal, and UV passes through it nearly untouched. On a hazy-bright day the UV index sits within a few percent of the clear-sky value while the sun looks weak enough to ignore. This is the day people burn at the beach without ever feeling hot.

A grey overcast cuts it to about a third, not to zero

Water droplets scatter UV rather than absorb it. What a low stratus deck does is take the direct beam away and redistribute it as diffuse light from the whole sky. Roughly a third of the clear-sky value still reaches the ground under a solid overcast, less under a deep storm deck. Zero happens at night, not on an ordinary grey afternoon.

Broken cloud can push UV above clear sky

When the sun shines through a gap between cumulus clouds, you get the full direct beam plus light scattered off the bright edges of the clouds around it. For a few minutes the ground receives more UV than under a clear sky. Reviews of the measurement literature put common enhancements around a quarter above the clear-sky value, with larger spikes on record (Calbo 2005). This is the answer to "why is the UV index higher on cloudy days": on a broken-cloud day it sometimes is.

UVA and UVB, and where the light comes from

Cloud does not change the ratio of UVB to UVA very much. That ratio is set by the sun's angle and the ozone column, which is why a low winter sun makes no vitamin D whatever the sky is doing. What cloud changes is direction. Under overcast, UV arrives from every part of the sky rather than one point, so a hat brim and the shade of a wall protect less than they do on a clear day.

Ground reflection adds to the load. Fresh snow reflects up to 80 percent of UV, sea foam about 25 percent, dry sand about 15 percent, in the WHO's figures. A cloudy day at a ski resort delivers UV from below as well as above.

"It said UV of 1 or 2 and that I was safe for the day"

The practical rule: cloudy does not mean UV 0. Sunburn on overcast days is common because the warmth you feel is infrared, and a cool grey day removes the warning signal while leaving most of the UVB dose intact. People stay out longer and the burn arrives that evening. If you take a photosensitizing medication, a tetracycline or a thiazide diuretic for instance, the burn times in any app do not apply to you, cloud or no cloud.

How to read a UV app on a cloudy day

Find out whether the number is clear-sky or cloud-adjusted. If the day's maximum never moves while the sky changes, you are looking at clear-sky or a noon forecast.
Check the hourly cloud cover next to the hourly UV. Thirty percent cloud at 2 PM means the index is mostly intact then.
Use the shadow rule whenever the sun shows. A shadow shorter than your height means UVB is strong enough to burn and to make vitamin D, whatever the app says.
Treat a solid grey overcast as roughly a third of the clear-sky value, and thin bright cloud as nearly all of it.

Check the UV index in your city →

What Sunlight is Life does

Sunlight is Life's UV is cloud-adjusted and comes from a solar physics engine that models the actual sunlight reaching you, shown as UVA and UVB separately. What it cannot do is see the one cloud over your head right now, because no forecast can.

The through line

The UV index is a forecast of the sky, not a reading of it. Clouds are the biggest thing a forecast gets wrong at the scale of your backyard. Your shadow is a measurement, and it is free.

Common questions

Do clouds block UV rays?: Only partly. Water droplets scatter UV more than they absorb it, so the direct beam becomes diffuse light from the whole sky but the energy still arrives. Thin high cloud passes most of it, a solid grey overcast passes roughly a third, and only a thick storm deck brings it near zero. You can burn on a day when you never see the sun.
Is the UV index a measurement of sunlight?: No. The UV index is a sunburn-weighted score: UV weighted by how well each wavelength reddens skin, which makes it a proxy for UVB and nearly blind to UVA. Cloud changes how much light arrives and from which direction, but the index still only tells you about burning. Two overcast afternoons with the same index can carry very different spectra.
How does Sunlight is Life calculate UV on a cloudy day?: It runs a solar physics engine that models the actual sunlight reaching your coordinates. The result is cloud-adjusted, so it falls under a grey deck and recovers in a gap, and the app shows the UVA and UVB reaching you separately, updating while it is open. It is a model, not a sensor reading of your sky.
Why is the UV index higher on cloudy days?: Broken cloud can push UV above the clear-sky value for minutes at a time. The direct beam through a gap adds to light scattered off the bright edges of nearby clouds. Enhancements of a quarter or so over clear sky are commonly reported.
Do clouds protect from UV rays?: Not reliably. A cloudy sky cools your skin, so you feel less warning and stay out longer, while the UVB dose keeps accumulating. Use your shadow, not the temperature, to judge exposure.
Can you get vitamin D on a cloudy day?: Yes, when the cloud-adjusted UV index is still around 3 or above. Under thin summer cloud that is usual. Under heavy overcast in spring or autumn at mid-latitudes it usually is not, and you should assume little to no production.