SunticCloudy Weather and UV Rays
A grey, overcast day feels like a break from the sun, but your skin may not get one. Cloud cover hides the brightness and takes the edge off the heat, yet a large share of the sun's ultraviolet rays passes straight through. That is why people are caught out every summer, coming home pink after a cloudy afternoon at the beach or a hike under hazy skies. The sky is a poor guide to UV; the UV index is the measure you can trust.
Do clouds block most UV?
Clouds scatter and absorb some of the sun's visible light, which is why the day looks duller. But UV behaves differently from the light and warmth you can sense. Much of it travels through cloud, so even when the sun is hidden, meaningful UV can still reach your skin. Thin or hazy cloud blocks very little, and even thicker cover lets a notable amount through.
Because you cannot feel UV the way you feel heat and brightness, an overcast day can lull you into skipping protection. The strength of the sun's UV is set by the same factors on any day: the time, the season, your latitude and altitude. To understand what drives it, see our guide to the UV index.
How much UV gets through cloud?
There is no single figure, because it depends on how thick and complete the cloud is. As a rough guide, drawing on Cancer Council and meteorological sources:
| Cloud cover | Roughly how much UV gets through | What it means |
|---|---|---|
| Thin or hazy cloud | ~90% or more | Barely reduces UV. Treat a hazy day like a clear one. |
| Scattered or broken cloud | Variable, sometimes over 100% | Gaps can scatter extra UV downward, briefly pushing it above clear-sky levels. |
| Heavy, complete overcast | Often around half, but still significant | The biggest reduction, yet enough UV gets through to burn over time on a high-UV day. |
So even thick cloud rarely makes a high-UV day safe; it just takes the edge off. A bright but overcast summer day can easily keep the UV index in the range where unprotected skin burns.
Broken cloud can even boost UV
Counterintuitively, a partly cloudy sky can sometimes deliver more UV than a clear one. When the sun shines through gaps between bright, white clouds, those clouds can scatter extra UV down towards you on top of the direct rays from the sun. This cloud-enhancement effect means a sky of scattered fluffy clouds can briefly push UV above the clear-sky level.
So a day that looks safer than a cloudless one is not necessarily so. This is one of the reasons sun-safety advice keeps coming back to a single point: judge risk by the measured UV index, not by how the weather looks.
Snow, water and sand reflect UV onto you
Whatever the cloud is doing overhead, the ground around you can add to your exposure. Some surfaces reflect UV back up at you, so you get hit twice, from the sky and from below. This effect applies on cloudy days too, since the reflected rays are still UV.
- Snow is the most reflective common surface and can sharply increase exposure, even under cloud on a winter or mountain day.
- Water reflects UV and provides no shade, so time on or near it raises your dose.
- Sand bounces a meaningful share of UV upward, adding to exposure at the beach.
Check the UV index, not the sky
The takeaway is simple: do not let an overcast or hazy day talk you out of sun protection. Because UV is invisible and cloud is unreliable, the only way to know the real risk is to check the number. When it is 3 or above, use broad-spectrum SPF 30+, seek shade and cover up, just as you would in full sun.
Suntic shows the live UV index for your exact location on your iPhone, so a glance tells you whether a cloudy day is genuinely low risk or quietly high. Check today's UV index before you head out and let the number, not the sky, decide your protection. For more myths worth retiring, see our piece on common UV index myths. This is general information, not medical advice.
Frequently asked questions
Do clouds block UV rays?
Only partly. Thin or hazy cloud blocks very little UV, and even thicker cover lets a notable amount through. Broken cloud can sometimes scatter extra UV downward, briefly raising it above clear-sky levels.
How much UV do clouds block?
It varies with thickness: thin or hazy cloud lets roughly 90% or more of UV through, while heavy, complete overcast can cut it to around half. Even then, enough UV passes through to burn over time, so cloud rarely makes a high-UV day safe.
Can UV go through clouds?
Yes. UV behaves differently from the visible light and heat you can sense, so much of it travels straight through cloud. That is why you can burn on a grey, overcast day without ever feeling the sun.
Can you get sunburnt on a cloudy day?
Yes. Much of the sun's UV passes through cloud, so unprotected skin can still burn on an overcast day. Cloud reduces brightness and heat far more than it reduces UV, which catches many people out.
Should I wear sunscreen when it's cloudy?
If the UV index is 3 or above, yes. The sky is an unreliable guide because UV is invisible, so check the UV index and use broad-spectrum SPF 30+, shade and cover-up clothing whenever it is high.
