Reviewed October 17, 2026

Sunscreen on cloudy days and through windows: the UVA reality

Clouds block some UV, but the rays that age skin pass through cloud cover and window glass far more easily than the ones that burn. When protection still matters indoors and on grey days.

Two kinds of ultraviolet, two different behaviors

Sunlight reaching your skin carries UVB, the shorter rays mainly responsible for sunburn, and UVA, longer rays that penetrate deeper and drive photoaging and pigmentation. UVB intensity swings a lot with season, time of day, and cloud cover. UVA is steadier: present from dawn to dusk all year, and much less impressed by a grey sky.

That difference is why a cloudy day can leave you unburned yet still deliver a meaningful UVA dose over hours. Standard window glass tells the same story in miniature: it blocks nearly all UVB, which is why you do not burn at a desk, but lets a large share of UVA through.

What clouds actually filter

A fully overcast sky cuts total UV significantly, but thin or broken cloud can let most of it through, and scattered cloud edges can briefly focus light and spike exposure above the clear sky value. As a rough guide, light cloud cover may block only a fifth of UV, while heavy storm cloud blocks much more. The catch is that your eyes cannot grade cloud density as a UV meter.

Brightness is a poor guide too, because visible light and UV part company under cloud. If you will be outside for an extended stretch on a bright grey day, treating it like a mild sunny day for exposed skin is the reasonable default.

The window question, answered practically

If you sit beside a large sunny window for hours daily, the skin on the window side gets a real cumulative UVA dose over months and years. Commuters see the same pattern from car glass, especially on the arm and face side facing the window on long drives. Laminated windshields filter UVA well; side windows usually filter far less.

For a normal desk set back from the glass, the dose is small and sunscreen is a personal judgment call rather than a duty. For the window seat lifer, a morning sunscreen application is cheap insurance, and UV film on the glass is a one time fix that ends the question permanently.

A sensible rule for grey days

Match effort to exposure time. A ten minute walk under cloud needs no strategy. Two hours outside, a long drive, or a day on the water under bright overcast all deserve normal sunscreen habits, because cumulative UVA is the currency those days pay in.

Remember that sunscreen under cloud still degrades on schedule. Reapply after roughly two hours of outdoor time or after sweating and swimming, exactly as you would in sunshine. The rays you cannot feel are the ones doing the quiet work, in both senses.

Frequently asked questions

Can you get sunburned on a cloudy day?

Yes. UVB is reduced by cloud but not eliminated, and thin or broken cloud lets most of it through. Burns on overcast beach days are a classic because people skip sunscreen and stay out longer in the comfortable temperature. Treat bright grey days with respect.

Does window glass block all UV?

No. Ordinary glass blocks essentially all UVB but transmits a substantial portion of UVA, the deeper penetrating aging rays. Laminated glass and UV films block far more. The practical effect is slow cumulative exposure for skin that sits by sunny glass for hours a day.

Do I need sunscreen indoors if I work from home?

If your desk is away from large windows, the dose is negligible and most people can skip it. If you work right beside a big sunny window all day, a morning application is a sensible, low effort habit. Position relative to the glass is what decides.

What SPF makes sense for cloudy days?

The same broad spectrum SPF 30 or higher you would use in sun, applied in the same amount. People underapply on grey days because the ritual feels less urgent. The product does not know the sky is grey; the dose on your skin is set by time outside and how much you applied.