Do you need sunscreen in winter?

For UVA, yes — if you spend meaningful time outdoors. At mid-latitudes, midday winter UVA is typically a quarter to a third of the midsummer value, whereas the sunburning UVB falls by roughly a factor of ten. The exposure that drives photoaging and pigmentation continues year-round even when burning effectively stops.

Is UVA the same all year round?

No — that claim is too strong. UVA does fall in winter, because the sun is lower and its light travels a longer path through the atmosphere. At 50 degrees latitude, clear-sky midday UVA in December is roughly a quarter of its June value. The point is that UVB falls two to three times more steeply, so winter sunlight is proportionally far richer in UVA.

Why does a low UV Index not mean low UVA?

The UV Index is weighted for sunburn and is therefore mostly a UVB measurement. Every condition that lowers it — a low sun, winter, early morning, cloud, glass — suppresses UVB more than UVA. So the lower the UV Index, the larger the share of the remaining ultraviolet that is UVA, and the less well the index represents your actual UVA dose.

Why UVA changes far less with the seasons than UVB

A widely repeated line holds that UVA is "constant year-round." That is not quite true, and the accurate version is more interesting. UVA does fall in winter — but UVB falls two to three times more steeply. The result is that winter sunlight, and early-morning and late-afternoon sunlight, is disproportionately UVA. This is the single strongest reason a sunburn-weighted UV Index cannot represent your real ultraviolet exposure.

The correction worth making: UVA is not seasonally constant. At 50° latitude, clear-sky midday UVA in December is roughly a quarter of its June value. Over the same swing, the UV Index typically drops by a factor of eight to ten. It is the ratio that shifts, not the absolute UVA that stays put.

The cause: ozone absorbs UVB, and ignores UVA

Ozone's absorption of ultraviolet is not a gentle gradient — it falls off a cliff. Across the UVB band the absorption cross-section drops by several orders of magnitude from 300 nm to 320 nm, and by the time you reach the middle of the UVA band ozone is effectively transparent. Ozone is, in practical terms, a filter that acts on UVB and leaves UVA alone.

Now add geometry. When the sun is low — in winter, or early and late in the day — its light enters the atmosphere at a shallow angle and travels a much longer path to reach you. At a solar zenith angle of 73°, that path is roughly 3.4 times longer than with the sun overhead.

Put the two together and the asymmetry becomes stark. A longer path through a strong absorber removes UVB exponentially. A longer path through something that barely absorbs your wavelength costs you comparatively little — UVA loses mostly to scattering, which is a much gentler penalty. The same low sun that decimates UVB merely trims UVA.

The numbers, at 50° latitude

A clear midday in June against a clear midday in December, at a mid-latitude location such as Prague, Vancouver or Frankfurt:

	June noon	December noon	Winter / summer
Solar zenith angle	~27°	~73°	—
Relative path length	~1.1×	~3.4×	3× longer
UV Index (erythemal, mostly UVB)	~7–8	~0.7–1	÷ 8–10
UVA irradiance	~50 W/m²	~13 W/m²	÷ 4
UVA Index	~8	~2	÷ 4

Read the last column across. Winter does not spare UVA — it cuts it to about a quarter. But it cuts the sunburning band by roughly ten. Per unit of UV Index, a winter noon is delivering somewhere around twice the UVA that a summer noon does.

Two smaller effects push in the same direction. Total-column ozone at mid-northern latitudes peaks in late winter and spring, adding further UVB suppression exactly when the path is longest. And the Earth is closest to the sun in early January, raising top-of-atmosphere irradiance by about 3.4% — a small bonus that reaches the ground almost entirely as UVA, since the extra UVB is absorbed on the way in.

The same logic runs through every day

Season and time of day are the same phenomenon: both are just the solar zenith angle changing. So the daily curve mirrors the annual one.

Midday, midsummer — high sun, short path. The most UVB-rich sunlight you will ever stand in, and the moment the UV Index is most informative.
Mid-morning and mid-afternoon — UVB is already dropping quickly; UVA is still near its peak. The classic "I was only out at four o'clock" exposure.
Early morning, late afternoon, all winter — UVB is largely gone. What remains is close to pure UVA, at a level that is low but far from zero.

This is why the UVA Index through the day chart on the calculator has visibly broader shoulders than a UV Index curve for the same day. Both peak at solar noon, but the UVA curve rises earlier, falls later, and never drops as sharply.

A common example, corrected: people sometimes contrast "UV Index 2 in January" with "UV Index 2 at 7 am in July" as if they were different exposure profiles. In fact they are strikingly similar — at 50° latitude both occur at a solar zenith angle near 73–74°, so both carry the same UVA-rich spectrum. That is the real lesson. A low UV Index almost always means a UVA-rich sky, whatever the reason for it being low, because everything that lowers the index — low sun, cloud, glass — suppresses UVB preferentially.

Why this is the case for measuring UVA separately

The UV Index answers one question well: how fast will this sunlight burn me? Since burning is nearly all UVB, the index tracks UVB. That is a legitimate design choice, and for beach-in-August decisions it is the right tool.

But it means the index systematically fails in exactly the conditions where UVA dominates — and those conditions are most of the year, most of the day, and most of the time people are actually outdoors. The photoaging, pigmentation and cumulative damage UVA drives are not seasonal in the way sunburn is. A number weighted for sunburn will always under-represent them.

A dedicated UVA Index is not a replacement for the UV Index. It is the second number you need in order to read the first one correctly.

What to do differently

Do not put sunscreen away in October. If you are outdoors regularly, a UVA-rated product year-round is the practical response. Reading UVA labels matters more than the SPF number here, since SPF is a UVB measure.
Treat winter mountain days as high-exposure. Winter suppresses UVA by four; altitude and snow albedo both push it back up hard. Ski conditions are the one winter setting where UVA genuinely rivals a summer day.
Stop treating "outside the midday window" as safe for UVA. The 10 am–4 pm rule is a UVB rule.
Check the actual number. Winter and early-morning UVA is low but not negligible, and that distinction is precisely what the UVA Index exists to make legible.

See how your UVA Index moves through the day →