How much UV reaches you through the day?

Of the ultraviolet light that reaches the Earth's surface, roughly 95% is UVA and 5% is UVB at midday — and effectively none is UVC, which the ozone layer absorbs almost entirely. But that split, and the total amount, swing through the day with the height of the sun. This model computes where the sun is from your latitude, the season and the time, and shows the clear-sky UV that follows — so you can treat UV exposure as a variable that changes by the hour, not a constant.

Which types of UV actually reach the ground?

Three bands of ultraviolet leave the sun: UVA (315–400 nm), UVB (280–315 nm) and UVC (100–280 nm). UVC is the most energetic and the most dangerous — and the atmosphere absorbs it almost completely, so essentially none reaches the surface. Most UVB is filtered by stratospheric ozone too, leaving only a few percent. UVA passes through largely unimpeded. The result at ground level is roughly 95% UVA and 5% UVB at midday, with UVC effectively absent.

Why does UV change so much through the day?

Because the sun's height changes. When the sun is high, its light takes the shortest path through the atmosphere; when it is low — early, late, or in winter — that path is far longer, and more UV is absorbed and scattered along the way. UVB, with its shorter wavelengths, is filtered out faster than UVA, so low-sun light is not just weaker overall but proportionally even more UVA-dominated. This model captures the geometry exactly; the absorption is approximated for clear skies.

What does this mean for training?

It turns UV from a vague worry into a schedulable variable. The same outdoor session carries very different UV exposure at 8am versus solar noon, and at 56°N in December versus June. If UVB exposure matters to you — for vitamin D, or as a tissue cost — the model shows when there is meaningfully any to be had. For an actual dose, pair it with a UV-index forecast or an on-body sensor.