Conditions & environment
Knowledge Base articles tagged Conditions & environment.
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What time is UVA highest? The daily curve
UVA peaks at solar noon — but its curve is far flatter than the UV Index's, so mornings and late afternoons carry proportionally much more UVA. Hour-by-hour numbers, why the shadow rule fails, and why solar noon is rarely 12:00.
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UVA on cloudy days: how much gets through
Cloud dims UVA far less than it dims sunlight — and broken cloud can briefly push UVA above clear-sky levels. Transmission by cloud type, why UVA penetrates cloud better than UVB, the cloud enhancement effect, and how the UVA Index models it.
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Why UVA changes far less with the seasons than UVB
Ozone absorbs UVB strongly and UVA barely at all, so a low winter sun collapses UVB while UVA falls only modestly. Why winter and early-morning UV is disproportionately UVA, with the numbers — and why a low UV Index never means low UVA.
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UVA around the world: why latitude matters less than you think
Ozone barely absorbs UVA, so the equator-to-pole gradient is far shallower for UVA than for the UV Index. Peak clear-sky UVA Index for cities worldwide, why the southern hemisphere runs hotter, and what actually drives local differences.
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UVA at altitude: mountains, hiking and flying
UV rises with elevation — but UVA rises more slowly than the often-quoted 10% per 1000 m, which is an erythemal figure. Why the altitude gradient differs by wavelength, how snow and clean air compound it, and what the cockpit UVA and pilot melanoma research shows.
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Reflected UVA: snow, sand, water and concrete
Ground reflectance adds to the UVA you receive — and UV albedo is not visible albedo. Reflectance values by surface, why skiers and boaters burn under the chin, why shade fails on a beach, and how the UVA Index models surface albedo.
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