What is UVA radiation?
UVA is the long-wavelength band of ultraviolet light, spanning roughly 315–400 nanometres (nm). It sits just beyond the violet edge of visible light — too long to see, but energetic enough to reach deep into your skin. It's also, by a wide margin, the most abundant ultraviolet radiation that actually reaches the ground.
Where UVA sits in the spectrum
Ultraviolet is the slice of sunlight with wavelengths shorter than visible light (about 100–400 nm). By convention it's split into three bands:
| Band | Wavelength | Reaches the ground? |
|---|---|---|
| UVA | 315–400 nm | Yes — the large majority of surface UV |
| UVB | 280–315 nm | Yes — a small fraction, mostly midday |
| UVC | 100–280 nm | No — absorbed by ozone and the atmosphere |
Shorter wavelength means higher energy per photon, so UVC is the most biologically damaging — but it never reaches us, because the ozone layer and atmosphere absorb it completely. UVB is partly absorbed. UVA passes through almost untouched.
UVA is sometimes split further
Dermatology and photobiology often divide UVA into two sub-bands, because the longer wavelengths behave differently from the shorter ones:
- UVA II (315–340 nm) — closer to UVB and slightly more energetic.
- UVA I (340–400 nm) — the longest wavelengths, which penetrate skin most deeply.
(You'll also see UVA quoted as 320–400 nm in older sources; the 315 nm boundary is the more modern convention.)
How UVA behaves
Three properties make UVA distinctive — and easy to underestimate:
- It's remarkably steady. UVB swings dramatically with the time of day and the season because low sun angles force it through more ozone. UVA varies far less: it's present from soon after sunrise to near sunset, and stays relatively high outside the summer midday peak.
- It passes through clouds and glass. Ordinary window glass blocks most UVB but transmits a large share of UVA, so you can accumulate UVA indoors by a sunny window or while driving. Light cloud also lets much of it through.
- It penetrates skin deeply. Because of its longer wavelength, UVA reaches the dermis — the living layer beneath the surface where collagen, elastin and blood vessels live.
How much UVA actually reaches the ground?
For an overhead sun on a clear day, surface UVA irradiance is on the order of ~66 W/m² — far larger in raw energy than surface UVB. That number falls as the sun gets lower (longer path through the atmosphere), and is further trimmed by clouds, haze and aerosols, then nudged up slightly at altitude or over reflective surfaces like snow. Those are exactly the factors this site's UVA Index calculator models to estimate the UVA irradiance for a given place and time.