Sun
Most people treat the UV index as a colour-coded suggestion. It is a weighted integral of solar spectral irradiance, and it tells you considerably more than whether to bring sunscreen.
The number your weather app displays as a single digit is not a comfort rating. It is the output of a defined calculation, standardised by the World Health Organization and the World Meteorological Organization, and it can be reasoned with.
The UV index is derived by taking the solar spectral irradiance arriving at ground level, multiplying it wavelength by wavelength against the erythema action spectrum, integrating across the ultraviolet range, and dividing by 25 milliwatts per square metre.
Two parts of that deserve unpacking. The erythema action spectrum is a weighting curve describing how efficiently each wavelength produces skin redness. It is not flat. A photon at 298 nanometres is roughly a thousand times more effective at causing erythema than one at 330 nanometres. The weighting therefore concentrates almost all of the index's sensitivity in the UVB band, even though UVB is a small fraction of total ultraviolet energy reaching the surface.
The divisor of 25 mW/m² is simply a scaling convention chosen to put typical midday tropical values in the range of 10 to 12, producing a number people can hold in their heads.
Solar elevation dominates everything else. The index tracks the sine of the solar altitude angle far more closely than it tracks air temperature, which is why a cool clear day in May can carry a higher index than a hot hazy day in August. Elevation angle depends on latitude, date and time of day, all three of which are known in advance and none of which are weather.
Ozone column thickness modulates UVB specifically. Total column ozone varies seasonally and with weather systems, typically by ten to fifteen percent, and a one percent decrease in column ozone corresponds to roughly a 1.2 percent increase in erythemal UV.
Altitude adds approximately eight to ten percent per thousand metres of elevation gain, because there is simply less atmosphere above you to scatter and absorb.
Surface reflection is the variable that catches people out. Grass and soil reflect under ten percent of incident UV. Water reflects five to ten percent at high sun angles but far more at low angles. Dry sand reflects fifteen to twenty percent. Fresh snow reflects up to eighty-five percent, which means a skier at altitude on a clear day can receive close to double the nominal index — from above and below simultaneously.
Cloud is unreliable in both directions. Heavy overcast can cut the index by seventy to ninety percent. Thin or broken cloud may reduce it barely at all, and scattered cumulus can briefly raise ground-level UV above the clear-sky value through edge scattering. A grey sky is not a safe assumption.
| Index | Category | 1 SED arrives in |
|---|---|---|
| 0–2 | Low | 33+ minutes |
| 3–5 | Moderate | 13–22 minutes |
| 6–7 | High | 10–11 minutes |
| 8–10 | Very high | 7–8 minutes |
| 11+ | Extreme | Under 6 minutes |
The categories are risk-communication devices, not physics. The underlying quantity is continuous, and the useful skill is converting the index into a time budget rather than reading a colour. An index of 6 is not qualitatively different from an index of 5; it simply means the clock runs about twenty percent faster.
Below an index of about 3, the atmospheric slant path has filtered out nearly all UVB. Erythemally weighted UV that remains is dominated by the long-wavelength tail, which produces almost no cutaneous vitamin D. This is the physical definition of what is loosely called vitamin D winter.
Above roughly 37 degrees latitude — a line running near San Francisco, Denver, Philadelphia and Athens — the noon UV index stays below 3 for most of November through February. No amount of outdoor time during those months produces meaningful synthesis. The relevant strategy shifts from exposure to stored status and dietary intake.
Because solar elevation is deterministic, the shape of the day's UV curve is known before the day starts. It rises steeply through the morning, peaks within about half an hour of solar noon, and falls symmetrically. Roughly half of the day's total erythemal dose arrives in the four hours centred on solar noon.
That gives you two clean planning levers. If the goal is vitamin D, take a short exposure near the peak, when the UVB fraction is highest and the required time is shortest. If the goal is a long day outdoors with minimal damage, push the exertion into the morning and late afternoon shoulders and take the midday hours in shade.
Note also that solar noon is frequently not clock noon. Daylight saving time shifts it an hour later, and position within a time zone can move it by up to another half hour. The daylight window calculator returns actual solar noon for your coordinates, which is the anchor the UV curve is built around.