What time is UVA highest? The daily curve

UVA peaks at solar noon, for the same reason everything else in the sky does: that is when the sun is highest and its light takes the shortest path through the atmosphere. The interesting part is not the peak. It is the shape — UVA's daily curve is markedly flatter than the UV Index's, which means every piece of familiar timing advice you have absorbed is calibrated for the wrong waveband.

Why the curve has the shape it does

Both bands weaken as the sun drops, for two separate reasons. The first is purely geometric: a beam striking the ground at an angle spreads the same energy over a larger area. That effect is proportional to the cosine of the solar zenith angle and it treats every wavelength identically.

The second reason is absorption, and it does not treat wavelengths equally. As the sun drops, sunlight travels through progressively more atmosphere — the air mass rises — and every extra kilometre of path is more ozone in the way. Ozone absorbs strongly in the UVB and only very weakly in the UVA. So the lengthening path punishes the burning band far more than the ageing one.

Combining both, surface irradiance falls roughly as a power of the cosine of the zenith angle, with a different exponent per band. This site's model uses cos(z)^1.2 for UVA. Erythemal UV behaves more like cos(z)^2.2, because the ozone term compounds with the geometry. That one-exponent difference is the whole story.

Hour by hour

The table below is a clear-sky midsummer day at 40° N — southern Europe, the northern Mediterranean, New York, Beijing. Values are percentages of each band's own noon peak, so the columns are directly comparable in shape rather than magnitude.

In absolute terms that noon peak is a UVA Index of about 10.5 under a clear sky at sea level — firmly in the Very High band. Three hours later it is still around 7.8 — the top of the High band. Even four hours out it reads about 6.0, still scraping High, and only at five hours does a clear midsummer sky finally fall to 4.1, or Moderate. The band descriptions spell out what each of those levels is worth doing about.

Why "avoid 10 to 4" is a UVB rule

The standard advice to stay out of the sun between 10:00 and 16:00 is sound public-health messaging and it is aimed squarely at sunburn. Read the erythemal column: the window really does capture the great majority of burning UV, so as burn-avoidance advice it works.

As UVA advice it is much weaker. Take a mid-latitude midsummer day where solar noon falls at 13:00. The six hours from 10:00 to 16:00 contain most of the erythemal dose — but the hours from 08:00 to 10:00 and 16:00 to 18:00, which the rule treats as free, are running at roughly 40–75% of peak UVA. An hour outside at 17:00 is not a UVA-free hour. It is most of a midday hour with the warning signals removed.

The same reasoning applies to the shadow rule — if your shadow is longer than you are tall, UV is supposedly low. That marks a zenith angle of 45°, which the table puts at about three and a quarter hours from solar noon. Beyond it, erythemal UV genuinely is falling away fast. UVA is still above half its peak.

Solar noon is not 12:00

Everything above is measured from solar noon — the instant the sun crosses your local meridian. On the clock this can sit a long way from midday, and three separate effects stack up:

• Daylight saving time moves solar noon an hour later, to around 13:00 through the whole summer. This alone means the true peak sits closer to the start of a typical lunch break than to noon.
• Your longitude within your time zone. Time zones are an hour wide but political. At the western edge of a zone the sun runs late. Spain keeps Central European Time despite sitting on Greenwich's longitude, so summer solar noon in Galicia arrives after 14:30. China runs a single time zone across five zones' worth of longitude; in Kashgar solar noon is close to 15:00.
• The equation of time — the Earth's orbit is elliptical and its axis tilted, so solar noon drifts by up to about ±16 minutes across the year regardless of anything else.

The practical consequence: do not assume the UV peak is at midday. In most of Europe in summer it is nearer 13:30, and in parts of Spain and western China considerably later than that. The calculator on this site works from your actual coordinates and the true solar position, so the hourly chart shows where the peak really falls for you rather than where the clock suggests.

What changes the shape

The clean curve above assumes a clear sky. Reality distorts it in predictable ways:

• Season flattens or sharpens it. In winter the sun never gets high, so the whole curve is compressed and low — but disproportionately UVA, because low sun means a long ozone path all day. Seasonal UVA works through the numbers.
• Latitude sets the ceiling. Nearer the equator the noon peak is higher and the curve broader. UVA by latitude has the comparison.
• Cloud adds noise, not a trend. A passing cumulus field can move the reading by a factor of two within minutes, and broken cloud can briefly push it above the clear-sky line — see UVA on cloudy days.
• Terrain truncates it. In a valley or a city street the sun disappears behind an obstruction long before the geometric curve says it should — though diffuse UVA from the open sky continues, which is why shade is a partial measure rather than a complete one.
• Morning and afternoon are not quite symmetric. Convective cloud typically builds through the day and aerosol loading often differs between morning and afternoon, so the real afternoon limb usually sits a little below the morning one.

Using this

If you are trying to avoid burning, time-of-day avoidance is efficient: the erythemal curve is sharply peaked, so a few hours of shade removes most of the risk. If you are trying to limit UVA — for photoaging, melasma, a photosensitivity condition, or eye exposure — timing is a much blunter instrument, because the curve you are fighting is broad. There the answer is not scheduling but barriers: sunscreen, clothing and UV400 lenses, which work equally well at 09:00 and 13:00.