Where is UVA highest in the world?

Wherever an overhead sun combines with high altitude and thin air: the tropical Andes and the Andean altiplano top the list, followed by the Tibetan plateau, tropical east Africa's highlands, and high-altitude tropical islands. At sea level the maximum is set by the overhead sun alone, so anywhere in the tropics reaches roughly the same peak.

Does UVA change much with latitude?

Less than the UV Index does. Between the equator and London the peak clear-sky UV Index falls by roughly 40%, but the peak UVA Index falls by only about 14%. The reason is ozone: the atmospheric column absorbs UVB strongly and UVA barely at all, so the extra path length at higher latitudes penalises the burning band far more than the ageing one.

Why is UV higher in Australia and New Zealand?

Three effects compound. The Earth is closest to the Sun in early January, during the southern summer, giving about 7% more solar irradiance than the northern summer receives. The southern mid-latitudes carry a thinner ozone column than their northern counterparts. And there is far less industrial aerosol over the southern oceans. Together these typically put southern-hemisphere UV around 10–15% above northern values at comparable latitude.

UVA around the world: why latitude matters less than you think

Everyone knows the tropics have brutal sun and northern Europe does not. For sunburn that intuition is accurate. For UVA it is much less so — the equator-to-pole gradient that makes the UV Index collapse as you travel north is largely an ozone effect, and ozone is almost transparent to UVA. The result is a world where the peak UVA is a great deal more uniform than the peak burning risk.

Why the two gradients differ

Two things change as you move away from the equator. The sun's maximum elevation falls, and the atmospheric path at that maximum lengthens. Both reduce ultraviolet at the ground — but by different mechanisms, with different spectral consequences.

Geometry is colour-blind. A lower sun spreads the same energy over more ground, and that dilution applies identically at every wavelength. This is the cos(zenith) term, and it hits UVA and UVB equally.
Absorption is not. A longer path means more ozone molecules in the way. Ozone's Hartley and Huggins absorption bands are powerful in the UVB and fall away sharply through the UVA. So the extra path length costs UVB heavily and UVA almost nothing.

On top of that, the ozone column itself varies with latitude — thinner over the tropics, typically around 250–280 Dobson Units, and thicker over the mid and high latitudes, often 300–400 DU and peaking in spring. That variation adds a second UVB-specific penalty on top of the path-length one, and again leaves UVA largely alone.

This is why this site's model gives ozone an exponent of just 0.05 — and in practice runs without live column-ozone data at all. Halving the ozone column would change modelled UVA by only a few percent, while roughly doubling erythemal UV. For UVA it is a rounding error; for the UV Index it is a first-order term.

Peak UVA Index, city by city

The table below gives each location's best-case UVA Index: clear sky, sea level unless noted, no aerosol, at solar noon on the day of year when the sun climbs highest. For tropical locations that is whenever the sun passes directly overhead; elsewhere it is the summer solstice. Values are computed with this site's model. The UV Index column is a rough figure for typical observed clear-sky summer maxima, included for shape rather than precision.

Location	Latitude	Elevation	Lowest solar zenith	Peak UVA Index	≈ Peak UV Index
Quito, Ecuador	0.2° S	2,850 m	0°	12.9	~14–16
Mexico City, Mexico	19.4° N	2,240 m	0°	12.5	~13–15
Nairobi, Kenya	1.3° S	1,795 m	0°	12.2	~13–15
Singapore	1.4° N	Sea level	0°	11.0	~12–13
Miami, USA	25.8° N	Sea level	2°	11.0	~11
Cairo, Egypt	30.0° N	Sea level	7°	10.9	~11
Sydney, Australia	33.9° S	Sea level	10°	10.8	~11–12
Los Angeles, USA	34.1° N	Sea level	11°	10.8	~10
Athens, Greece	38.0° N	Sea level	15°	10.6	~9–10
New York, USA	40.7° N	Sea level	17°	10.4	~9
Rome, Italy	41.9° N	Sea level	18°	10.3	~9
Toronto, Canada	43.7° N	Sea level	20°	10.2	~8–9
Paris, France	48.9° N	Sea level	25°	9.7	~8
London, UK	51.5° N	Sea level	28°	9.5	~7
Berlin, Germany	52.5° N	Sea level	29°	9.4	~7
Stockholm, Sweden	59.3° N	Sea level	36°	8.5	~6
Reykjavik, Iceland	64.2° N	Sea level	41°	7.9	~4–5

Read the two right-hand columns against each other. From Singapore to Reykjavik the peak UV Index falls by something like 60%. The peak UVA Index falls by 28%. From Singapore to London, the UV Index roughly halves while UVA drops just 14% — London's best summer day is, in UVA terms, a tropical day.

Two further things the table shows. Altitude beats latitude. Quito, Mexico City and Nairobi top the list not because of where they sit north to south but because of how high they are; the model adds about 6% per kilometre, and every one of them clears the notional top of the scale. UVA at altitude covers why. And the tropics saturate. Once the sun can pass overhead, extra proximity to the equator adds nothing — Singapore and Miami reach the same peak, on different days.

The southern hemisphere runs hotter

At matched latitude, southern-hemisphere ultraviolet exceeds northern by roughly 10–15%. Three independent effects stack:

Perihelion. Earth's orbit is elliptical and it is closest to the Sun in early January — the height of the southern summer. That delivers about 7% more solar irradiance to the southern summer than the northern summer receives at aphelion in July. Unlike almost everything else here, this applies to every wavelength equally, so it lifts UVA as much as UVB.
Thinner ozone. The southern mid-latitudes carry a lower ozone column than the corresponding northern latitudes, partly for reasons of atmospheric circulation and partly as a residual influence of the Antarctic ozone hole. This one is UVB-specific.
Cleaner air. Far less industrial and continental aerosol sits over the southern oceans, so less is scattered and absorbed on the way down.

This is a large part of why Australia and New Zealand have the world's highest melanoma rates, and why their sun-protection campaigns are the most developed anywhere.

What actually varies locally

Latitude sets the ceiling. Day to day, what moves your reading is everything else — and mostly not the things people expect:

Factor	Effect on UVA	Effect on UV Index
Time of day	Large	Larger
Season	Moderate	Large
Cloud	Large, variable	Larger
Altitude	~6% per km	~8–10% per km
Ozone column	Negligible	Large
Aerosol, haze, dust	Moderate	Moderate
Snow or sand underfoot	Moderate addition	Moderate addition
Latitude	Modest	Large

The bottom two rows are the point of this page. If you want to know your sunburn risk, where you are on the planet is among the first things to ask. If you want to know your UVA dose, it is one of the last — what time it is and what the sky is doing matter far more.

A caveat on these numbers

Every value in the city table is an upper bound: a perfectly clear, aerosol-free sky on the single best day of the year. Real readings sit below it, often well below, and the gap is larger in humid, hazy or cloudy climates than in dry clear ones. Singapore's actual daily peak is frequently far under 11 because of convective cloud, while Cairo's routinely approaches its ceiling.

For an actual number where you actually are, the calculator pulls live cloud, aerosol and elevation data for your coordinates rather than assuming the best case.

Get the UVA Index for any location on Earth →