Sun

When you need sun protection

The UV index is a measure of the ultraviolet radiation that burns skin. WHO advises protection when it is 3 or higher. Here it is calculated for your place and day with an open clear-sky formula: you can see the peak, the hours that need protection and how the index changes over the year.

This is a model, not a forecast: the clear-sky formula (Madronich, 2007) from the sun’s elevation and the typical ozone for the latitude and month, plus an altitude correction. Clouds, haze and smog usually lower the index; snow, sand and water add reflected UV. For today’s forecast, check your city’s weather service.

The WHO scale

One scale for the whole world: the number is the strength of UV, the colour is the category.

Low0–2

No protection needed: you can safely stay outdoors. The exception is snow and water, which reflect UV.

Moderate3–5

Protection needed: around midday, seek shade and wear sleeves, a hat and sunglasses, plus sunscreen.

High6–7

Protection needed: around midday, seek shade and wear sleeves, a hat and sunglasses, plus sunscreen.

Very high8–10

Extra protection needed: better not to go out around midday; outdoors, shade, clothing, a hat, sunglasses and sunscreen are a must.

Extreme11+

Extra protection needed: better not to go out around midday; outdoors, shade, clothing, a hat, sunglasses and sunscreen are a must.

Sun by month

The peak clear-sky index in the middle of each month. In Moscow you need no protection in winter, while by the sea in Vietnam the index is above 10 almost all year round.

Place	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sept	Oct	Nov	Dec
Moscow150 m above sea level	0	1	2	4	6	7	7	6	4	2	1	0
Saint Petersburg10 m above sea level	0	0	1	3	5	6	6	5	3	1	0	0
Krasnaya Polyana560 m above sea level	1	2	4	7	8	10	10	9	7	4	2	1
Sochi30 m above sea level	1	2	4	6	8	9	10	9	7	4	2	1
Novosibirsk150 m above sea level	0	1	2	4	6	7	7	6	4	2	1	0
Hanoi15 m above sea level	8	10	12	13	13	13	13	13	12	10	8	7
Da Lat1500 m above sea level	12	14	16	16	16	15	15	16	16	14	12	11
Phu Quoc5 m above sea level	11	13	14	14	14	13	13	14	14	13	11	10
Antalya30 m above sea level	3	4	6	8	10	11	11	10	9	6	4	2
Dubai5 m above sea level	6	8	10	12	12	13	13	13	12	9	7	6

Clear-sky model for 2026, the 15th of each month, the highest value of the day. Real values are usually lower because of clouds and haze.

SPF: what the number means

SPF shows protection against UVB, the rays that burn. UVA, which reaches deeper, needs separate protection in the formula: look for “broad spectrum” or the UVA mark.

SPF 30 and SPF 50

SPF 15 blocks about 93% of UVB, SPF 30 97%, SPF 50 98%. The difference in what is blocked is small, but in what gets through it is one and a half times: 3% against 2%. No sunscreen blocks 100%.

Skin Cancer Foundation · 01.06.2022

How much to apply

SPF is measured with 2 mg of sunscreen per 1 cm² of skin. In studies, people apply 0.4–1 mg, and protection falls at least by half, by some accounts far more. The Cancer Council Australia guide: a teaspoon for the face, neck and ears, and a teaspoon each for every arm and leg, the front of the body and the back: about 35 ml for the whole body.

Cancer Council Australia; Petersen, Wulf 2014; Faurschou, Wulf 2007 · 2007–2014

When and how often

20 minutes before going out and then every 2 hours, and straight after swimming, sweating or towelling off. “Water resistant” means 40 or 80 minutes in water in a test, not all day; in the US, “waterproof” and “sunblock” are not allowed on labels.

FDA; Cancer Council Australia · 2011–2014

UVA and broad spectrum

UVB burns; UVA reaches deeper and also ages the skin. Look on the pack for “broad spectrum”, the UVA mark in a circle (EU) or PA+…PA++++ (Asia).

AAD: Sunscreen FAQs · 11.02.2025

Sunscreen is not the first line

According to WHO, protection starts with shade around midday, clothing, a hat and sunglasses; sunscreen is for whatever stays uncovered. The FDA does not recommend sunscreen for babies under 6 months: they need shade and clothing.

WHO 2002; FDA: sunscreen · 2002; 16.10.2026

Sunscreen expires

UV filters break down over time. The FDA considers sunscreen with no date on the pack good for 3 years from purchase; once it is open, check the period-after-opening symbol and keep it out of the heat.

FDA: sunscreen · 16.10.2026

Where UV is stronger than it seems

Around midday

About 60% of the day’s UV arrives between 10:00 and 14:00 (WHO). The WHO rule: if your shadow is shorter than you, seek shade.

Clouds

Thin clouds barely block UV and sometimes even intensify it by scattering (WHO). Getting sunburnt on an overcast day is common.

Snow

Fresh snow reflects up to 80% of UV and almost doubles the dose. On the slopes, in Krasnaya Polyana for example, you need protection even in freezing weather, and your eyes need it most.

Water and sand

Sand reflects about 15% of UV, sea foam about 25%. Half a metre under water, about 40% remains (WHO): swimming does not shield you from the sun.

Mountains

With every 1,000 m of altitude, UV rises by about 10% (WHO). Da Lat, Sa Pa and ski slopes are a reason to add protection.

The tropics

The closer to the equator, the higher the sun. In Nha Trang, Phu Quoc and Bali the clear-sky index is above 10 almost all year: you need protection in the morning and in the afternoon alike.

A tan is not protection

A tan is the skin’s response to UV damage. A dark tan protects roughly like SPF 3–4 (WHO, Skin Cancer Foundation). WHO classifies sunbeds as Group 1 carcinogens.

Oils and homemade creams

In the test tube, oils give SPF 1 to 8: that is not protection. Homemade “sunscreen” recipes never go through testing: in a review of recipes from social media, 68% gave insufficient protection.

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