The UV index is a measure of the strength of ultraviolet radiation reaching the ground, expressed on an open-ended scale where higher numbers mean stronger radiation. It is not a measure of heat, which is why it can be high on a cold day and modest on a hot one.
What is the index a measure of?
Ultraviolet radiation is part of the light arriving from the sun, at wavelengths shorter than the eye can see. The UV index expresses how much of it is reaching the ground, weighted according to how strongly different wavelengths affect skin.
That weighting is the interesting part. Not all ultraviolet wavelengths act on the skin equally, and the index is not a raw measurement of radiation but a measurement adjusted for biological effect. It is closer to a forecast of consequence than a reading of quantity.
The scale runs upward from zero and is open-ended, in the sense that it is not capped at a round number for tidiness. Higher means stronger. What the various bands mean in terms of what anybody should do is published by health authorities, varies by country, and is deliberately not paraphrased here.
Who decided what it means?
The index was not invented by any one weather service. It was developed jointly by the World Health Organization, the United Nations Environment Programme, the World Meteorological Organization and the International Commission on Non-Ionizing Radiation Protection, which is why a given number carries the same meaning in Manchester as it does anywhere else publishing it.
That international agreement is easy to overlook and it is the reason the index is useful at all. A national scale would have made forecasts incomparable across borders and would have left travellers guessing.
Why is it not about temperature?
Warmth and ultraviolet radiation arrive together from the same source, which makes them feel like the same thing. They are not, and the difference has practical consequences.
Temperature at ground level depends on air mass, wind, humidity and season. Ultraviolet intensity depends chiefly on how high the sun sits in the sky, on how much ozone lies above you, and on what the cloud is doing. Those inputs are only loosely related to how warm it feels.
A crisp, clear day in spring can therefore carry a higher index than a muggy, overcast day in summer. Snow and water complicate matters further by reflecting radiation upward, so exposure can exceed what the sky alone would suggest. Skiers discover this reliably, and usually the hard way.
The practical upshot is that the sensation of heat is a poor instrument. If you want to know what the ultraviolet is doing, the forecast is the thing to look at rather than the thermometer or your own impression of the weather.
What affects it on a given day?
Sun angle dominates. It is why the index peaks around the middle of the day and why it is far higher in summer than in winter at the same latitude, and why equatorial regions differ so markedly from northern Europe.
Ozone matters, since it absorbs a substantial share of the incoming radiation before it arrives. Forecast models account for it, sometimes using an average value for the time of year rather than a live measurement, which is a detail the Met Office is explicit about in its own description of how the forecast is produced.
Cloud has a mixed effect. Thick cloud reduces the index substantially; thin or broken cloud reduces it much less than most people assume, and scattered cloud can occasionally increase readings briefly through reflection.
Altitude increases it, because there is less atmosphere above you to absorb anything on the way down. Surfaces that reflect, particularly snow, sand and water, raise effective exposure without changing the forecast figure at all.
What this page does not do
It gives no numerical thresholds, no categories and no advice about what to do at any given index value. Those are published by health authorities, they differ between countries, and they are revised. They are linked below so they can be read where they are maintained.
Common questions
Is a high UV index the same as a hot day?
No, and this is the most common misunderstanding about it. The index measures ultraviolet radiation, not temperature. A clear spring day can carry a higher index than a humid overcast day in high summer, because what drives it is sun angle and atmospheric conditions rather than warmth.
Who devised the UV index?
It was developed jointly by several international bodies, so that a single scale would mean the same thing everywhere rather than each country publishing its own. The list of organisations involved appears on this page, taken from the World Health Organization's own account.
Does cloud cover mean the UV index is low?
Not reliably. Cloud absorbs some ultraviolet radiation but not all of it, and thin or broken cloud can let a great deal through. The forecast index accounts for expected cloud, which is a better guide than looking at the sky and drawing conclusions.
Why does the index vary through the day?
Because the sun's angle determines how much atmosphere the radiation passes through before reaching you. When the sun is high, the path is shorter and less is absorbed. That is the whole reason the middle of the day is discussed differently from early morning.
Where can I find the index for where I am?
National meteorological services publish it as part of the weather forecast. In the United Kingdom that is the Met Office, whose forecasts are linked in the sources below. Local figures are more useful than any general statement about the season.
Sources
- Radiation: the ultraviolet (UV) index — World Health Organization (International) · accessed
- UV and sun health — Met Office (United Kingdom) · accessed
- What you need to know about UV (ultraviolet) radiation — UK Health Security Agency (United Kingdom) · accessed
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