The ultraviolet index is the only number in this entire field that describes your environment rather than your product, and it is the one people ignore. That is a shame, because it is free, published daily, and directly actionable.
What the index is
The global solar ultraviolet index is a standardised measure of ultraviolet intensity at ground level, developed under World Health Organization guidance with international meteorological and radiological bodies, so that a value means the same thing in Aberdeen as in Auckland. It is expressed as a number with defined bands running from low, through moderate, high and very high, to extreme.
The bands carry recommended protective behaviours, which is the part that makes it useful. The number itself is abstract. The behaviour attached to it is not.
In the UK, the Met Office publishes a UV index forecast alongside its weather forecasts.
What drives the number
Sun angle, which means time of day and time of year
The higher the sun in the sky, the shorter the path ultraviolet takes through the atmosphere and the less of it is absorbed on the way. This is why the index peaks around solar noon and why it is far higher in June than in October at the same latitude.
Latitude
Closer to the equator, the sun climbs higher and the peak is greater and wider. Our conditions file on tropical travel deals with what that means for a British traveller.
Altitude
Less atmosphere above you means less absorption. World Health Organization guidance describes ultraviolet increasing with altitude, with a figure of around ten per cent per thousand metres commonly cited. On a British hill this is a modest addition. In the Alps it is not.
Cloud
Cloud reduces ultraviolet, but far less reliably than it reduces visible light and warmth. Thin or broken cloud can let a great deal through, and a bright overcast day can still carry a meaningful index. This is the single most common reason people are caught out in Britain.
Ozone
Stratospheric ozone absorbs ultraviolet and varies naturally, which the forecast accounts for.
Surface reflection
Not part of the index itself, and this matters. The index describes what arrives from the sky. It does not add what bounces back off snow, sand or water. Our note on reflected ultraviolet covers the addition.
Reapplication clock: How the index moves through a day
A general shape, not a forecast. Real values depend on date, latitude, cloud, ozone and altitude.
Using it to make a decision
The mistake is to read the index as a permission or a prohibition. It is neither. It is an input to three decisions.
1. How much textile
A higher index moves items from optional to necessary: the long sleeves, the brimmed hat, the rash vest. See UPF clothing.
2. How often to reapply
A higher index compresses the interval at which a degrading film becomes a problem. See reapplication.
3. When to go out
The strongest control of all is timing, because it reduces the dose rather than blocking it. See shade and timing.
The British problem
British ultraviolet is genuinely lower than in southern Europe or the tropics, and British weather is genuinely worse, which produces two opposite errors.
The first is complacency: people assume the UK sun cannot do much, and get caught on the handful of clear high summer days when the index is meaningful, or on a Scottish hill where altitude and a clear day combine.
The second is bad calibration abroad: a person whose entire experience of sun is British has no reference for a tropical index, and applies British habits to a fundamentally different environment.
Both are fixed by looking at the number rather than at the sky.
What a forecast is and is not
High UV index today
What it certifies
A forecast of ultraviolet intensity at ground level for a location, expressed on the standardised international scale. The Met Office publishes it for the UK, and the World Health Organization defines the bands. It is the most useful quantitative input available to anyone planning a day outdoors.
What it does not
It is a forecast for a location at a broad scale, not a measurement at your position. It does not account for the altitude you climb to, the snow or water reflecting under you, or the length of time you stay out, all of which change your actual dose.
Apps, watches and personal sensors
Various consumer devices claim to measure or estimate personal ultraviolet exposure. We do not test or recommend any of them and we make no comment on their accuracy, because we have not assessed a single one. What we would say is that a device measuring at your wrist is measuring at your wrist, and your face, ears and shoulders are pointing in different directions. Treat any personal figure as indicative rather than as a dose measurement.
The local factors a forecast cannot know
A published forecast is produced for an area, and several things that change your actual exposure operate at a scale far below that.
Aspect and terrain. A south facing slope receives a different dose from a north facing one at the same altitude. A gorge, a wood or a north side street removes a large part of the sky.
Surface. The forecast describes what arrives from the sky. It does not add what bounces up off snow, water, pale sand or a white deck.
Cloud at your location. Forecasts model cloud, but broken cloud is genuinely difficult, and above a cloud base you can be in full sun while the valley is grey.
Your posture and coverage. A stationary snorkeller, a seated paddler and a walking hiker present very different surfaces to the sky for the same index.
None of this makes the forecast less useful. It makes it an input to be adjusted rather than a figure to be read literally, and the adjustments run in one direction, upwards, for almost everyone reading this site.
The British season, briefly
Ultraviolet in the UK follows the sun angle, so it rises through spring, peaks around midsummer and falls away through autumn, with the winter months low enough that the practical concern for most people is small. The two exceptions worth naming are altitude and snow, where reflection and elevation change the picture regardless of the calendar, and any travel to lower latitudes, where the British seasonal intuition simply does not apply.
What is not verifiable
We do not publish index values, forecasts or destination figures, because we are not a forecasting service and the numbers change daily. We also cannot tell you your personal dose, because that depends on posture, coverage, reflection and time in a way no published forecast models. The forecast is an input. Your behaviour is the rest.
The one line version
Look at the forecast before a day outdoors, add altitude and reflection to it in your head, and let it decide your clothing, your timing and your reapplication interval rather than whether you are allowed outside.