The UV index and session timing
Forecast index and intended duration. Those two numbers between them determine almost everything, and neither of them is the temperature.
The two inputs that should set an outdoor session plan are the forecast ultraviolet index for the day and how long you intend to be out. Air temperature is a poor proxy for either, because ultraviolet and warmth do not move together. The World Health Organization index is forecast in the UK by the Met Office, and protective measures are advised from index 3 upwards.
People plan outdoor sessions on temperature because temperature is what they can feel. It is a reasonable instinct and a poor input. A clear cold morning in April at altitude can present a higher ultraviolet load than a muggy overcast afternoon in August, and nothing about how the two days feel would tell you that.
There is a purpose-built alternative and it is free. The global solar ultraviolet index is an internationally agreed scale set out in World Health Organization guidance, and it is forecast for the United Kingdom by the Met Office alongside the ordinary weather forecast. It is designed for exactly this decision.
How the index is meant to be used
The index is a simple open-ended scale beginning at zero, with higher numbers meaning more ultraviolet reaching the ground. The internationally recommended threshold at which protective measures are advised is index 3, and that threshold is the useful thing to remember. In the United Kingdom the index is at its highest around the middle of the day in late spring and summer, and much lower in winter.
What the index does not tell you is duration, and duration is the other half of the equation. An index reading applies to a moment; your session applies to hours. Two hours at a moderate index and forty minutes at a high one are different problems, and the plans for them are different shapes.
| Forecast index | Short session, under an hour | Long session, over two hours |
|---|---|---|
| Low | Ordinary sense. Nothing special needed | Cover and a hat still worth having on a long day |
| Moderate, from index 3 | Hat, and an application before you go | Cover, hat, and move the session out of the middle of the day if possible |
| High | Hat, cover, application, and prefer the shaded route | Move the session, or accept that cover has to do the whole job |
| Very high, which is uncommon in the UK | Treat as a high day even for a short session | Reschedule if the session is genuinely movable |
The hour of the day
Ultraviolet reaching the ground depends on how high the sun is in the sky, which is why it peaks around the middle of the day and falls away sharply towards morning and evening. This is the mechanism behind the standard advice to avoid the strongest hours, and it is also the reason moving a session is such an effective control.
For most recreational athletes the session is movable. An easy run happens when you decide it happens. A ride, a walk, a round of golf and a session in the garden are all schedulable. The exceptions are the ones covered elsewhere on this site: fixtures, court bookings, work shifts and group sessions. For everybody else, moving the session is free and outperforms everything else on the list.
| Surface | Angle to the sun | Exposed for | What removes protection | Control that holds |
|---|---|---|---|---|
| Early morning | Sun low, long shadows, plenty of natural shade | 60-120 | Very little | Almost nothing beyond ordinary sense |
| Middle of the day | Sun at its highest, shortest shadows, index at its peak | 60-120 | Everything | Full plan: cover, hat, application, shaded route |
| Evening | Sun low again, load much reduced | 60-120 | Very little | Ordinary sense, plus awareness that low sun hits the face directly |
| Middle of the day in December | Sun very low even at noon | 60-120 | Very little in terms of ultraviolet | Reflection off snow is the exception worth watching |
Why cloud is a bad input
Cloud is the input people actually use, and it is unreliable. Thick overcast reduces ultraviolet considerably, but thin, high and broken cloud transmits a great deal while making the day feel cool and unthreatening. Broken cloud with sunny intervals is a common British summer pattern and it can present a substantial load across a long session.
There is also the psychological effect. On a bright hot cloudless day people take precautions because the day announces itself. On a cool bright day with high cloud, nobody does anything, and the session may be long precisely because the conditions are comfortable for training. That combination, comfortable conditions and a long session under broken cloud, produces more unplanned exposure than heatwaves do.
General sun advice offers a rule of thumb about shadows: if your shadow is shorter than you are, the sun is high and the load is significant. It is a useful field check and it works on land as well as anywhere.
What does not transfer is the beach version of the same advice, which is to go indoors between certain hours. If you are training, working or competing, that option does not exist, so the shadow check becomes a prompt to change what you are wearing and which route you take rather than to stop.
Making it a habit rather than a decision
The reason this fails in practice is that it requires a check the night before, and nobody adds a new check to their evening. The way round it is to attach it to something you already do. If you already look at the weather forecast before an outdoor session, the index is on the same page. If you already plan the week's training on a Sunday, the forecast for the week is available then.
- Look at the forecast index at the same moment you look at the rain forecast.
- Note the duration you intend. Round it up, because sessions overrun.
- If the index is moderate or above and the session is long, either move it or plan cover.
- Use the shadow check as a field prompt during the session itself.
- Review at the end of a block. If something burned, the plan was wrong, and it will be wrong again next time unless it changes.
Sunlight exposure has benefits as well as risks, and NICE has published guidance covering both sides of that balance. Nothing on this site argues for avoiding daylight. It argues for not accumulating large unplanned doses during long sessions in the middle of the day. Questions about vitamin D and supplementation are for the NHS or a GP.
What this looks like across a season
The people who handle this well are not more careful on any given day. They have simply made one structural decision at the start of a season, which is that long sessions happen in the morning or the evening and the middle of the day is for short ones, shaded ones, or the ones that cannot be moved. That single decision does more than everything else on this site combined, and it is available to almost everybody who trains recreationally.
Questions
What UV index should I start taking precautions at?
The internationally recommended threshold is index 3 and above, which is the point at which the World Health Organization guidance advises protective measures. In the United Kingdom that is reached routinely through the late spring and summer around the middle of the day.
Where do I find the UV index for today?
The Met Office publishes a UV index forecast alongside its ordinary weather forecast for the United Kingdom, using the internationally agreed scale.
Is temperature really that poor a guide?
Yes. Ultraviolet depends on sun angle, atmosphere, altitude and cloud, none of which is what makes the air feel warm. A cold bright spring day at altitude and a muggy overcast August afternoon can sit at opposite ends of the index while feeling the opposite way round.
Does the shadow rule work?
It is a reasonable field check. When your shadow is shorter than you are, the sun is high and the load is at its most significant. It is a prompt to check what you are wearing and which side of the road you are on rather than a precise measurement.
Should I avoid daylight altogether then?
No, and nothing here suggests it. Sunlight exposure carries benefits as well as risks and NICE has published guidance on that balance. The argument here is narrower: do not accumulate large unplanned doses during long sessions at the strongest hours, when moving the session is free.
This article has no commercial links. Nothing in it is sponsored, no product is named and no payment of any kind was received for anything written here. Water Resistant SPF is published by Northbank Media and recommends no products at all. Our funding is set out in full on the about page.
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