When Is the Sun Actually Strongest? Ask Your Shadow.

You look out of the window on the way to the car. Grey, close, no glare — so the cap stays on the hook and you go. Most of us decide about sun the same way: by looking at the sky. The people who actually forecast it do not. Their number is not built from what the sky looks like at all.

When is UV strongest during the day?

Late morning through mid-afternoon — and the reason has nothing to do with heat.

The UV number in a weather app is not read off the sky — it is worked out. The environmental agency that publishes it lays out how. It starts from how high the sun is sitting, which in their words comes down to "latitude, day of year, and time of day" — how far you are from the equator, what date it is, what time it is. Then it gets adjusted for the ozone overhead, how much cloud there is, and how high up you are. Nowhere in any of that is how bright it looks to you, or how warm it feels on your arm.

Which is why the official advice is written in hours, not in weather. Once the number is into the middle of the range, the advice is to get into shade "during late morning through mid-afternoon". Same wording again at the top of the range. Not "when it is sunny". Not "when it is hot".

Closer to home, the Malaysian Meteorological Department says the same thing in its own words: the higher the sun in the sky, the higher the UV level — so it varies with time of day and time of year — and the closer you are to the equator, the higher it goes, because the rays have a shorter distance to travel through the atmosphere. MetMalaysia reports the index hourly from 11am to 4pm. That is the window our own national service watches.

So the thing most of us go by — glance out, decide — is not what any of this is built on.

Does a cloudy sky mean less UV?

Some, and much less than it looks like it should.

Cloud does soak up some UV, and the agency publishes how much still gets through at each kind of cover. A clear sky lets nearly all of it through. Patchy cloud lets through most of it. Cloud with gaps in it still lets through about three-quarters. Only a thick, completely grey sky cuts it right down — to under a third. The exact figures are in the sources below.

The gap between the first three and the last one is where people get caught, because a bright grey sky and a heavy dark one look much more alike than they behave. They work an example through on their own page: cloud with gaps in it overhead, and the number still comes out in their "very high" band.

MetMalaysia is blunter still about the two things we tend to go by. Thin or light cloud, they say, barely reduces what reaches you and can even push UV levels up by scattering it around. And UV cannot be seen or felt, and does not depend on the temperature — which is why skin can take damage on a mild day as easily as a hot one.

If you want the version of this told as somebody's actual mistake rather than a calculation, there is one here: Cold Country, Cloudy Sky — the One Step I Thought I Could Skip.

How can you tell without checking anything?

Look at your shadow. That is not an old wives' tale — it is on the agency's own page, and it is the closest thing to a free UV meter you already own.

Their rule, in their words: if your shadow is taller than you are, early morning or late afternoon, your UV exposure is likely to be lower. If your shadow is shorter than you are, around midday, you are being exposed to higher levels. It works because your shadow tells you how high the sun is sitting — which is exactly where their whole calculation starts — and it does not care what the sky looks like.

The scale is worth knowing once, and it is built to match the international standard. It says you need nothing at the bottom of the range, you need protection from the middle upward, and extra at the top. What they tell you to do at those levels: get into shade, cover up with clothes and a wide-brimmed hat, and put sunscreen on whatever skin is still out.

None of which needs an app, a subscription or anything new in the house. It needs you to stop asking the sky a question it cannot answer.

Stop deciding by how the sky looks. Look down instead — a shadow shorter than you means the sun is high, whatever the weather is doing.

This article is general skin and sun-safety information, not medical advice. The cloud-transmission figures quoted come from a foreign environmental agency, named in the sources below; the reporting hours and the latitude and sun-elevation statements are MetMalaysia's. Live readings for where you are come from MetMalaysia. Last reviewed: July 2026.