We might still be experiencing some cold, frosty mornings but the sun is now as strong as it is in September. With the sunshine returning this week, the UV index can reach moderate from late morning until mid-afternoon meaning it's a good time to start applying suncream.
A commonly held belief is that the strength of the sun is related to the temperature of the air which is often a reason why people are caught out at this time of year. In reality, it is a combination of the angle of the sun, cloud cover, and the amount of ozone in the atmosphere above. With the sun now taking a similar path across the sky as it does in late September, the sun is just as strong then even if it doesn't feel like it.
What is UV?
Ultraviolet (or UV) radiation makes up part of the total energy that is naturally emitted from the sun. You can't see UV with your eyes, but your skin can still be affected by exposure to it. Prolonged and/or frequent exposure to UV without suitable protection can lead to sunburn, premature aging and an increased chance of developing skin cancer.
The good thing is that the atmosphere contains a gas called ozone which at least reduces the amount of UV that reaches the surface of the earth. A concentrated layer of ozone exists at around 15 to 35 kilometres above the ground in a region called the stratosphere, but the concentration of ozone varies around the world and throughout the year. When the ozone concentration decreases, the amount of UV that reaches the surface increases.
What is different currently?
The amount of ozone in the stratosphere over the UK is lower than normal which means the amount of UV reaching the ground has been higher on several days during March, including through this week. With a normal amount of ozone in the atmosphere, the UV index may only just reach 3 (moderate) for an hour or two around lunchtime in southern England. With 20-30% less ozone in the stratosphere, moderate levels of UV are possible for longer and across a larger part of the UK.
Forecast UV index for Exeter on 18 March 2025 assuming clear skies and an ozone layer that is 22% thinner than normal. Moderate UV requiring protection is possible from late morning to mid-afternoon. Data courtesy of Environment Canada.
Why has this happened?
During the winter, the air over the Arctic cools as it is plunged into darkness. This year, weather patterns have meant that very cold air has been allowed to develop and persist over the Arctic; some of the coldest on record. Temperatures in the stratosphere below a chilly -78 °C support the development of clouds (called 'polar stratospheric clouds') which support the breakdown of ozone. This has resulted in below normal ozone over the Arctic. In Spring, the wind patterns over the Arctic begin to change and this has resulted in the area of cold stratospheric air and low ozone to be pushed off the pole and, on this occasion, towards the UK. This is expected to last through the rest of the week.
False-colour view of total ozone over the Arctic pole on 15 March 2025. The purple and blue colours are where there is the least ozone, and the yellows and reds are where there is more ozone. Data courtesy of NOAA Ozone Watch.
Can I see what the UV index is in real-time?
The UK Health Security Agency (UKHSA) and the Department for Environment, Farming and Rural Affairs (Defra) provide near-real-time monitoring of UV across the UK on the UK AIR portal.
Several weather websites and apps provide UV forecasts but it is worth highlighting that these rely on accurate cloud and ozone forecasts. If the forecast is cloudy when it is actually sunny, the UV index is likely to be higher. Similarly, some UV forecasts do not account for differences in the amount of ozone. A good rule of thumb for southern England is that at least some protection from sun exposure is required between the months of March and October.