UV levels

Increased UV Levels

The main factors affecting the strength of UV radiation reaching the Earth's surface are:

• the elevation angle (height) of the sun in the sky
• the amount of cloud, dust and pollution in the atmosphere
• the amount of ozone gas in the stratosphere

The presence of ozone in the stratosphere is important because it absorbs much of the UV radiation before it reaches ground level. Our UV model currently accounts for sun angle and forecast cloud amounts, but uses a "climatological" value (i.e. the average concentration for this time of year over the UK) to estimate the total ozone concentration.

During the dark polar winter, the concentration of ozone in the upper atmosphere over the pole typically decreases because sunlight is a critical ingredient in making ozone in the first place. At the same time the circulation of air around the North Pole keeps all this low ozone air at higher latitudes near the pole. In March and April, as the sun moves north, this polar circulation begins to break down and occasionally allows pockets of low ozone air to break away. These can sometimes pass over the UK.

In these situations there will be less ozone in the high atmosphere available to absorb the UV compared to the average amount used in our forecasts. This means that the UV index could be higher than currently indicated in our forecasts particularly if forecast cloud amounts are low. We do round up our UV values but due to this low ozone event, the UV forecast can still be lower than the actual levels. We are currently exploring ways to incorporate low ozone events such as this into our models to improve our UV index forecast.

Enjoy the sun safely

It is important that, if you are in an area that is particularly sunny over the next few days, you take steps to ensure that you and your family are protected from these increased UV levels.

You can also get UV forecasts for UK and worldwide locations on the Met Office app for the next 7 days.

• For iPhone the app is available from the App store.
• For Android the app is available from the Google Play store.

Yinka Ebo, Senior Health Information Officer at Cancer Research UK said: "It's great to have such lovely weather, and we all need some sun to keep us healthy, but it's important to stay safe when the sun is strong and take care not to burn. In many cases sunburn actually happens in the UK, often when people are out and about. The sun's rays can be strong enough to burn in the UK from around April to September. You can protect yourself and your family from sunburn by using a combination of shade, clothing and at least SPF 15 sunscreen when enjoying the sunshine."