Even on cloudy days the sun can be strong enough to cause sunburn, in fact over 90% of UV can pass through light cloud. Use the UV index to see how strong the suns UV rays are and when theres risk of burning. The higher the value, the greater the risk. When its above 3, the sun is strong enough to cause damage. Dont get caught out, protect your skin when the sun is strong by covering up with clothing, spending time in the shade and using a sunscreen with at least SPF 15, and 4 or more stars. The UK sun can be strong enough to burn from the start of April to the end of September.