Although we see a lot of sunshine in April, it's in May that the strength of the sun really becomes apparent if you stay out in it for any length of time.

The sun is at its strongest at the time of the summer solstice, around June 21st but by early May it is as strong as in mid August and quite capable of causing sunburn, if you don't take the right precautions.


The average sunshine amount during May across England and Wales is about 200 hours, equating to around 7 hours per day but the sunniest Mays' can see more like nine or ten hours on average each day. As the far north sees increasingly long days, in the right conditions parts of Scotland can see a lot of warm sunshine; for example in May 1975 there were 329 hours recorded on the island of Tiree in the Outer Hebrides, Scotlands sunniest month ever.

Whilst we record the total hours of sunshine it is the strength of the sun that actually causes most problems, combined with how long you are exposed in total. On a day when the sun shines through high upper cloud it can reduce its strength, although often not by as much as we might imagine. We use an UV scale to measure the strength of its potentially damaging ultra violet radiation, also known as radiant energy. On earth, most radiant energy comes from the sun and is responsible for warming the air and the land. The majority of the sun’s energy is reflected back into the atmosphere, but some is absorbed by land, water, air and clouds. There are several measuring instruments available to do this, including a handy portable UV monitor by Oregon, sold by the Weathershop.

Ultraviolet rays create heat as the energy contained in solar infrared rays causes the molecules of the substance it hits (in this case your skin) to vibrate back and forth. In fact they vibrate so much that it can knock electrons away from the atoms, causing the molecules to split, resulting in a change in the chemical structure of the molecule. The change is detrimental to living organisms, as it can cause cell damage and deformities by causing a mutation in the genetic code, something which can lead to skin cancer. Young people's skin is especially sensitive to this process.

A variety of factors affect the amount of UV radiation that reaches us. High amounts of ozone present in the stratosphere will mop up some of the UV radiation from the sun before it reaches the ground but in areas where there is a reduction in the ozone layer, an atmospheric layer that contains a high proportion of oxygen that exists as ozone (O3). It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 15 to 20 kilometers above the earth's surface. '); return false">Ozone layer this can cause problems. An ozone 'hole' has been noted around the South Pole and in the northern hemisphere ozone thinning has occured around the polar regions and as far south as 60 degrees north, partly as a consequence of the effects of pollutants and aerosols.

UV radiation also varies with altitude, by around 4% for every 300m (1000ft), so for example, skiing in the Alps at a height of 5,000ft would equate to receiving a 20% higher level of radiation than at sea level, due to the thinner atmosphere. This is the name we give to the gaseous envelope gravitationally bound to any celestial body, such as a planet. Our own planet Earth's atmosphere is different to all other known atmospheres by having a very active cycle in which water evaporates from the oceans and the land surface, is carried across the earth as water vapour, precipitates out again as rain or snow, is intercepted by trees and vegetation, runs off the land surface, (infiltrates into soils and recharging groundwater levels) and then flows out into the oceans, from which it will eventually evaporate again. The water contained in the Earth's atmosphere plays a vital role as far as nearly all living organisms on the planet are concerned. As incoming sunlight passes through the atmosphere it is absorbed by the ground as heat energy. This heat energy is then re-transported back up through the atmosphere and released by condensation processes, as water vapour. Most solar radiation is absorbed in the tropics then the atmosphere and the ocean transports this heat poleward. It is these fundamental motions (which are heavily altered by the effects of planetary rotation) that determine the atmospheric circulation. Dynamical instabilities which can be modeled using fluid dynamics on weather maps play a large role in this circulation and are crucial in determining the fluctuations in this circulation that we call “weather.” We divide the atmosphere into several layers (although there are no distinct separation zones) according to differences in both its thermal structure and the amount of ionization present. The region where the temperature decreases gradually is called the troposphere. Above this there is a layer in which temperature actually increases upward (primarily because of the absorption of ozone by solar radiation) the stratosphere. Above this region the temperature decreases with height again as we enter the mesosphere. Above this again we have the mesosphere and the thermosphere. Finally at the highest outer levels of our atmosphere an area with high levels of energetic radiation above this is known as the exosphere. '); return false">Atmosphere. Equally, the earth's albedo has an affect, this is the ability of certain surfaces to reflect more light and heat than others. Sand, snow and water have high albedos which tend to scatter the UV rays back into the air where they are then scattered down to earth again by molecules or clouds.

This is why when skiing, even in January in relatively weak sun, with snow on the ground you can still easily burn. Another factor that can increase sunburn is the wind. A 15kt mean wind can decrease the time it takes to burn by as much as half.

The solution of course is to wear enough a good quality protective sunblock; SPF (sun protection factor) 15 is the minimum that kids should have on, whilst topping up with a total sunblock (SPF 30 or higher) on sensitive areas like the nose, the back of the neck and forehead is always a good idea.

Whatever May brings, we hope you enjoy the start of the summer sunshine season!