UV index is simplification
We tend to consider suntan good looking. It gives us the feeling of healthiness and energy, although everyone knows that too much sun is bad for you. UV index tells us when sunbathing is or isn’t safe, but how is it calculated?
Photo Evil Erin / Flickr Creative Commons.
UV index is a handy way of examining the risks of sunbathing. Meteorologist Henri Nyman from the Finnish Meteorological Institute tells us that one should use sun protection when the UV index is between 3 and 5. Usually it doesn’t rise above that level in Finland.
On a sunny summer day the index is 5 in the afternoon. The angle of the sun has a great effect on the UV index. On more southern latitudes UV index may be around 10 or 11. Also in the morning and in the evening the index is much lower than at solar noon.
The index value given in weather reports is usually an estimate of the possible maximum. Cloudiness will affect the actual index, although UV radiation easily passes through thin clouds.
But how come the meteorologists count values that don’t even have a unit? Why can’t we just get the intensity of the radiation? The answer has to do with wave lengths; the harm that UV radiation causes is greatly dependant on the wave length. Shorter wave lengths are much more dangerous than longer wave lengths. On the other hand longer wave lengths pass the atmosphere. So, the numbers the weather forecast would give, would be difficult to comprehend for most television viewers. But I’m still curious, what is hidden behind the index?
It turns out that the UV index forecast is made based on the forecast of the ozone concentration in the high altitudes of atmosphere. According to the ozone concentration and information about sun’s position, a meteorologist can calculate the intensity of UV radiation that is going to hit the Earth’s surface in a certain area. Then the intensities of several wave lengths are multiplied with a factor representing their harmfulness to humans. These numbers are then summed up and multiplied by two factors. One that is dependent on the altitude of the geographical area and one that depends on the cloudiness of the area. The result is then divided by 25 and rounded to the closest whole number to give the index.
Can we trust the UV index?
The arbitrary division by 25 in the end doesn’t seem very physicist-like to me. Nyman assures me that the calculations are based on studies on the effects of UV radiation to human skin. Nyman points out that people can get a nice tan in northern locations in the early spring when the UV index is very low because snow reflects UV radiation. Reflection from any surface may increase the amount of UV radiation, and this isn’t shown in the UV index.
On the internet I found sites where you can give your location and calculate the safe sunbathing time for the day. Nyman thinks that the numbers can probably be trusted if the time of the day and cloudiness are taken into account.
Safe sun bathing time is dependent on many variables. People have very different skin types and it affects the results, for example. In Finland the Meteorological Institute doesn’t offer information on “safe times” but instead encourages using adequate sun protection when the UV index is 3 or above.
More information on UV index: U.S. Environmental Protection Agency and World Health Organization.
Jan Jansson is a chemistry teacher that spends his little free time doing molecular gastronomic experiments.