How do we find the UV-Index level?

In the spring and summer months it is natural for us all to check the UV-Index level and take precautions as needed. Do you know how it is calculated?

Some may think that it is possible to measure the amount of UV hitting the planet but it is actually more complicated than this.

In fact, the level is determined by a number of factors. This includes: atmospheric ozone levels, angle of sunlight, cloud cover, time of the year, and elevation.

The formula begins with the ozone layer. More dangerous shortwave UV is absorbed by the ozone layer so depending on how thick the layer is at any given time ground-level radiation may be stronger or weaker.

Multiple wavelengths are given a strength, from here a weighting factor is assigned using another calculation which determines the danger of each.

Three wavelengths will be measured, today we will use 280nm, 340nm, and 400nm. The strength is calculated from the ozone thickness and sunlight angle.

Once weighted we multiply the two. If the 280nm wavelength has a strength of 5 and a weight of 15 it would be 75.

This is done for all three wavelengths, the results are then added together to find a total UV effect, for now let’s say that this amounted to 280.

Now, we look at elevation and clouds. UV intensity rises by around 6% each kilometre above sea level and clouds absorb UV rays.

For this example, lets say we are at 1km elevation with 73% cloud cover. Now we multiple them all together, 280 x 1.06 x 0.73 which equals 216.7.

From here we divide by 25 and round to the nearest number. From our example the UV-Index level would be 9. So while it is easy to check the levels each morning, there is a lot of hard work that goes into finding that number!