The UV index is a simple measurement of the strength of the sun’s UV radiation and its impact on the skin (in other words, sunburn). UV forecasts for the current and following day are available on the MeteoSwiss website and in the app.
UV radiation is a type of short-wave (100-400 nm) electromagnetic radiation that is invisible to the naked human eye. The UV radiation spectrum is divided into three bands: UVA (315-400 nm), UBV (280-315 nm) and UVC (100-280 nm). UVC and 90% of UVB radiation are mostly absorbed by the ozone layer, but also by the oxygen, hydrogen and carbon dioxide in the atmosphere. The UV radiation that reaches the Earth’s surface consists of UVA and a small proportion of UVB.
There are various environmental factors that influence UV radiation:
A collection of parameters is used to calculate the UV index, including the spectral irradiance of the sun with a wavelength range between 250 and 400 nm. The irradiance of the different wavelengths, from UVB (since UVC does not reach the Earth’s surface) to UVA is weighted based on its potential to cause burns (sunburn or erythema) on the skin. While over 90% of UVB radiation is absorbed by the atmosphere, it is 1,000 times more effective at causing sunburn than UVA radiation. A person’s sensitivity to UV radiation depends on the amount of skin pigmentation. The dose required to have a noticeable effect on the skin (the minimal erythemal dose) for the European population ranges between 200 and 500 joules per square meter.
MeteoSwiss calculates the UV index daily for the following day. This is an automatically generated forecast [Internal link: UV index] based on a radiation model that accounts for the environmental factors mentioned above. Cloud cover is also incorporated into the forecast, and the data is sourced from the ICON model of MeteoSwiss. The index is calculated for the maximum radiation intensity (approximately between 11:00 and 15:00 hours).
The UV-index forecast allows the intensity of solar radiation to be estimated so that people can take appropriate protection measures. The index applies to Switzerland, and it ranges from 1 to 11, the latter indicating extreme radiation intensity (Fig. 3). The recommended protective measures vary with the intensity level. UV rays pose a health risk, with frequent and prolonged exposure leading to faster skin ageing and potentially causing damage that can result in skin cancer. UV radiation can also affect the eyes and weaken the immune system.