MeteoSwiss - UV erythemal radiation

The UV-B part of the solar radiation spectrum represents a very small part of the total solar radiation intensity. However, it is of great importance, because it has a significant influence on life and may in some cases be a threat to public health (skin cancer, eye damage, etc.) MeteoSwiss measures UV radiation at four stations of the Swiss Alpine Climate Radiation Monitoring network (SACRaM). Two stations are located in the Swiss Alps - at Jungfraujoch and Davos - and the others in the plains - at Payerne and Locarno-Monti. The UV radiation measurements are done with UV erythemal biometers. Such instruments measure the intensity of UV radiation using an erythemal filter whose response mimics the sensitivity of the skin: mostly to UV-B with a small contribution of UV-A. UV radiation is measured by MeteoSwiss since May 1995 at Davos, November 1996 at the Jungfraujoch, November 1997 at Payerne, and May 2001 at Locarno-Monti.

UV erythemal yearly cycle at MeteoSwiss SACRaM stations

UV erythemal irradiance daily averages at Payerne, Locarno-Monti, Davos and Jungfraujoch, corresponding monthly moving averages (31 days) and average yearly cycles computing using data for 1996-2009 (Davos), 1997-2009 (Jungfraujoch), 1998-2009 (Payerne) et 2001-2009 (Locarno-Monti). Comparing the monthly moving average to the average yearly cycle shows that in 2009 UV radiation was slightly higher to the norm in August and beginning of September at Davos, Locarno-Monti and Payerne, and significantly higher at Jungfraujoch. The same type of difference shows up for solar irradiance (not restricted to the UV, see the GAW-CH Atmospheric Radiation Monitoring Program), which suggests that a reduced cloud cover is responsible for them. At Payerne and Davos, measurements for June and July are slightly below the norm. For the rest of the year, the UV erythemal irradiance is similar to the average yearly cycle.

uv_av_cl_en.jpg, 200 KB

The 2009 yearly cycle of UV erythemal irradiance at the SACRaM stations is shown beside. Each dot represents a daily average and the red curves are corresponding monthly moving averages. The daily averages include night values (zero). The monthly moving averages are computed for each day of the year using the 31 surrounding days. Average yearly cycles (black curves) are also included. These are obtained similarly to monthly moving average, except that they are computed on days-of-year averages using all years on record: For a given day of year (e.g., March 25) the data of all years on record are averaged together. Then, the monthly moving average is computed to obtain the average yearly cycle. Comparing the monthly moving average for 2009 to the average yearly cycle allows uncovering the deviations with respect to the average cycle.

In general, the intensity of UV radiation depends on many factors including sun elevation, cloudiness, ground albedo (reflectance strongly influenced by the presence of snow), and the amount of atmospheric components interacting with UV radiation, especially ozone. The importance of the sun elevation is clearly demonstrated by comparing the average intensity in summer and winter. The intensity of the radiation is higher at Jungfraujoch and Davos during winter months mainly due to the presence of snow at Jungfraujoch and Davos, and lower at Payerne due to frequent winter clouds and much smaller quantity of snow at this site. The intensity of radiation at Davos progressively becomes lower than at Payerne in summer 2009, because of the disappearance of snow, and the presence of frequent convective clouds developing at this period above Davos. At Jungfraujoch, snow is present yearlong, at least on one side, which faces the Aletsch glacier.

For further information on MeteoSwiss UV measurements contact Laurent Vuilleumier.