Assessment of Erythemal UV Level in Nepal Based on Solar UV Estimates from Total Ozone Mapping Spectrometer

Nepal lies on the southern slope of Himalaya in Asia. In a width rangingbetween 150 and 250 km, the altitude varies greatly from about 100 m at itssouthern border to a maximum of 8848 min the northern part. Like thevariation in altitude, climatic condition varies quite a lot. Long-termmonthly mean erythemal UV daily dose values for Nepal are evaluated usingTotal Ozone Mapping Spectrometer (TOMS) estimation from the time of itsoverpass between 1996 and 2003. The results are presented as summer andwinter maps of mean UV levels in each satellite grid. The mean wintererythemal UV daily dose ranges between 2.1 and 3.6 kJ m^-2 whereassummer values are found to lie between 4.6 and 9.7 kJ m^-2. The altitudevariation increases the UV levels by about 0.2 kJ km^-1 in wintermonths, and 0.9 kJ km^-1 in summer. A multiyear monthly averageerythemal daily dose in most of the areas shows that the summer value isabout three times higher than that in winter. Although year-to-yearvariation is not pronounced in high- and mid-elevation regions, UV levelsseemed to decrease from 1997 to 2002 in the southern part of the country inthe low elevation region by about 5.35%. Due to the combined effects ofthe altitude, low ozone concentration in the troposphere, and thin air,surface UV radiation at higher altitudes is found to be higher than in thesurrounding regions.