This study examines the distribution of long-term trends in ground level erythemally weighted ultraviolet (UV) exposures in the northern latitudes for the period 1979–1991 using measurements from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) instrument. A new erythemal UV data set (now available to the public via World Wide Web) was produced recently by NASA and has been tested by NASA at the Goddard Space Flight Center against a previous NASA erythemal UV product, which was used in a former study that included similar adjustments for aerosols and clouds but not aerosol absorption. Zonal mean erythemal UV data from both products show similar, ∼3–7% per decade, increases in the midlatitudes to high latitudes. The detection of regional patterns in trends in erythemal UV favors summer months when surface UV is strongest and noise factors such as clouds and aerosols are not as influential. Analysis of the zonal patterns in trends around summer months indicates that most of the regional increases (exceeding 6% per decade) in the Northern Hemisphere in the latitude range 30°N–40°N originate from the Pacific and Atlantic oceanic regions. Increases (also exceeding 6% per decade) in latitudes 40°–60°N appear to originate from the North American and Asian continents and also central Europe. Trends over the east Asian continent in high latitudes indicate increases exceeding 10% per decade for May-August. The important conclusion is that positive trends in the northern subtropical latitudes originate mostly over oceanic regions, whereas positive trends at higher latitudes originate mostly over landmasses. Some of the increases in erythemal UV over central Europe and the east Asian continent in summer months can be attributed to decadal decreases in cloudiness for the 1979–1991 time period.
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