Latitudinal and Seasonal Variation in Calculated Ultraviolet‐b Irradiance for Rice‐growing Regions of Asia*

Ultraviolet‐B (UV‐B,280–320 nm) irradiance was calculated for more than 1200 sites in Asia to characterize the spatial and temporal variation in the present UV‐B climate for rice‐growing regions. The analytical model of Green et al. ( Photochem. Photobiol. 31 ,59–65, 1980) was used to compute UV‐B irradiance for clear skies using satellite‐observed ozone column thickness and local elevation data. Ground‐based observations of cloud cover were then used to approximate the average effect of cloud cover on UV‐B irradiance using the approach of Johnson et al. ( Photochem. Photobiol. 23 ,179–188, 1976). Over the geographic range of rice cultivation, the maximum daily effective UV‐B irradiance (UV‐B BE ), when weighted according to a general plant action spectrum, was found to vary approx. 2.5‐fold under both clear and cloudy sky conditions. Under clear skies, the timing of maximum solar UV‐B BE changed with latitude and varied from February‐March near the equator to July‐August at temperate locations. Cloud cover was found to alter the season of maximum UV‐B BE in many tropical regions, due to the pronounced monsoonal climate, but had little effect on UV‐B seasonality at higher latitudes. Under a climate resulting from a doubling of atmospheric carbon dioxide, estimated UV‐B using predicted cloud cover was found to change by up to 17% from present conditions in Thailand. Both latitudinal and seasonal variation in solar UV‐B radiation may be important aspects of the UV‐B climate for rice as cultivars differ in sensitivity to UV‐B and are grown under diverse conditions and locations.