Ultraviolet Index (UVI)is 0—15,a unitless value.At night,UVI is 0.On tropical plateau,UVI is 15 on clear sky.The WMO standard UV index unit is equal to 25mW/m² erythemally weighted dose rate.The major methods of forecast of surface sunny Ultraviolet Index consist of statistics forecast and model forecast.The statistics forecast mainly depends on the observed UVI and cloud amount data,and the model forecast mainly depends on forecasting the ozone field and applied radiative transfer model.