The diagnosis and forecast of clear sky ultraviolet levels at the Earth's surface

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Abstract
A climatology of clear sky ultraviolet (UV) levels, weighted by the sensitivity of human skin, is presented. Away from the tropics the values are shown to be strongly dependent on the time of year, with peak values occurring in midsummer. Near the equator two annual maxima occur due to the twice‐yearly passage of the sun over the equator. The second factor influencing clear‐sky UV levels is the total ozone amount. In mid latitudes, because of lower ozone amounts, UV levels are generally higher around the autumn equinox than at the spring equinox. High UV levels also occur locally in spring due to the Antarctic ozone hole, although Arctic spring values have not yet been significantly affected. Next, total ozone amounts are shown to be related to meteorological parameters. Using a simple representation of the UV level in terms of the solar zenith angle and total ozone amount, an operational procedure for forecasting UV levels is outlined Finally, some comparisons with observed UV levels are presented and a sample UV forecast is shown.