People usually wear less clothing at the beach and spend a lot of time in a horizonal position. Each factor may increase the area and directness of the UV radiation with respect to your skin. The UV Index forecast is VERY dependant upon the cloudiness forecast. However, there are several factor that modify UV radiation, including haze, pollution, and aerosols. If you are travelling to lower latitudes or to the southern hemisphere, you can call your local forecast office for UV information prior to travelling.
Seasonal variability can be quite high.
Late spring to summer typically produces the highest UV level, while in the early winter, levels are lower. However, the exact impact of ozone depletion or other local effects such as haze is not yet fully understood. UV radiation has been related to cataracts and possible suppression of the immune system so proper precautions should always be observed. The NWS UV Index forecast is produced by the NWS Climate Prediction Center in Camp Springs, Md., approximately a day in advance of the time the forecast is effective for in each area. This is done using existing data and technology, at little extra cost to taxpayers.