To avoid sunburns, sunblock and hats are recommended on high UV days. The ultraviolet index is a forecast of the ultraviolet intensity at the Earth's surface, taken over a one-hour period around noon. It is measured once a day. Clouds will affect the intensity, measured on a scale of 0-15. People need to have skin block and dress properly to avoid sunburn.
Hats are a good idea on high-UV days.
The heat index and temperature can affect the UV Index, but not directly. Back to the Top Visibility Definition Visibility is the distance one can see at ground level. On clear days, visibility can stretch for dozens of miles. But haze, smog, fog and clouds all reduce visibility at ground level. Visibility is particularly important for pilots, drivers and boaters, because decreased visibility makes it more difficult to spot potential obstacles, giving the operator far less reaction time.