While a little sunlight can seem harmless and enjoyable, too much exposure to the sun can be dangerous to ones health. Overexposure to the sun causes sunburn and sun poisoning. Longer-term effects include skin cancer and cataracts. In order to avoid these dangerous risks, before spending the day in the sun, one should check the daily UV Index. The National Weather Service and the U.S. Environmental Protection Agency (EPA) created the UV Index. The Index works as follows: ranging from a scale of 1 to 11+, the levels of UV intensity are rated. The lower numbers indicate a lower risk of overexposure to the sun, and a higher rating warns against an extreme risk of overexposure. The UV Index is calculated for every ZIP code in America, for the next day, and takes in to account factors such as clouds and other environmental conditions that affect the UV exposure.

This and other relative information is available at the  EPA