Sun Strength at Different Temperatures

Say that at 2 p.m. on May 31, 2015, the temperature is 60 degrees in my backyard. Now suppose that on the same date and time, the temperature is 90 degrees instead. All other conditions are identical. At which temperature is the risk of sunburn greater?

—John P., Maynard, Mass.

Surprise: They’re the same. Air temperature has no effect on the strength of the ultraviolet (UV) radiation that reaches the ground, and it is UV rays that cause sunburn and contribute to skin cancer. Instead, major factors are the time of day and year, the elevation of the location, the thickness of the ozone layer over the area and the cloud cover that day.