Suppose in summer, two consecutive days, the first was hot at 30 centigrade, the second at 20 centigrade. Both was cloudless in the afternoon. There haven't been any rain prior to the days nor after for about a week. What caused the second day so much cooler then the first even though they looked identical. How would the UV radiation compare for both days. I assumed that since the second day was so much cooler, no sun protection was needed. Is that wrong?