Why is getting a tan in the evening less likely than getting a tan in the morning? When I was young I read one book in which is written that you get more tan on the morning than on evening even light angle from Sun to Earth is the same. Don't remember exact reason, I think because ultraviolet is more absorbed on the evening because air is more humid or something like this. Is it true?
 A: This sounds false to me if it's proposed as a general statement rather than something about weather patterns that operate on a 24-hour cycle. Disregarding weather, the intensity of sunlight depends on (1) the angle at which the solar radiation intercepts the landscape and the surface of your skin, and (2) the thickness of the atmosphere traversed by a ray of sunlight, which causes absorption. Both of these factors depend only on the height of the sun in the sky. The claim would obviously be silly, for example, if applied to the arctic -- you wouldn't be able to get a tan in October, when the sun was setting, but you would be able to get one in April, when it was rising??
And if it is a statement about weather patterns, then it's certainly not something that can hold true in all places. E.g., on a lot of the California coast a common summer weather pattern is for there to be fog in the morning, which burns off later in the day -- this would be the opposite of what was claimed.
A: The only explanation I can think of for such an observation would be simply that it is generally hotter at the same Sun angle after noon than before noon, due to the thermal inertia of the atmosphere and surface.  (This ignores other effects like differences in cloud cover or marine layer between the morning and the afternoon.)  The result is that you won't want to stay out as long in the afternoon as you would in the morning.  Assuming that you respond to your discomfort, you will spend less time at the same Sun angles in the afternoon than in the morning.
