People ask often about "Angles Share" or have never heard the term and ask "what the?"... Here is my summary or attempt at a public schooled explanation;
During the whiskey aging process, the alcohol/water mixture in the barrel warms up and expands in the summer months. In the colder months the mixture contracts again. This is known as "breathing"; the surrounding air impacts the pores of the oak wood. Barrels are water tight, not air tight. The whiskey can lose a little of its alcohol due to "breathing", or evaporating. Somewhere between 0.5% and 1% of its alcoholic strength per year is lost during aging. This is known as the "Angels' Share", since it evaporates up into the sky. There is a reason why the alcohol decreases more as opposed to the water; something with each having different "vapor pressure", its too complicated for me. The longer you age, the more you lose is a good way to remember it.
This is why some distilleries fill their whiskey into the barrel with an alcohol content of 63.5%, a compromise between aging time and alcohol loss.
Hope that helps explain Angels Share .... up next; I will attempt to explain the "Devil's Share".
Cheers & Stay Silly!