The Angel's Share is the amount of distilled spirits lost to evaporation from the barrel or cask into the air as the spirit ages.
Climate, cask and storage conditions all impact how much spirit is lost, but on average around two percent of the total liquid in the cask is lost for Scotch producers each year, while for Bourbon producers it’s not uncommon for up to 10% oof the volume of the barrel to evaporate in the first year, reducing to between 3-5% each year thereafter.
The angel’s share doesn’t just impact the volume of liquid in a cask, it also affects the ABV. In low humidity climates like Kentucky more water will evaporate than alcohol, increasing the overall ABV, while for Scotch producers, around 0.5% ABV is lost to the angels each year it is in left to mature in a cask.