Wood is porous, so over time it will breathe in air from the surrounding environment in which it is stored. This will also give the whisky some unique characteristics. If the distillery storage facilities are next to the sea, on an island or in the middle of the Highlands then the air quality, temperature and humidity will be different and influence the end product. During each year of maturation about 2% of the spirit is lost through natural evaporation. This is called the 'angel's share' and explains why older whiskies are less readily available and more expensive to buy. There is simply less whisky in the cask to bottle.