but in the simplest terms the following happens: Firstly you have to keep in mind the fact that alcohol evaporates at a lower temperature than water. Secondly it's worth mentioning that water molecules are much smaller than alcohol molecules. This means that in a barrel stored on a low floor of the warehouse, the temperature doesn't usually rise to a point where water starts to evaporate, but it does get warm enough for some of the alcohol molecules to break free. Over time alcohol molecules work their way through the wood in the barrels and escape, so bit-by-bit the alcohol content of the Whiskey decreases. The opposite happens in barrels stored on the top floors. Here the temperature rises to the point where both alcohol and water start to evaporate, but as the water molecules are smaller, it is much easier for them to escape through the grain of the wood, which means that more water escapes than alcohol and so the strength of the Whiskey increases. With this in mind, if a barrel is stored high up in a warehouse for enough years the strength of the Bourbon can rise dramatically, and in fact if you look at products such as George T Stagg, it's not uncommon to find that the abv has increased by 10 to 15%. I could keep writing on this topic for much longer as there is so much history and heritage to this category of spirits, but if I'm honest I think you'll agree