We’ve come a long way from the cane field and it is time to see what happens to the liquid during cask maturation.
Generally, the spirit that goes into a barrel is proofed down to 60-65% ABV. This alcohol level provides the best interaction with the wood without extracting too many harsh compounds from the oak, while still being financially sustainable.
The spirit does not sit passively inside the vessel, but moves in and out of the staves to extract color and aroma. Rather simplified, the spirit enters the staves during the day (warm) and exits during the night (cold). This also works on a macro scale so aging in a warmer climate will increase the interaction and extraction. Since the staves are not completely airtight, some oxygen will enter the barrel to react with the spirit, and some alcoholic vapor will evaporate. This evaporation is called the angel share.
In a warm climate, there is more evaporation than in a colder climate. Humidity also plays a big part. In high humidity the angel's share will consist of more alcohol, so the alcoholic strength will fall over time. In a drier climate, more water will evaporate and the alcoholic strength will rise.
Since both temperature and humidity can even vary in different parts of a warehouse, many distillers will move barrels around to balance these intricate processes. More common is to keep track of hot/cold/dry/humid zones in the warehouse and blend barrels during or at the end of maturation.
Aromas and flavors will also develop considerably during aging. Fatty acids and alcohol will continue to form new esters. Wood extracts and oxidization can develop earthy, funky aromas referred to as rancio. Think leather, tobacco & varnish. Even at a glance, you can see the myriad of variables that come into play during maturation and hopefully give you a better appreciation for the true mastery it takes to make sure that you will get a tasty tot down the line.