A cask is not a sealed system. Oak expands as temperature rises and contracts as it falls, and spirit moves into and out of the wood with each cycle. The annual range of temperature and humidity a warehouse experiences therefore determines how much interaction occurs in a given year, which is why identical spirit in identical casks matures at different rates in different countries.
Scotland: Cool and Maritime
Scotland's maritime climate produces a narrow annual temperature range and consistently high humidity. Traditional dunnage warehouses, built of stone with earth floors and casks stacked three high, moderate that range further. Interaction is correspondingly slow, and the three-year legal minimum is in practice far exceeded, with standard expressions commonly matured for ten to eighteen years.
High humidity also affects the composition of the loss. Where the atmosphere is damp, alcohol evaporates preferentially and cask strength falls slowly over time. The conventional Scottish figure of approximately two per cent annual volume loss reflects those conditions.
The same spirit in the same cask matures differently in Kentucky and in Islay, and the difference is climate alone.
Kentucky: Continental Extremes
Kentucky experiences hot summers and cold winters, a continental range far wider than Scotland's. Traditional rickhouses are tall timber-framed structures, and the temperature difference between the upper and lower floors is significant enough that warehouse position is recorded as a production variable. Distillers have historically rotated casks between floors to manage that variation.
Interaction is rapid under those conditions, and bourbon reaches a documented maturity in far fewer years than Scotch. Drier air means water evaporates preferentially and strength tends to rise during maturation, the inverse of the Scottish pattern.
Japan: Four Distinct Seasons
Japan's four clearly separated seasons produce a wider annual range than maritime Scotland while retaining high summer humidity. Production literature documents this combination as a distinctive maturation condition. Warehouses at different altitudes, notably at Hakushu in the Southern Japanese Alps, are used to vary conditions systematically within a single company's stock.
Warmer Climates
In tropical and subtropical conditions, annual evaporation losses recorded by producers can exceed ten per cent. Maturation is rapid, and extended ageing becomes economically impractical. Producers in India, Taiwan and parts of Australia have documented maturation programmes measured in a small number of years rather than decades, and age statements from those regions are not directly comparable with Scottish ones.
An age statement describes time, not condition. Climate determines what that time achieves.
Warehouse type compounds the effect. Dunnage, racked and palletised warehouses hold different numbers of casks at different heights and with different airflow, and each produces a measurably different maturation profile from the same climate.