The temperature profile varies depending on the type of warehouse, with the options including brick, stone or metal clad warehouses.
"We only use metal clad warehouses, which transmit temperature more readily than any other type. We're looking for as much heat in summer, and as much cold in winter," says Rick Robinson, Plant Manager, Wild Turkey.
Location also matters. Warehouses on hill tops are more exposed and experience greater temperature extremes than warehouses in valleys, which are more sheltered.
Changes in temperature prompt a vital process known as a 'cycle.' As the temperature rises in spring and summer the spirit expands within the cask and penetrates into the oak staves, which contain various flavour compounds. As the temperature cools, during the autumn and winter, the spirit contracts and exits the oak, carrying flavour compounds (which add vanilla notes, for example) back into the 'bulk' of the spirit.
A related factor is the height of a warehouse, which can comprise seven floors or more. Additionally, each floor holds three tiers of barrels, sitting on rails, which means a warehouse with seven floors can store barrels 21 high. This scale also means the temperature varies in different parts of a warehouse.
"During the winter it's marginally warmer at the top than the ground floor, with the warehouse essentially reflecting the ambient temperature. However, in the summer when the ambient temperature is 90° F, it can be cooler at the bottom of the warehouse, perhaps 70° F, with the temperature rising progressively as you go up the warehouse, and at the top of a nine floor warehouse it may be 120° F. This creates a series of micro-climates between the ground floor and the top," says Fred Noe, Master Distiller, Jim Beam.
These micro-climates exert their own particular influence.
"The higher the temperature the more intense the cycle, with the spirit achieving a greater depth of penetration into the oak staves. This means bourbon ages faster in barrels at the top of the warehouse, developing deeper flavours and greater complexity than in barrels lower down the warehouse," says Harlen Wheatley, Master Distiller, Buffalo Trace.
As the temperature rises so does the rate at which water and alcohol evaporate from the barrel, helping to develop complexity (the exact influence of varying evaporation rates is still being researched). Annual evaporation rates are around 3-5% of the contents of the barrel, depending on the barrel's location. Evaporation at the top of the warehouse could be up to one per cent higher than at the bottom.
Traditionally barrels were rotated around a warehouse to experience a range of micro-climates, which Maker's Mark continues to do.
"Our barrels first go to the top floors of the warehouse for a more intense start to the ageing process, then a taste panel decides when each barrel is ready to be moved to the lower floors where the ageing process is slower. It's all about achieving a consistent taste profile, rather than having a set amount of time for barrels to spend in different parts of the warehouse," says Greg Davis, Master Distiller, Maker's Mark.
Most distillers now leave barrels in one location for the duration, and blend bourbon aged in different parts of a warehouse to create a consistent flavour profile. Not rotating barrels also provides other opportunities.
"We bottle a huge variety of brands offering different styles of bourbon matured for varying periods. This is only possible because we don't rotate barrels, which gives us a comprehensive inventory that spans a range of flavour profiles to choose from," says Larry Kass, Communications Director, Heaven Hill.
The ageing warehouse clearly has a significant influence, but is it possible to quantify?
"We know how each warehouse and how each floor in each warehouse influences the bourbon. The warehouse accounts for 25% or more of the character of the bourbon," says Harlen Wheatley.
Heating the warehouses
The annual temperature changes that prompt a 'cycle' can be replicated by using steam-heated radiators to heat the ageing warehouses, and consequently increase the number of cycles per year.
"This is a traditional approach taken by some distilleries, with a patent for this concept taken out in 1874. Woodford Reserve, for example, is aged in heated warehouses, built from stone or brick," says Chris Morris, Master Distiller, Brown-Forman.
"We heat warehouses in winter then turn off the heat, open windows and turn on fans to draw cold air in. The temperature falls quite quickly. Then we close the windows, stop the fans and put the heat on again. During the aging process Woodford Reserve gets more spice and fruit character from the barrel. It's expensive to build and operate heated warehouses, but it's worth the expense as the product is rich, complex and balanced," says Chris Morris.