Humidity
High humidity minimizes evaporation. Wine makers consider humidity over 75% for reds and over 85% for whites to be ideal for wine aging and barrel storage. Humidity in wine caves ranges naturally from 70 to 90%.
In Northern California, wine barrel evaporation in a surface warehouse is on the order of 4 gallons (15.1 liters) per each 60 gallon (227 liter) barrel per year. In a wine cave, barrel evaporation is reduced to about 1 gallon (3.8 liters) per barrel per year.
Since red wines are usually barreled and aged for two years, this represents a 10% gross volume loss difference. For white wines, which are barreled and aged for about one year, a 5% loss difference is realized, a significant savings.
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