humidity, or moisture content, of the atmosphere has considerable implications both for vine growth and for storing barrels and wine, whether in bulk or bottle. Humidity is normally measured as per cent relative humidity (% RH): the amount of water vapour a given volume of air holds, as a percentage of the maximum it could hold at the same temperature. The latter amount increases with temperature, so the RH of air containing a constant amount of water vapour falls as temperature rises, and vice versa. Humidity can also be expressed as saturation deficit, which is a direct measure of the evaporative power of the atmosphere. Actual evaporation is further influenced by wind and sunlight.