Term (also humidity) for the proportion of water vapour in the gas mixture of the earth's atmosphere or in rooms. Liquid water such as raindrops, fog droplets, ice or snow crystals are not included. Humidity is one of the many factors in weather or in the long-term course of the climate. From the free surfaces of water bodies (rivers, lakes, oceans), individual water molecules always transfer from the water volume to the air volume. Absolute humidity is the gaseous water vapour contained in one cubic metre of air, usually expressed in g/m3.
The most frequently used measure, however, is relative humidity (rLH), which is the ratio of the actual amount of water vapour contained in the air to the maximum possible amount, measured in %. However, the air can only absorb a limited amount of water vapor. The warmer the air, the higher this saturation amount: at 40 °Celsius 50 g, at 65 °Celsius 200 g and at 100 °Celsius 600 g. When the saturation level is exceeded, condensation forms tiny water droplets in the form of mist, as well as dew on surfaces close to the ground. Air humidity is an important parameter for meteorological processes. A relative humidity between 55 and 65% is optimal for humans, animals and most plants.
In viticulture, air humidity is very important for the growth of the vines. Sufficient relative humidity increases the absorption of nutrients and growth, and reduces the release of water. Plants release a lot of water through their leaves in dry air or when the relative humidity is too low, in order to cool the air. This process, known as evaporation, can lead to water stress. As a rule, during the fruit set (phase of the development of the flowers into berries), high humidity with sufficient sunlight, especially in the afternoon, has a positive effect on the quality of the wine through continuous photosynthesis with optimal formation of sugar and aromatic substances.
Such conditions exist in Germany, Austria and Switzerland, for example. Regions with low humidity usually have a hot climate. However, increased dew formation and high humidity, especially in combination with high temperatures, can favour a number of fungal diseases. The critical value for the formation of various moulds is around 70% rLF. Humidity is also very important for storage and maturing of wine in barrels or bottles. In the wine cellar the ideal value is 70 to 80%. See also under ageing, cellar mould, bottle ageing, wine climate chamber and wine temperature.