Air humidity and precipitation

There is always invisible water vapour in the troposphere; it rises by evaporation from oceans, lakes, soil and plants. The warmer the air, the more vapour it holds: 1 m³ of air holds up to about 5 g at 0 °C, 9 g at +10 °C, 17 g at +20 °C and 30 g at +30 °C. The amount of vapour in 1 m³ of air (in grams) is absolute humidity; expressed as a percentage of the most the air can hold it is relative humidity (if there are 15 g at 30 °C, 15 : 30 × 100 = 50 %). Humidity is measured with a hygrometer. When air cools and vapour turns into tiny droplets or ice particles, a cloud forms; there are three main kinds: cumulus, stratus and cirrus. When droplets grow large they fall as rain; in cold air as snow, and from tall cumulus clouds as hail. Dew (frost in winter) forms from clear air. A rain gauge measures precipitation in mm. In Tashkent yearly precipitation is about 400 mm, in the western plains 100–200 mm, in the eastern mountains 800–900 mm and more; in the wettest places in the world (Mawsynram in India) it exceeds 10 000 mm a year. Moistening degree = yearly precipitation : yearly evaporation; on Uzbekistan’s plains evaporation is several times greater than precipitation (for example 300 : 1500 = 0.2), so there is a lack of moisture and irrigation is needed.