Water in the Atmosphere — Class 11 Geography

Water is present in the atmosphere in gaseous, liquid and solid forms, derived from water bodies by evaporation and from plants by transpiration. The water vapour present is called humidity: absolute humidity is the weight of water vapour per unit volume, while relative humidity is the percentage of moisture present compared to the air's full capacity at a given temperature. Air holding moisture to its full capacity is saturated, and the temperature at which saturation occurs is the dew point. Evaporation transforms water from liquid to gas using heat (latent heat of vaporisation), while condensation transforms vapour into water through loss of heat, usually around hygroscopic condensation nuclei. After condensation, moisture takes the forms of dew, frost, fog, mist and clouds. Clouds are grouped by height and form into cirrus (high, feathery), cumulus (cotton-wool, flat-based), stratus (layered) and nimbus (dark rain clouds). Precipitation is the release of moisture as rainfall, snowfall, sleet or hail. Rainfall is of three types: convectional (heated air rises), orographic or relief (air forced up a mountain, giving a rain-shadow on the leeward side) and cyclonic or frontal. Rainfall generally decreases from the equator towards the poles, and coastal areas receive more than continental interiors.