page_title: Precipitation
section_title: Cooling air to its dew point
hierarchical_section_title: Precipitation / How the air becomes saturated / Cooling air to its dew point
context_page_description: In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravity from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation but colloids, because the water vapor does not condense sufficiently to precipitate. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called "showers."
  Moisture that is lifted or otherwise forced to rise over a layer of sub-freezing air at the surface may be condensed into clouds and rain. This process is typically active when freezing rain occurs. A stationary front is often present near the area of freezing rain and serves as the focus for forcing and rising air.
context_section_description: The dew point is the temperature to which a parcel of air must be cooled in order to become saturated, and (unless super-saturation occurs) condenses to water.  Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds.  An elevated portion of a frontal zone forces broad areas of lift, which form cloud decks such as altostratus or cirrostratus.  Stratus is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass.  It can also form due to the lifting of advection fog during breezy conditions.
  There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling. Adiabatic cooling occurs when air rises and expands. The air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain (orographic lift). Conductive cooling occurs when the air comes into contact with a colder surface, usually by being blown from one surface to another, for example from a liquid water surface to colder land.  Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath.  Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or until it reaches saturation.
caption: Late-summer rainstorm in Denmark
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