According to its appearance

1. Liquid precipitation
   
   
   1. rain: drops of liquid water; most begins as snow
   2. drizzle: droplets (<0.5 mm in diameter)
   3. dew: condensation of water vapour onto a cool surface
   4. fog-drip (occult precipitation): accumulation of fog droplets on vegetation and other obstacles (horizontal interception)
      
2. Solid precipitation
   
   
   1. snow grains: small crystals of ice; solid equivalent of drizzle
   2. snow flakes: agglomeration of grains; solid equivalent of rain
   3. sleet: frozen rain
   4. graupel: pellets of ice 2-5 mm in diameter formed by collision of snow crystals and rain drops when cloud temperatures are near the freezing point
   5. hoar frost: solid equivalent of dew; formed by sublimation of water vapour onto cold surfaces as feature-like crystals
   6. rime (occult precipitation): freezing of water droplets from fog onto cold surfaces; includes artificial snow; crystals tend to be larger than in snow
   7. hail: spherical lumps of ice composed of concentric layers; distinct from other forms of solid precipitation because it form in the warmest season and thus begins to melt immediately upon contact with the ground

According to its place of origin

1. adjacent to cold ground: dew, fog-drip, rime and frost
   
   
2. above the condensation level in the atmosphere, where water vapour, in air below the dew point temperature, condenses onto hygroscopic nuclei or sublimates to form ice crystals

• outside the tropics, most precipitation begins as ice crystals which melt as they fall through warmer air
• cirrus clouds composed of ice crystals often precede a storm
• sun dogs are light refracted by ice crystals
• ice crystals grow at the expense of water droplets because the colder air next to the ice has low vapour pressure (hold less water) and thus supercooled water droplets freeze to the ice crystals
• whereas dew point temperatures are achieved near the ground by radiative cooling, they occur at higher altitudes during the rising, expansion and adiabatic cooling of moist air; therefore precipitation from above the condensation level can be further classified by mode of uplift:
  
  
  • convergence of tropical and subtropical air masses (hurricanes)
  • mid-latitude frontal (cyclonic) precipitation where warmer air is diaplced along fronts between air masses (tornados)
  • differential heating of the earth and therefore thermal convection lower atmosphere (thunderstorms)
  • orographic: rise of air masses over mountains (Figures 4-11 in Dingman shows mean annual precipitation and topography across southern B.C.)
• these mechanisms are superimposed on one another (e.g. monsoons (convergent precipitation) rising over northern India (Himilaya) to produce highest annual precipitation in the world, 10800 mm)
• precipitation requires cooling of air to the dew point temperature, condensation on nuclei, coalescence of droplets to form rain drops, and a continuous source of vapour to sustain the process
• prior to the 1940's, the source of water vapour was thought to be local and this notion was the basis for proposals to modify land use to ameliorate the dust bowl
• however, Holzman (1937) and subsequent studies have shown that most precipitation comes from maritime air masses and most water evapotranspiring from land into continental air masses moves off the continent (see maps on pp. 78-79 in Dingman)
• thus little precipitation falls where the water originates, except over large evaporating surface (e.g. Amazon basin, northern wetlands, great lakes)

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