2.3 Weather processes and phenomena

• Air mass


  large body of air with relatively similar temperature and humidity characteristic
• Melting


  The change in state from a solid to a liquid
• Sublimation


  solid to gas
• Deposition


  the direct placement of water vapor onto a solid such as ice or vegetation.
• snow


  single flake or mass of flakes or frozen water
• sleet

  partly melted snow
• Convectional rainfall (5)


  Most common in hot climates. air that has been warmed at ground and rises. When it reaches high altitudes it cools quickly. vapour condenses and falls heavily. usually associated with a thunderstorm
• frontal rainfall (3)

  when warm and cold air meets. warm air (lighter, less dense) forced to rise over cold air. if it continue to rise it will condense and turn to rain.
• Orographic rainfall (2)

  air might be forced to rise over a barrier (eg mountain). air rises and forms rain.
• rain shadow


  leeward side of hills has relatively light rainfall compared to windward side
• Thunderstorm


  severe heavy rainfall thunder and lighting. usually caused by intense heating of ground during summer
• conditional instability


  air that continuous to ride and becomes unstable
• front


  boundary between a warm air mass and cold air mass. turned to cyclonic rainfall
• fog

  clouds at ground level. water vapor that condenses on dust.
• radiation fog


  forms on clear nights and calm when land loses heat rapidly by radiation.
• dew point


  temperature at which relative humidity is at 100%
• radiation cooling

  cooling off earths surface abs adjacent air caused by loss of heat due to surface emission of long wave emissions
• smog


  fog containing impurities. causes illness
• inversion


  situation where it at surface is replaced by warm air
• advocation fog


  formed by warm air flowing over cold air. drew point is reached and condensation takes place close to ground
• steam fog


  fog which cold air blows over warmer surface. evaporation from water quickly saturates air resulting in condensation leads to steam
• what states does atmospheric moisture exist in? (3)

  vapour, solid and liquid
• what are the main factors affecting evaporation (3)

  initial humidity of the air (air dry = lots of evaporation), supply of heat (hotter air = more evaporation), wind strength (calm conditions = air saturates faster)
• what are the main factors affecting condensation (3)

  radiation cooling of the air, contact cool of the air (rests on cool surface), expansive cooling (when rises)
• what must be first be produced to form any type of precipitation (1) and what is the exception (1)


  clouds except dew
• explain the forming of clouds and how precipitation occurs (4)

  minute droplets of water are condensed from vapour. they float in atmosphere as clouds. they form large droplets, when heavy enough they fall as rain
• Explain the Bergeron theory on precipitation (2)

  rain come from clouds that are well below freezing at high altitudes, the snow/ice melt as dropping down to warmer altitude
• what are the stages of thunderstorm (3)

  1 - developing stage: updraught (an upward current of air) caused by uplift.

  2 - mature stage: sudden onset of heavy rain and maybe lightening and thunder

  3 - dissipating stage: downdraught prevent any further convection instability.
• what are clouds formed by

  millions of tiny water droplets held in suspention
• what are the most important ways of classifying clouds (2)

  form or shape: like stratiform (layers) and cumuliform (heap shaped)

  height: low(<2000m), medium/alto (2000-7000) and high (7000-13000)
• what clouds form in unstable conditions

  dominant form of uplift, may produce cumulus clouds
• what clouds form in stable conditions

  stratiform clouds
• what clouds formed when a variety of fronts are involved

  variety of clouds exist
• what are the high clouds and explain characteristics? (3)

  cirrus - wispy, feathery, and composed entirely of ice crystals

  cirrostratus - transparent high clouds, which cover large areas of the sky

  cirrocumulus - made up of lots of small white clouds
• what are the medium clouds and their characteristics? (2)

  altostratus - sheets of thin cloud

  altocumulus - layers or patches of clouds
• what are the low clouds and their characteristics? (3)

  strato-cumulus - clumps or patches of cloud varying in colour

  stratus - uniform and flat

  nimbostratus - dark, grey, featureless layers of cloud, thick enough to block out the Sun
• what are the clouds that develop vertically and their characteristics? (2)

  cumulonimbus - menacing looking multi-level clouds, extending high into the sky in towers or plumes

  cumulus - detached, individual, cauliflower-shaped clouds
• what are the only two types of clouds to produce rain?

  cumulonimbus and nimbostratus