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Unit 4 – Atmospheric Processes. Thunderstorms. Necessary Atmospheric Conditions. Water vapour must be available in the lower atmosphere to feed clouds and precipitation as the storm forms A temperature gradient must exist so that rising air cools off rapidly with height

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necessary atmospheric conditions
Necessary Atmospheric Conditions
  • Water vapour must be available in the lower atmosphere to feed clouds and precipitation as the storm forms
  • A temperature gradient must exist so that rising air cools off rapidly with height
  • An updraft must force moist air up to colder levels of the atmosphere
the 2 kinds of thunderstorms
The 2 Kinds of Thunderstorms

Air mass thunderstorms

  • Usually created by convective uplift of warm, moist, and unstable air.
    • Have you ever been surprised by a sudden downpour of thunderous rain on what was up to that point a pretty nice day?
  • Air mass thunderstorms typically do not have very high winds, hail, or much lightning associated with them.

Severe thunderstorms

  • Have very high winds, hail, or much lightning associated with them
  • May even spawn tornadoes
  • Tend to form along strong cold fronts where the air on either side is very different, the atmosphere is very unstable, and wind shear aloft is prevalent.
    • Wind Shear – the change in wind speed or direction with height in the atmosphere over a relatively short horizontal distance
stages of thunderstorm development
Stages of Thunderstorm Development


  • During this stage warm, moist, and unstable air is lifted from the surface.
  • In the case of an air mass thunderstorm, the uplift mechanism is convection.
  • As the air ascends, it cools and upon reaching its dew point temperature begins to condense into a cumulus cloud.
  • Near the end of this stage precipitation forms.


  • Warm, moist updrafts continue to feed the thunderstorm while cold downdrafts begin to form.
  • As rain falls through the air it drags the cool, dry air that surrounds the cloud into it.
  • As dry air comes in contact with cloud and rain droplets they evaporate cooling the cloud.
  • The falling rain drags this cool air to the surface as a cold downdraft.
  • In severe thunderstorms the region of cold downdrafts is separate from that of warm updrafts feeding the storm. As the downdraft hits the surface it pushes out ahead of the storm.
    • Sometimes you can feel the downdraft shortly before the thunderstorm reaches your location as a cool blast of air.


  • Thunderstorm dissolves away.
  • By this point, the entrainment of cool air into the cloud helps stabilize the air.
  • In the case of the air mass thunderstorm, the surface no longer provides enough convective uplift to continue fueling the storm.
  • As a result, the warm updrafts have ceased and only the cool downdrafts are present.
  • The downdrafts end as the rain ceases and soon the thunderstorm dissipates. 
severe thunderstorms
Severe Thunderstorms

Conditions necessary:

  • Winds blowing in different directions producing wind shear
  • high water-vapour content in the lower troposphere
  • uplift of air
  • the existence of a dry air mass above a moist air mass
supercell storms
Supercell Storms
  • Most damaging of all severe thunderstorms
  • Smaller than the other types of thunderstorms (mesoscale convective complexes (MCCs) and squall lines) they are extremely violent and the breeding ground for most large tornadoes
  • Range from 20-50km in diameter
  • Last from 2-4 hours
  • Can bring high or low amounts of precipitation, create strong downbursts, flash floods, large hail and tornadoes
  • MCCs
    • Most common; very large clusters of self-propagating storms; downdraft in one cell leads to the formation of a new cell nearby
  • Squall Lines
    • Average 500km in length; long lines of individual storm cells; parallel to cold fronts approx 300-500km ahead of the front