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Thunderstorms

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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Thunderstorms

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  1. Unit 4 – Atmospheric Processes Thunderstorms

  2. 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

  3. 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.

  4. 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

  5. Stages of Thunderstorm Development CUMULUS STAGE • 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.

  6. MATURE STAGE • 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.

  7. DISSIPATING STAGE • 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. 

  8. 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

  9. 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 http://www.nebraskastorms.com/video.htm

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