1. Unit 4 – Atmospheric Processes

2. Necessary Atmospheric Conditions 1. Water vapour must be available in the lower atmosphere to feed clouds and precipitation as the storm forms 2. A temperature gradient must exist so that rising air cools off rapidly with height 3. 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