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EESA09H3 (185)
Lecture 4

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Environmental Science
Tanzina Mohsin

Lecture 4 Midlatitude Cyclones  Occur within the moving boundary of the Ferrel and Polar cells referred to as the Polar front  Commonly referrod to os low pressures, are the major source of weather variation in the midlatitudes 30 to 60  These storms occur apporx every four to seven days  In North America, they occur at the boundary of two major air masses, a cold, polar air mass (cP) and a moist, tropical air mass (mT)  The warmth and latent heat in the water vapor of the mT air provide the nergy for the storms  In North America, there are four major regions of cyclone development. Two of the regions are on the lee-side (eastern side) of the Rocky Mountains producing Alberta Clippers and Colorado Lows. The other two regions have cyclones developing over ocean bodies. Gulf Lows begin over the Gulf of Mexico and Hatteras Lows (Nor‟easters) off the eastern coast of the United States.  Typically 100s to 1000s km in extent (larger than hurricanes)  Have less intense winds than hurricanes  May have thunderstorms and tornadoes associated with them (along with a cold front) How are they formed?  Surface conditions are not the dominant mechanism for midlatitude cyclone formation  Upper level flow triggers storm formation, particularly the jet stream *Midlatitudes are battle ground between cP and mT Fronts  Division between air masses  Storms often referred to as low pressures or extratropical storms that dominate the weather in midlatitudes  Four types of fronts o Stationary front  exists between air masses but lacks the instability or energy for a storm to develop  low pressure trough  horizontal wind shear o Cold front  is typically the division between cP and mT air, although it can occur between mP and mT air  Designated by blue line with triangles facing warm air  Frontal slope is 1:50  15-25 knots (7-13 m/s)  Heavy precipitation along the front where mT air is forced up o Warm front  Warm air pushing into a cold air mass  Designated by red line with semi-circles pointing toward cold air  Slope: 1:150-1:300  Gentle precipitation (drizzle) o Occluded front  Not directly obs
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