Unformatted text preview: s). Faster winter speed due to stronger upper level flow in winter 1 knot is 1 nautical mile per hour, = 1.1 (statue) mile per hour Wrap Up Polar Front Theory Midlatitude cyclones begin from polar front when local low pressure area develops Cyclones life cycle spans six stages from stationary front to matured low pressure center. Cyclones or storms in U.S. form in some preferred regions and typically move eastward or northeastward Cyclones move at an average of 16 kts summer, 27 kts winter 12.2 Cyclone Structure and Development What controls surface pressure total mass of air column above surface? If there is a net removal of air from the column, Warming the column causes it to expand "spill out" or removal of air lowers surface pressure Recall surface pressure decreases; and vice versa Click here to activate th Cooling the column eventually increase in surface pressure Convergence and Divergence Convergence (Con) is local piling up of air, it increases air density in the column Divergence (Div) spreading out of air
http://www.islandnet.com/ ~see/weather/elements/wh atgoesup3.htm Con/Div has lots to do with storm development If upperlevel pressure systems were always arranged this way, pressure systems would always die rapidly Convergence and Divergence cont'd Con/Div can occur due to both change in wind direction and in speed Either way air piles up or spreads out Think of cars on a highway Merge: convergence Branch: divergence Idealized Vertical Structure for Cyclone Behind cold front is cold air both at surface and aloft A cold column of air is "compressed" meaning most air molecules are near the surface Thus an area of relatively low pressure will be present aloft above the cold surface air Upper low is located to the north and west of the surface low Above the closed surface low is a trough Idealized Structure cont'd
The surface low is located directly beneath an area of divergence downstream from the base of the trough When upper level divergence is stronger than surface convergence: surface pressure dro...
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This note was uploaded on 03/18/2012 for the course UNDERSTAND 212 taught by Professor Qi during the Spring '12 term at Saint Louis.
- Spring '12