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Chapter 9 Weather Patterns Why front is tilted? Fronts Warm Fronts Normal situation 1:200 Warm seasons 1:200 Cold Front 1:100 Cold Fronts and Warm Fronts Stationary Front The surface position of the front does not move, or moves very slowly. Dryline Oklahoma Occluded Front Warm and Cold-type Occlusions Cool air (b) Warm-type Cold air Midlatitude Cyclones Polar-Front Theory (Norwegian Cyclone model) Extratropical Cyclone Category One Hurricane: Winds 74-95 mph Category Two Hurricane: Winds 96-110 mph Category Three Hurricane: Winds 111-130 mph Category Four Hurricane: Winds 131-155 mph Category Five Hurricane: Winds > 155 mph Cloud Patterns Associated with a Mid-latitude Cyclone Stages in the Life Cycle of a Mid-latitude Cyclone Theories of the development of a Mid-latitude Cyclone Mid-latitude Baroclinic instability density surface . warm cold S N Release of potential energy stored in a tilted system to kinetic energy cold L H warm The energy for the growth of cyclone and anti-cyclone is from the potential energy stored in a tilted system. Heat transport Heat flux unit : J2 W2 H ms m Specific heat at constant pressure Kinematic sensible heat flux, sh sh H , C p J m 2s kg J kgK 3 m ms K Velocity X Temperature Cp 1004 kgJK Mid-latitude: mainly through the transient weather systems. North warm northward cold southward warm northward v' t' 0 v' 0 t' 0 v' 0 t' 0 v' t' 0 v' 0 t' 0 East North cold southward v' 0 t' 0 L v' t' 0 warm northward v' 0 t' 0 H cold southward v' 0 t' 0 v' t' 0 East Major Sites of Cyclone Formation Typical Paths of Cyclonic Storms Steering of Mid-latitude Cyclone Cold Anticyclone ― Blocking High Omega blocks Omega blocks are so-named because the high pressure system resembles the Greek letter omega. The typical pattern for this is low-high-low, arranged in the west-east direction. Rex blocks A Rex block resembles a "half figure 8" or "backwards S“. The Rex block in this case is hugging the West Coast of the U.S. A Rex block sets up with a strong high-pressure ridge adjacent to a strong low-pressure trough. This causes the air to swing in loops near the same longitude. The wind flow makes little progress to the east. RING OF FIRE / CUT-OFF HIGH High pressure is common across the Southeast US in the summer time. The high pressure cell, which can be an extension of the Bermuda-Azores high, can become fixed over the area for several days, especially if the jet stream is weak and with a lack of Canadian cool fronts. The air is most stable at the center of the high pressure. At the edges of the high pressure, the cap will be weak enough to allow afternoon convection to occur. SPLIT FLOW A split flow occurs when the jet stream branches into two separate branches. Weather systems flow quickly through each branch of the jet but the weather pattern becomes stagnant in the region between the two jets. A Modern View: The conveyor Belt Model