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Fronts & Cyclones FRONTS & CYCLONES “Storm of the Century” Severe Blizzards from Cyclonic Storm Polar Front Theory Norwegian Cyclone Model (1921) Weather for Middle Latitudes… • Between South Florida & Alaska • Storms driven across U.S.A. by Westerlies Polar Front Theory Fronts between Air Masses… Stronger during Cold Winter… • Colder Polar (Continental) • Warmer Tropical (Maritime) Fronts Boundaries separating Air Masses… Different Densities… • Temperature –Warm –Cold • Moisture (Humidity) Fronts Air Mass collision similar to “Battlefronts” One Air Mass is displaced over another • Warm over Cold Fronts are Mid-Latitude source for… Precipitation Severe Weather Fronts 4 Types… Warm Front Cold Front Stationary Front Occluded Front Fronts on Weather Map Warm Fronts Ground Air movement… Warm Air occupies where… • Formerly Colder Air Red Line with Half-Circles on Map... • Facing into displaced Cold Air Warm Fronts Warm Air… Less Dense than Cold Air & cannot displace it Rises gently over Cold Air (wedges)… •Gradual Slope •Lower Height Warm Fronts Warm Air… Cools Adiabatically as rises over Cold Air • Moisture condenses into Stratus Clouds • Precipitation for large area in advance of edge Wind shifts from East Æ Southwest Warm Fronts Hot Summer Season… Warm Air Mass is very Moist Unstable Warm Air is forced aloft… • Becomes Buoyant & Rises • Cumulonimbus Clouds & Thunderstorms Warm Fronts Where? East of Rockies • Maritime Tropical (mT) from Gulf of Mexico… – Overruns receding Cool Air Raises Air Temp. of area displaced Cold Fronts Cold Air Æ Area with Warmer Air Blue Line with Triangles on Map... Facing into displaced Warm Air Cold Fronts Subsiding dense Cold Polar Air… Pushes into Warm Air… •Steep Slope •Greater Height = 2 X Warm Front Cold Fronts Move faster than Warm Fronts… Cold = 20-35 mph Warm = 15-20 mph Wind shifts from Southwest Æ Northwest Cold Fronts Cumulonimbus Clouds Heavier Precipitation… • Shorter distance/area & duration at edge… – Narrow Width due to steeper slope Cold Fronts More Violent Weather… Stationary Fronts Neither Cold nor Warm Air is aggressive Air flows parallel to line of front Front does not advance Line on Map with opposite side… Red Half-Circles… • Ahead of Warm Air Blue Triangles… • Ahead of Cold Air Occluded Fronts Involves 3 Air Masses… Fast Cold Air Slower Warm Air Cool Air Purple Line with Alternating… Half-Circles & Triangles Occluded Fronts Rapidly moving Cold Front… Overtakes slower Warm Front Occluded Fronts Warm Air wedged upwards New Front between… Slower Cool Air Faster Cold Front Occluded Fronts Warm Air forced Aloft Rain with Warm Front… Heavy at first Lightens with Occlusion Occluded Fronts Cold Type… Polar Cold Air Æ Cool Air East of Rockies Warm Type… Maritime Cool Air Æ Polar Cold Air Pacific Coast Drylines Fronts between… Humidity differences Denser Dry Air displaces lighter Moist Air Moist Air is forced Aloft Advancing Front brings drop in Humidity • Little change in Temperature Drylines Occurs in Summer… Great Plains Æ lower Mississippi Southwest (cT) Æ Gulf of Mexico (mT) Mid-Latitude Cyclones Storm Track along Polar Front Propelled West Æ East by Jet Stream aloft Mid-Latitude Cyclones Cyclogenesis – Cyclone formation Waves form as Jet Stream moves East Mid-Latitude Cyclones Waves reflect competing… Cold Air from North Warm Air from South Mid-Latitude Cyclones Waves caused by surface irregularities… Topography (Mountains) Sea-Land contrasts in Temperature Ocean currents Cyclone has 6 Stages in Life Cycle Cyclone Life Cycle Stationary Front develops… Parallel opposing winds… • Invading Cold Polar Air È South •Warm Air Ç North Cyclone Life Cycle Wave develops… Front bends to opposing winds… Cyclone Life Cycle Cyclone develops… Around Low Pressure center • Updraft of Warm Moist Air… –Clouds & Rain Cyclone Life Cycle Occlusion begins… Cold Air begins to overtake Warm Air Cyclone Life Cycle Occluded Front develops… Low Pressure Center… • Forces Warm Air aloft Cyclone Life Cycle Occlusion exhausts energy… Low Pressure weakens… • Cyclone & Storms dissipate •Warm Air above •Cold Air below Fronts gone Cyclone Life Cycle 6 Stages proceed from West Æ East… Along Storm Track… • Propelled Eastward by Westerlies Cyclone on Weather Map Cyclonic Storm Weather Warm Front… mT from South Falling Air Pressure Cirrus Æ Stratus... • Light Rain Temperature rises Winds change… •From East Æ South Cyclonic Storm Weather Between Storms… Maritime Tropical Clear skies Wind from South Cyclonic Storm Weather Cold Front… cP from North Rising Air Pressure Cumulonimbus… •Heavy Rain Temperature drops Winds change… •From S Æ N Cyclonic Storm Weather Occlusion… Center of Cyclone • Warm Air aloft • Temperature remains cool at surface Air Pressure continues to fall Cyclonic Storm Weather Occlusion… Farther North Nimbostratus… •Winter Snow & Ice Moves slowly… •Cold Weather remains longer Cyclones & Anticyclones Aloft Ground Cyclones & Anticyclones Wind circulation propelled by Winds aloft Surface Convergence Convergence Aloft • Divergence Aloft • Surface Divergence Also Wind blows… High Æ Low Cyclones & Jet Stream Divergence Aloft not in all directions Flow from West Æ East due to Jet Stream Mechanisms… Speed Divergence • Acceleration Directional Divergence • Horizontal Spread Vorticity •Rotation Cyclones & Jet Stream Low Pressure continues at Ground… When Divergence Aloft… • Exceeds Convergence at Ground Cyclones & Jet Stream Convergence Aloft into Anticyclone… Downwind from Ridge Divergence Aloft out of Cyclone… Downwind from Trough Cyclone Formation Areas Leeward of Mountains Coastal Regions East of Rocky Mtns Southeast USA Where great Temperature differences Cyclonic Storm Tracks Cyclones travel Æ East Æ Northeast Most Storms… • Great Lakes • Northeast Cyclonic Storms “Alberta Clipper” “Nor-easter” Wind & Snow Perfect Storm movie Cyclone Migration Cyclone migration… Æ Southeast • Æ North Anticyclones Polar High… Frigid Cold • È Gulf of Mexico “Blocking Highs” = Stagnant Anticyclone… Blocks Cyclone migration Anticyclones Subtropical Inversions – Subsiding Air… Traps Pollution • Los Angeles • Mexico City Conveyor Belt Model Modern view… Circulation Conveyor Belt Model 3 Interacting Airstreams… 2 Surface that Rise… •Warm Conveyor Belt (Humid) –Most Rain with rise • Cold Conveyor Belt (Dry) – Saturates with rise 1 Aloft… • Dry Conveyor Belt (Cold) Mature Cyclone Mid-Latitude Cyclone “Comma” shape •Dry slot from South Æ center (Low) •Comma Tail Weather Forecasting Indicators for Weather Forecasting… Barometric Pressure •High = Clear •Low = Rain Wind Direction • Air Mass source.
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