Severe Weather

Air masses are classified on the basis of their source region

Air masses and weather Continental polar (cP) From northern Canada and interior of Alaska Winter – brings cold, dry air Summer – brings cool relief Responsible for lake-effect snows cP air mass crosses the Great Lakes Air picks up moisture from the lakes Snow occurs on the leeward shores of the lakes

Air masses and weather Maritime tropical (mT) From the Gulf of Mexico and the Atlantic Ocean Warm, moist, unstable air Brings precipitation to the eastern United States

Air masses and weather Continental tropical (cT) Southwest and Mexico Hot, dry Seldom important outside the source region

Air masses and weather Maritime polar (mP) Brings precipitation to the western mountains Occasional influence in the northeastern United States causes the "Northeaster" in New England with its cold temperatures and snow

Fronts Types of fronts Warm front Warm air replaces cooler air Shown on a map by a line with semicircles Small slope (1:200) Clouds become lower as the front nears Slow rate of advance Light-to-moderate precipitation

Warm Front / High pressure system Cold Front / Low pressure system Rain!

Fronts Types of fronts Cold front Cold air replaces warm air Shown on a map by a line with triangles Twice as steep (1:100) as warm fronts Advances faster than a warm front Associated weather is more violent than a warm front Weather behind the front is dominated by Cold air mass Subsiding air Clearing conditions

Warm Front / High pressure system Cold Front / Low pressure system Warm Front / High pressure system Rain/Snow Thunderstorms

Fronts Types of fronts Stationary front Occluded front Flow of air on both sides of the front is almost parallel to the line of the front Surface position of the front does not move Occluded front Active cold front overtakes a warm front Cold air wedges the warm air upward Weather is often complex Precipitation is associated with warm air being forced aloft

Middle-latitude cyclone Primary weather producer in the middle-latitudes

Middle-latitude cyclone Idealized weather Middle-latitude cyclones move eastward across the United States First signs of their approach are in the western sky Require two to four days to pass over a region Largest weather contrasts occur in the spring

Cloud patterns typically associated with a mature middle-latitude cyclone

Stages in the development of a thunderstorm http://www.nws.noaa.gov/om/brochures/tstorm.pdf http://www.agiweb.org/earthcomm/fluidspheres/weather.html

Severe weather types Tornadoes A rapidly spinning column of air descending from a large thunderstorm Have the highest wind speeds of any other weather phenomenon. In the U.S., tornadoes move northeast at speeds up to 100 km/hr (62 mph) Rotating wind speeds sometimes in excess of 500 km/hr (310 mph)

Severe weather types Tornadoes How a Tornado Forms Moist air from Gulf of Mexico Fast moving cold, dry air mass from Canada Jet stream moving east at 150 mph Sets up shearing conditions

Severe weather types Tornadoes How a Tornado Forms Warm moist Gulf air releases latent heat, creates strong updraft Updraft sheared by polar air, then twisted in a different direction by jet stream

Severe weather types Tornadoes Why do some thunderstorms spawn tornadoes while others do not? Super Cell Thunderstorms

Severe weather types Tornadoes Scales The Fujita-Pearson Scale measures wind speed The size of a tornado is not necessarily an indication of its intensity Large tornadoes can be weak, and small tornadoes can be violent

http://www.tornadoproject.com/fscale/fscale.htm

Severe weather types Hurricanes Only natural disaster that is given a human name Actually large tropical cyclones Convert heat in the ocean into winds Exports excess heat from the tropics to the midlatitudes

Severe weather types Hurricanes How a Hurricane Works Tropical disturbance Low pressure zone develops and draws in clusters of thunderstorms and winds

Severe weather types Hurricanes How a Hurricane Works Tropical disturbance Tropical depression Surface winds strengthen, move about the center of the storm Central core funnels warm moist air up towards stratosphere Air cools, vapor condenses, latent heat released Fuels more updrafts, cycle repeats, storm grows

Severe weather types Hurricanes How a Hurricane Works Tropical disturbance Tropical depression Tropical Storm Storm has sustained surface wind speeds of +39 mph

Severe weather types Hurricanes How a Hurricane Works Tropical disturbance Tropical depression Tropical Storm Hurricane Surface winds consistently over 74 mph

Severe weather types Hurricanes How a Hurricane Works Tropical disturbance Tropical depression Tropical Storm Hurricane The Eye As wind speed increases, winds are spiraled upwards prior to reaching the center A distinctive clear “eye” is formed Strongest winds are located on the walls of the eye

Hurricane Origins Form in the tropics ~ 5° and 20 ° latitude Cannot form at the equator (Coriolis effect = 0)

Hurricanes Hurricane Damages Storm Surges Large mound of water builds up beneath the eye Reaches land as a surge of water

Hurricanes Hurricane Damages Storm Surges Wind speed varies depending upon which side of the hurricane you’re on Amount of damage on the coastline will vary accordingly

Hurricanes Hurricane Damages Heavy Rains Mudflows and Debris Avalanches Flooding

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