WEATHER PATTERNS AND SEVERE STORMS

AIR MASSES

• Body of air of similar temperature and moisture content

• Creates fairly constant weather over an area

• As an air mass moves, the conditions of the air mass change

CLASSIFYING AIR MASSES

• Air masses are classified by the source region

• Eastern North America is influenced by continental polar and maritime tropical air masses

Continental Polar – cP – uniformly cold and dry in winter bringing cold temperature and clear skies to an area

• cool and dry in summer bringing cool temperatures

Maritime Tropical – mT – warm, loaded with moisture, and usually unstable

• Source of much of the precipitation in the Eastern two-thirds of the US

• In summer maritime tropical brings hot temperatures with high humidity!

Maritime Polar – mP – in winter the maritime polar that affects the US is from the Pacific NW

• Cool, moist and unstable air mass, low clouds and showers

• Produces heavy rain and snow on windward side of mountains

• Maritime Polar in the Atlantic can form a low pressure storm called a nor’easter

Continental Tropical – cT – form in Southwest US and Mexico in summer, brings very hot and dry weather

• In fall the movement brings warm conditions in fall called the Indian Summer

3. FRONTS

• A boundary that separates two air masses

Warm Front – warm air moves into area formerly covered by cool air

• Slope is gradual

• Sequence of clouds: cirrus, cirrostratus, altostratus, nimbostratus

• Produce light-to-moderate precipitation

• Gradual warming of temperature

• Wind shift from east to southeast

Cold front – forms when cold, dense air moves into a region occupied by warmer air

• Twice as steep as warm fronts and advance more rapidly

• Associated with violent weather, thunderstorms and tornadoes

• Forms cumulonimbus clouds

• The temperature drops with the passage of a cold front and the weather clears

• Stationary Front – the cold air mass and warm air mass move parallel to each other; brings gentle to moderate precipitation

• Occluded Front – an active cold front overtakes a warm front and wedges the warm front upward

4. MIDDLE-LATITUDE CYCLONES• Main weather producers in the US• Large centers of low pressure that travel from

west to east and cause stormy weather• Rotate counterclockwise• Cold front and warm front, abundant

precipitation• Forms when a cold front overtakes a warm

front and lifts the warm front forming an occlusion

• As the occlusion begins, the storms gets stronger, the pressure falls, and the wind speed increases

5. THUNDERSTORMS

• Storm that generates thunder and lightning; can produce gusty winds, heavy rain and hail

• Associated with cumulonimbus clouds

• Thunderstorms are common

• Thunderstorms form when warm, humid air rises in an unstable environment

• Strong updrafts cause the cloud to grow vertically

6. TORNADOES• Tornadoes are violent windstorms that take the

form of a rotating column of air called a vortex. Te vortex extends down from a cumulonimbus cloud

• Frequency is greatest from April through June• Most tornadoes are associated with severe

thunderstorms• A mesocyclone begins when strong winds aloft

cause the surface winds to roll. Updrafts cause the rolling winds to tilt up

7. HURRICANES• Tropical cyclones that produce winds of at least 119

km/hour• Growing threat because more and more people live

near coast• Form between 5 and 20 degrees north and south

latitude• Develop most often in late summer when surface water

temperatures are warm enough to provide the necessary heat and moisture to the air

• Eye wall has the highest wind speeds and heaviest precipitation

• Eye is a zone where precipitation and winds stop• Most damage occurs with storm surge• A hurricane weakens over cold water or over land

2008 Hurricanes for Atlantic

Arthur MarcoBertha NanaCristobal OmarDolly PalomaEdouard ReneFay SallyGustav TeddyHanna VickyIke WilfredJosephineKyle