document7

Chapter 7: Air Masses and Weather Systems

Physical Physical GeographyGeographyNinth EditionNinth Edition

Robert E. Gabler

James. F. Petersen

L. Michael Trapasso

Dorothy Sack

Ch. 7: Air Masses and Weather Systems

7.1 Air Masses

• Air Mass: large body of air whose temperature and moisture characteristics are similar.

• Due to General Atmospheric Circulation, air masses move and cause changing weather.

• Source Region:m = maritime (sea)

c = continental (land)

E = Equatorial (very warm)

T = Tropical (warm)

P = Polar (cold)

A = Arctic (very cold)

7.1 Air Masses

7.1 Air Masses

Q: Which air mass affect your location? Are there seasonal variations?

7.1 Air Masses

• Air Mass Modification– cP or CA moves over Great Lakes and picks up

moisture, and may cause Lake effect snow.

7.1 Air Masses

• Continental Arctic (cA): Dry and very cold– occasionally impacts U.S. in the winter– it often produces record breaking cold.

• Continental Polar (cP): Dry and cold. – May reach Gulf of Mexico in winter. – Rarely affects west coast.

• Maritime Polar (mP): moist and cool– Westerlies bring air mass to west coast,

especially in winter. Occasionally affects east U.S.

7.1 Air Masses

• Maritime Tropical (mT): moist and warm– May originates in Gulf of Mexico– Major impact on central and eastern U.S

• T-storms in the summer• Clashes with cP

• Continental Tropical (cT): dry and hot– Small source region (SW deserts and N. Mexico– Dry line– Smallest player in U.S. weather

7.2 Fronts

• Fronts– clash between air

masses– Generally move with

westerlies– 3-dimensional– Frontal uplift

7.2 Fronts

• Cold Front:– Cold air moves in on warm

air– Warm air (less dense) rises

above cold air– Steep slope– Cumulonimbus– may form a Squall line– Sharp changes in

temperature, pressure, and wind

7.2 Fronts

• Warm Front:– Warm air moves in on

cooler air– Warm air (less dense)

rises above cold air– Slope is not as steep– Light precipitation which

may last longer– Usher in warmer

conditions

7.2 Fronts

• Stationary Front– Boundary between air masses that is not

moving– Extended period of light precipitation and

occasionally strong T-storms

• Occluded Front– Cold air is overtaking warm air– Dying storm

7.2 Fronts

• Four Major Frontal Symbols used on weather maps.

7.3 Atmospheric Disturbances

• Middle Latitude Cyclones– Common location for a clash between cold,

polar air and warm, subtropical air– Convergence– Guided by polar jet stream– Variable weather


• Cyclones (L)– Move with jet stream/westerlies– Converging, rising air– Fronts– Precipitation

• Anticyclone (H)– Move with path of westerlies– Divergence, sinking air– Dry weather


• Horizontal and Vertical Structure of pressure systems• Where would the strongest and weakest winds be?


• Common track of storms (mid-latitude cyclones)• What storm track (jet stream) influences your location?


• Stages in the development of a mid-latitude cyclone• Q: Where would you expect precipitation. Why?


• Cyclones and Local Weather

• Q: Describe the front(s), temp, air mass type, wind direction, and precip. at Des Moines, St Louis, and Pittsburg.


• Cyclones and Upper Air Flow

• Polar jet stream• Major impact on

temperature and precip.

• Q: Where would you expect storms to develop?


• Polar Jet Stream Analysis• Anticyclones• Q: Which country does most of this pattern

occupy? Where is an anticyclone?


• Hurricanes– Circular, cyclonic

storm with wind speeds greater than 74 mph (118 km/hr)

– Smaller than mid-latitude cyclone

– Same air mass type– More destructive– Also called tropical

cyclones


• Cross section of a hurricane


• Storm Surge

Q: Why is the timing of landfall so critical to coastal areas?


• Major “Hurricane Alleys”• Which coastlines seem unaffected by these

tracks?


• Hurricane development– Warm water (>25oC or 77oF)– Most air– Coriolis force (do not form or survive near equator)

• Stages of Development– Tropical disturbance– Tropical depression– Tropical Storm (become named)– Hurricane (categorized by Saffir-Simpson

Scale)


• Saffir-Simpson Scale


• Three hurricane tracks

• Subtropical High (Bermuda High)


• Hurricane StrikesQ: What areas of our coasts seem to have escaped

landfall?


• Snow Storms and blizzards– Mid and high latitudes

• Blizzard– Severe weather event– Heavy snow and

strong winds (55 km/hr or 35 mph)

– Visibility reduced


• Thunderstorms– Low and mid latitudes– Lightning: intense

discharge of electricity– Thunder: sonic boom

created by the expansion of air around the lightning bolt

– Usually cover a small area


• Types of Thunderstorms– Convective (thermal)– Orographic– Frontal


• Tornadoes– Occur almost

anywhere but are most common in North America (Tornado Alley)

– Small intense, cyclonic storm of low pressure, violent winds, and converging air


• Destruction caused by an F5 tornado in Greensburg, Kansas on May 16, 2007


• Average annual # of tornadoes

and seasonal variation


• Doppler radar:– improves tornado

detection and forecasting

– Able to determine wind speed and direction

Q: How many squall lines can you see on this image?


• Fujita Scale (F-0 to F-5)

• Enhanced Fujita Scale (EF-0 to EF-5)


• Weak tropical Disturbances– Easterly Wave– Polar Outbreak


• Weather Forecasting– Doppler radar– Weather satellites (e.g. GOES East)– High speed computers– Improving!

Physical Geography

End of Chapter 7: Air Masses and Weather Systems

document7

Education

warm air warm air

polar air

cooler air warm air

expansion of air

air masses q

air dry weather

cold air slope

air mass type