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Unformatted text preview: Exam 3 Dec 10 th Air Masses and Fronts o Air masses Contain uniform humidity characteristics Affect vast areas o Fronts Boundaries between air masses Spatially limited Linked to mid-latitude cyclones o Source regions Topographically-uniform sites of air mass formation Typical formation occurs when air stagnates over large surface regions Overlying air gains temperature and moisture characteristics of surface o Air mass moisture classification, from wettest to driest: Maritime ( m )-wet Continental ( c )-dry o Air mass temperature classification, from warmest to coolest: Tropical ( T )-warm Polar ( P )-cold Arctic ( A )-very cold • Movement of Air Masses o Move with general atmospheric circulation o Displace existing air over locations and change surface weather characteristics (temp, humidity, etc) o Continually affected by surface influences • Continental Polar ( c P ) and Continental Arctic ( c A ) o Continental Polar ( c P ) Source region • Northern Canada Characteristics • Cold, dry, stable; slightly warmer and more humid during summer o Continental Arctic ( c A ) Source region • Above Arctic Circle Characteristics • Extremely cold and very dry o The boundary between c P and c A is the Arctic Circle • Maritime Polar ( m P ) Air Masses Source regions • Northern Atlantic and Pacific Basins Characteristics • Cool and moist o m P air masses are often found along the West Coast ahead of mid-latitude cyclones and along the East Coast behind cyclones Nor’easters • Continental Tropical ( c T ) Air Masses Source regions • Southwest U.S. and northern Mexico Characteristics • Hot and very dry o Very unstable but produce little precipitation due to lack of water vapor o Thunderstorms may occur with moisture advection or orographic lifting • Maritime Tropical ( m T ) Air Masses Source regions • Warm ocean basins (e.g., Gulf of Mexico) Characteristics • Very warm, very humid, very unstable o Advection of m T across continent during the summer Can lead to high heat indices Causes NAMS • North American Monsoon System • Fronts Separate air masses Bring about changes in temperature and humidity Associated with mid-latitude cyclones Naming reflects advancing air mass type o Cold Fronts Form when cold air displaces warm air steeply upward Associated with heavy, brief precipitation and rapid temperature drops • Cumulonimbus clouds Sharp cold front boundaries are evident on satellite and radar images o Warm Fronts Warm (usually moist) air moves aloft over cool (usually drier) air • Overrunning Light, steady precipitation from stratus-type clouds • Also possible fog, sleet, freezing rain o Stationary Fronts Two unlike air masses remaining side by side, with neither encroaching Identified by temperature differences on either side of the front May slowly migrate; warmer air may be displaced above cooler air...
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- Fall '07
- Tropical cyclone, Thunderstorm