AIR MASSES A large body of air with uniform temperature and moisture characteristics. They acquire their characteristics in source regions, because they move slowly when passing these regions Ex: A very cold air mass with low water vapor is generated over cold , snow-covered surface in artic region

AIR MASSES Air masses move from one region to another (pressure gradient, upper level winds, jet stream) When moving, the properties are influenced by the new environment

CLASSIFICATION OF AIR MASSES 1. Latitudinal position Air Mass Symbol Source region Artic A Artic region Antartic AA Antartica Polar P Continents and oceans (50-60°) Tropical T Continents and oceans (20-35°) Equatorial E Oceans close to equator 2. Type of surface Maritime m oceans Continental c continents

TYPICAL AIR MASSES Air mass Symbol Source region Properties Maritime equatorial mE warm oceans in equatorial region warm, very moist Maritime Tropical mT warm oceans in tropical region Warm, moist Continental tropical cT Subtropical deserts Warm, dry Maritime polar mP Midlatitude oceans Cool, moist(winter) Continental polar cP Northern continental interiors Cold, dry (winter) Continental artic cA Regions near north pole Very cold and dry Continental antartic cAA Regions near south pole

TYPICAL AIR MASSES See Figure 6.2 Page 155

NORTH AMERICAN AIR MASSES Strong influence on North American weather Continental Polar (cP) Tongues of cold dry air; periodically extend S,E producing cold temperatures and clear skies 2. Continental Artic (cA) When moving southward: severe cold wave 3. Maritime Polar (mP) Unstable in winter: heavy precipitation over coastal ranges 4. Maritime Tropical (mT) Moves northward bringing moist unstable air: thunderstorms 5. Continental Tropical (cT) Does not move widely, influence weather conditions over source region See Fig 6.3, page 156

FRONTS Transition zone between two air masses of different characteristics COLD FRONT Transition zone where a cold air mass invades a warmer air mass Colder air mass remains in contact with ground (because is denser) It forces warm air mass to rise If warm air is unstable: thunderstorms

PRECIPITATION IN A COLD FRONT (ANIMATION)

FRONTS WARM FRONT Transition zone where a warm air moves into a region of colder air Cold air remains in contact with ground (denser) It forces warm air mass to rise If warm air is stable: Steady precipitation If warm air is unstable: thunderstorms

PRECIPITATION IN A WARM FRONT (ANIMATION)

OCCLUDED FRONT When a cold front overtakes a warm front (cold fronts move at a faster rate than warm fronts)

STATIONARY FRONT A front that is not moving. Masses are not strong enough to replace each other There is a noticeable temperature change and shift in wind direction between two sides of front

THE PRESIDENT SNOW STORM (Feb 16-17 2003) Stationary front

WEATHER The condition of atmosphere at any particular time and place. It’s always changing Weather surface map, July 02 2008 CLIMATE The synthesis of weather, the average weather of a region over a period of time Annual average precipitation

WEATHER SYSTEMS Some patterns of wind circulation present recurring patterns of weather Weather systems: Few kilometers (tornado) Thousands kilometers (a large traveling cyclone)

CYCLONES ANTICYCLONES Air spirals inward and upward Air spirals outward and downward condensation, precipitation condensation cannot occur

TRAVELING CYCLONES AND ANTICYCLONES CYCLONES AND ANTICYCLONES: masses of air moving in a spiraling motion Most of them are large features moving slowly across Earth’s surface and bringing changes to weather. TRAVELLING CYCLONES AND ANTICYCLONES TYPES OF CYCLONES: Wave cyclones (middle and high latitudes, ~1000km or 600miles) Tropical cyclones (hurricanes, typhoons) Tornado

WAVE CYCLONES The wave cyclone forms, intensify and dissolves along the polar front Conditions to form: two anticyclones, one with warm air, and the other with cold polar air are in contact in the polar front.

WAVE CYCLONE FORMATION See Fig 6.8 page 161, 162 Early stage: There is a wave formation in the polar front Cold air is going southward and warm air is moving northward B. Open stage: Wave is deepened and intensified C. Occluded stage: Cold front overtakes warm front (occluded front) Precipitation is intensified D. Dissolving stage: The cold front is reestablished

HOW DOES A WAVE CYCLONE AFFECT WEATHER? See page 162

MARCH 21, 22 2005 http://www.hpc.ncep.noaa.gov/dailywxmap/index.html

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