Chapter 8 Global Weather Systems
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Chapter 8 Global Weather Systems Global Weather Systems ` Low-latitudes – Hadley Cell Circulation ` Wet near the equator Dry near 20-30°N and 20-30°S Periods of wet and dry in between Easterly Winds (NE & SE Trades) High-latitudes – Polar Highs Dry conditions – polar deserts Easterly Winds (Polar Easterlies) Mid-Latitude Weather Systems ` Cyclone – Low-pressure system Rising air, precipitation Relatively small, compact weather system ` Anticyclone – High-pressure system Descending air, clear skies Relatively large, expansive weather system ` “weather” caused by the interaction of cyclone and anticyclone ` Usually see interactions of cyclones and anticyclones ` Air Mass – large body of air with fairly uniform temperature and moisture characteristics – 1000s of km across Air masses are labeled by their source region and also based on thermal and moisture characteristics: 1. Thermal • • Tropical (T) Air Mass – warm Polar (P) Air Mass – cold 2. Moisture • • Continental (c) Air Mass – dry Maritime (m) Air Mass - moist Air masses are labeled by their source region: 1. Polar Air Masses (Polar Highs) • • Arctic (A) Air Mass Antarctic (AA) Air Mass 2. Equatorial Air Mass (Equatorial Oceans) • Equatorial (E) Air Mass ` Seldom affect mid-latitude weather Air Mass Name Thermal Moisture. Continental Arctic (cA) Very Cold Very Dry Regions near North and South Poles (cAA as well) Continental Polar (cP) Cold Dry Northern Continental Interiors Maritime Polar (mP) Cool Moist Midlatitude Oceans Continental Tropical (cT) Warm Subtropical Deserts Maritime Tropical (mT) Warm Warm Oceans near 30°N and 30°S Maritime Equatorial (mE) Very Warm Warm Oceans near the Equator Dry Moist Very Moist What happens when air masses collide? When air masses collide….. ` Front – boundary between air masses having different thermal and/or moisture characteristics Cold Front: when a cold air mass collides with a warm air mass a the surface Cold Front ` Cold air is more dense, so the warm air is forced to rise along the front ` Lifting is strong – cumuloform clouds ` Thunderstorms are common along a cold front Cold Front When air masses collide….. ` Front – boundary between air masses having different thermal and/or moisture characteristics Cold Front: when a cold air mass collides with a warm air mass a the surface Warm Front: when a warm air mass collides with a cold air mass at the surface Warm front Warm Front ` Cold air is more dense, so it is difficult to force it to rise ` Lifting is weak – stratiform clouds ` Light, continuous rains are common along a warm front Warm Front ` Stationary Front – boundary between warm air and cold air where there is no movement ` Lifting is extremely weak ` Light, intermittent rains are common along a stationary front ` It will become a warm/cold front if there is movement (or it may dissipate) Stationary Front ` Occluded front – occurs when a cold front overtakes a warm front ` Warm fronts move more slowly than cold fronts ` Lifting is mixed as warm air is pushed aloft ` A combination of rainfall types occur along an occluded front ` Will dissipate rapidly when the warm air is lifted A sharp cold front boundary can be seen on both satellite pictures and radar composites Cyclones Three types of cyclones: 1. Wave cyclones: cyclone/low 2. Tropical cyclone: hurricane/typhoon 3. Tornado 1) Wave cyclones ` Occur in the mid-latitudes and polar regions ` Formed when air masses of different temperature and moisture characteristics meet along the polar front http://www.physicalgeography.net/fundamentals/7s.html Formation of a wave cyclone ` Form between the two high pressure cells in a low pressure trough ` when a cold, dry polar air mass meets a warm, moist, maritime air mass….. Airflow converges setting up an unstable situation Mid-latitude Cyclone • Cold and warm fronts meet at the center of low pressure • Cold air dominates larger area Characteristics of a wave cyclone ` Cyclones normally move eastward, propelled by prevailing westerlies aloft ` Cold front pushes south and east, supported by a flow of cold, dry polar air ` The wind direction and temperature change abruptly as the cold front passes ` Warm front is moving northeast, with warm moist maritime tropical air following ` Precipitation patterns: A broad zone near the warm front A central core of the cyclone, and A thin band extending down the length of the cold front ` Cloudiness generally prevails over much of the cyclone but may be more limited over the cold front Example of an Occlusion Cold Front Approaches a Warm Front Example of an Occlusion • Cold front meets the warm front ahead of it • Warm air is lifted upward, away from the surface • Occluded front becomes longer Æ more of the cold front converges with the warm front Example of an Occlusion • Eventually, the cold front completely overtakes the warm front • Entire system is occluded Cold front Æ cold air moves toward the warm air mass ahead Warm front Æ warm air mass moving toward a cold air mass Stationary front Æ two unlike air masses remain side by side, but neither air mass has recently undergone substantial movement Occluded front Æ cold front meets warm front, warm air gets lifted upward, away from the surface When Air Masses Collide… ` Dry Line – a front between warm, moist air and warm, dry air Lifting is extremely strong Heavy thundershowers, hail, and possible tornadoes are characteristic of a dry line Seen almost exclusively in the southern states west of the Mississippi cT air versus mT air masses Dryline Over Texas Notice the differences in the dew points on both sides of the dryline. Cyclone Tracks Wave cyclones tend to form in certain areas: North Pacific and Alaska Central U.S. east of the Rocky Mountains Gulf Stream Wave Cyclones travel common paths until they dissolve: North Pacific Ocean path Convergence of paths over Northeast U.S. that head toward Europe Cyclone Tracks In the Southern Hemisphere storm tracks are more nearly along a single lane, following the parallels of latitude This track is more uniform because of the uniform pattern of ocean surface circling the globe
