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The Atmosphere, Part 2: Winds & Storms v 3.1 Thomas V. Dagenhart, M.S. Storm over the Atlantic near the Sargasso Sea; Dagenhart, 2003 A Convection-Driven Circulation Cell • Similar to circulation on a non-rotating Earth. Equator Poles Garrison, 2005 Wind Circulation on a Non-Rotating Earth • • • • • Driven by unequal heating. Warm air rises at equator. Cool air falls at poles. Surface air flows directly to equator. Upper air flows directly to poles. Gross & Gross, 1996 The Coriolis Effect • Acts on winds, ocean Gross & Gross, 1996 currents, and anything in motion. • Acts to right in Northern • • Hemisphere and to left in Southern Hemisphere. Zero at equator and strongest at poles. Earth actually rotates out from beneath moving object. North Pole Reason for the Coriolis Effect • • Buffalo & Quito rotate through the same number of degrees each day. But Quito covers many more kilometers, i.e. Quito has a greater rotational speed. Garrison, 2005 Reason for the Coriolis Effect (continued) • Has Garrison, 2005 The Coriolis Effect • Objects veer to the right in the northern hemisphere. Garrison, 2005 Coriolis Effect on Moving Objects • Causes curved paths except at equator. • Effect increases with velocity, mass & • • distance. Ball moves horizontally on slope when Coriolis effect balances gravity (requires high velocities and ignores friction) Leads to geostrophic (“earth turned”) winds and currents Gross & Gross, 1996 Global Wind Patterns Horse Latitudes Horse Latitudes Garrison, 2005 Global Wind Patterns Gross & Gross, 1996 Intertropical Convergence Zone (ITCZ) • • • • • Meteorological equator, ~5o north of geographical equator. Marked by cloud bands. Moves N-S with seasons. Controls location of rain forests. Location of weak intermittent surface winds known as Doldrums. Gross & Gross, 1996 ITCZ Seasonal Variation • Average position deflected northward by: – greater land mass in Northern Hemisphere; land heats • faster than water. – greater reflectance by Antarctic snows than by Arctic Ocean ice cover; limits solar heat gain. Surface salinity diluted by heavy rain bands under ITCZ. Rainfall 1979-2001 Gross & Gross, 1996 Generalized Surface Winds over the Ocean in February • Local winds vary as storms pass through. • Wind bands migrate N-S with seasons. • Shifted S with Southern Hemisphere summer. Gross & Gross, 1996 Generalized Surface Winds Over Ocean in August • Note Northward shift of all belts compared to February. Gross & Gross, 1996 • Lines of equal temperature. • Lines are close together where the temperature Isotherms changes rapidly, a steep temperature gradient. X X X Wikipedia, 2009 Low and High Pressure Systems • Vertical cross section • Map view for N. hemisphere Gross & Gross, 1996 Formation of An Extratropical Cyclone, A & B • Front forms along boundary between two air masses, one • • • • warm & one cold, along Polar Front. Then a wave develops along the front (not well understood). Extratropical = “outside of tropics”, ~ 30 to 60o from Equator. Synonyms = low pressure system, low, mid-latitude storm, nor’easter. May generate huge ocean waves. Gross & Gross, 1996 Stationary Front Formation of An Extratropical Cyclone • • • • • Parts C & D Cyclonic circulation flows counterclockwise in N. hemisphere. Cyclonic circulation flows clockwise in S. hemisphere. Occlusion develops as the cold front catches up to the warm front. Lows usually move from W to E with Prevailing Westerlies. Steered by jet streams, cold core, more common in winter. Cold Front Warm Front Gross & Gross, 1996 Formation of An Extratropical Cyclone • • Parts E & F Note well developed comma shape of clouds in fully occluded cyclone. As cyclone dissipates warm air has moved aloft, cold is found below. Secondary low may develop here. Occluded front Gross & Gross, 1996 Another Look at Extratropical Cyclone Formation, A & B • Form along Polar Front especially during winter. Garrison, 2005 Another Look at Extratropical Cyclone Formation, C • Note the difference in width of cloud bands for cold vs. warm fronts. Vilhelm Bjerknes, a Norwegian, first theorized about air masses, fronts & cyclones. Garrison, 2005 Warm Fronts • Broad cloud & precipitation band, 100’s km wide. • Classic progression of clouds from high altitude cirrus along leading edge to low altitude nimbostratus as front passes. Gross & Gross, 1996 Cold Fronts • Narrow cloud & precipitation band, 10’s km wide. • Mostly cumulonimbus clouds. • Cloud band may be well ahead of actual front. • Sudden wind shift as front passes. Gross & Gross, 1996 Nor’easter Damage • A fierce extratropical • • cyclone. Ash Wednesday storm, Mar. 7, 1962. Wreaked havoc from NC to New England. Nor’easter Damage Outer Banks, NC Nov. 2009 Fire Island, NY Garrison, 2005 Fully-Developed Extratropical Cyclone • Following Prevailing Westerlies and approaching western N. • America on October 27, 2000. Visible-light photo by GOES-10 satellite, note commas shape. Cold Front Garrison, 2005 Warm Front Satellite Photo of Extratropical Cyclone • Which direction does the storm circulate? • In which • hemisphere is this storm located? Appears hurricanelike, but no eye & larger diameter. Gross & Gross, 1996 Hurricanes • AKA typhoons (W. • • • • • • Pacific, tropical cyclones (Indian O.), baguios (Philippines) CCW circulation in N. Hemisphere CW circulation in S. hemisphere Well developed eye. Formed in tropics inside warm mass, warm core. Derive energy from warm ocean waters. Rapidly dissipate over land. Hurricane Andrew approaches Gulf Coast of U.S., Aug., 1992 N.O. Worst Path! Storm Surge! Gross & Gross, 1996 Hurricane Formation Areas • Mostly in tropics 23.5o S to 23.5o N. • Formation zones (in orange) never within 5o of equator since • • Coriolis effect equals zero there. Note typical paths indicated by red arrows. Almost never in S. Atlantic or SE Pacific; water too cold. Garrison, 2005 Ocean Temperatures Limit Areas of Hurricane Formation Sea Temp. 1987-1999 • Reds & oranges indicate highest humidity & air temperatures, places favorable for hurricane formation. • This moist air lies above ocean waters warmer than 26oC. October 1992 Garrison, 2005 Track of Hurricane Georges, Sept. 1998 • Hurricane’s eye varies in definition. Garrison, 2005 Hurricane Alberto Viewed from Space • In this oblique view, note how thin the atmosphere appears relative to Earth’s size. Garrison, 2005 • • Hurricane Damage in Galveston, TX Sept. 8, 1900, little warning. Deadliest U.S. natural disaster (for now), ~8,000 dead. Garrison, 2005 Anatomy of a Hurricane • High level clouds exhausted from hurricane’s top & rotate CW. Garrison, 2005 • Most hurricane winds & clouds rotate CCW. Hurricane Near Hawaii • Directions of arrows • indicate directions of winds. Lengths of arrows indicate speeds of winds. Gross & Gross, 1996 Seasonal Wind Patterns • Caused by tilt of Earth’s rotational axis. • Zones of most intense heating & cooling shift N-S with seasons. Gross & Gross, 1996 The Summer • Warm air rises over Asia to be replaced by moist Indian Ocean air. Monsoon • Causes intense rains over India & SE Asia. Note shift in wind circulation as picture changes. Ocean currents change as a result. Gross & Gross, 1996 dry, dense air flows Southward off Asia The Winter • Cold, onto the Indian Ocean, dry season on land. Monsoon • Monsoons unique to Indian Ocean, no N half Note shift in wind circulation as picture changes. Ocean currents change as a result. Gross & Gross, 1996 The Sea Breeze • Forms in daytime as land heats faster than water. • Cloud bands form over land & parallel to shore. • In afternoon, helps fishermen sail home after a morning’s work. Gross & Gross, 1996 The Land Breeze • Forms at night as sea cools more slowly than land. • Cloud bands form over sea & parallel to shore. • In early morning, helps fishermen sail out to sea for another day of work. Gross & Gross, 1996 Cumulus Clouds Building over Land as Sea Breeze Reaches Maximum Development • Late afternoon facing the mainland. • Sea breeze hits photographers back. Emerald Isle, NC; Dagenhart, 2003 Island Effect • Heavy precipitation occurs on island’s windward (upwind) side. • A rain shadow forms on leeward (downwind) side of island. • Mainly on islands with significant elevation, not atolls. Gross & Gross, 1996 Island Effect: Dry Side of Island • Semi-arid conditions. • Intermittent streams. • Like the more extensive rain shadows found on leeward side of mountain ranges. Dry side of Kauai, Hawaii Gross & Gross, 1996 Island Effect: Wet Side of Island • Lush rain forest vegetation, perennial streams. Gross & Gross, 1996 wet side of Kauai, Hawaii References • Environment Canada • • (2002) Hurricane Terms. http://www.ns.ec.gc.ca/weather/hurricane/hurricanes9.html Garrison, T. (2005) Oceanography: An Invitation to Marine Science, 5th ed. Brooks/Cole Thomson Learning, Stamford, CT, 522 p. Gross, M.G. and E. Gross (1996) Oceanography: A View of the Earth, 7th ed. Prentice Hall, Upper Saddle River, NJ, 472 pp.