Atmospheric Circulation. Density of Air. Density of Air: H 2 O and Pressure effects

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1 Today s topics: Atmospheric circulation: generation of wind patterns on a rotating Earth Seasonal patterns of climate: Monsoons and Sea Breezes Tropical Cyclones: Hurricanes and typhoons Atmospheric Circulation Density of Air Varies with T, P & H 2 O T effect: hot air expands due to collisions between molecules Thus, as air warms it becomes less dense and rises Cold air has lower volume due to fewer collisions. Thus, denser and sinks Density of Air: H 2 O and Pressure effects H 2 O effect: Moist Air is less dense than dry air! Density of H 2 O molecule is less than the density of the air molecules (N 2 + O 2 ) it displaces. Moist air rises P effect: Atmospheric Pressure ~ weight of atmosphere (~14.7 lb/in 2 at sea level) P is greatest at the surface (compresses air) and decreases with elevation (air expands). Compressed air warms up (due to colliding molecules) Expansion of air causes cooling (reduced number of collisions between molecules) 1

2 Near Surface Atmosphere Heating from surface causes expansion Moisture from surface absorbed by air Combined effect: decrease density, and Thus, warm, moist air rises Rising air experiences lower pressure Thus, air expands and cools Cool air cannot hold moisture Thus, condensation of droplets, releases heat of vaporization to the air Causes warming, resulting in Continued rising and cooling and condensation If rapid enough = storm Eventually, at high altitude vapor water is lost and air mass is cooled Causes air mass to sink Compresses under pressure and Warms Vertical Circulation Draw vertical circulation of the atmosphere due to surface heating Which way does the wind usually blow in Illinois? A. North to South B. East to West C. West to East D. South to North 2

3 True or False: The direction of rotation (clockwise vs. counterclockwise) of tropical storm depends on the hemisphere. A. True B. False Take a minute to consider this. You may talk with your neighbor I will call on you Why? True or False: The direction of rotation (clockwise vs. counterclockwise) of water down a draining sink depends on the hemisphere. A. True B. False 3

4 Remember, Earth Rotates! Rotation toward East Angular velocity of Earths rotation = 15º/hr But arc-length is greatest at low latitude, thus faster rotational velocity at low latitude This results in Coriolis effect Which in combination with solar heating of surface results in atm. circulation pattern 15º/hr Plane leaves equator with a northward velocity But also, initial eastward velocity due to rotation of the Earth (1674 km/h at equator) This initial eastward velocity is greater than the rotation speed of Earth s surface at higher latitudes. So, relative to Earth s surface the plane veers eastward as it travels northward Coriolis Effect deflects flow paths to the right in N. hemisphere and to the left in S. hemisphere Note: this deflection is relative to Earth s surface. An observer from space would see the plane flow due north without deflection Show animation Coriolis 1 Coriolis Effect Draw atm circulation with rotation 4

5 Atm. Circulation & the Coriolis effect Vertical circulation model combined with Coriolis effect produces the pattern of observed circulation cells. Equatorial heating Near surface air masses heat, rise, expand, cool, and precipitate over tropics Air flows over surface to replace rising air mass Easterlies = surface flow deflected by Coriolis Circulation at Polar cells Polar cooling Air sinks Flow over surface toward lower latitude, deflected westward by Coriolis Polar easterlies = deflection Heats as it flow over surface and rises at border with temperate cell Temperate or Mid-latitude (30-60º ) cells Flow deflected by Coriolis = westerlies Result from neighboring cells: sinks at subtropic high pressure belt due to sinking of subtropical cell Flows over surface, heating and Rises at 60º due to meeting air mass from polar cell. Circulation Animation 5

6 Can you predict Wet vs Arid regions based on the pattern of circulation cells? Weather animation Effect of land on Circulation: Sea Breezes During day - on-shore breeze Heat land (low heat capacity) Rising air mass draws in cooler air from over the ocean During night = off-shore breeze Warm ocean relative to land (heat capacity again) Warm air over ocean rises as cool air over land sinks and flows out to ocean Monsoon ~ season long onshore breeze Heat continent over summer Continuously rising air mass Replaced by moist ocean air Continuous rain 6

7 Tropical Cyclones (wind speeds > 120 km/hr) Hurricane (Atlantic) = Typhoon (Pacific) = Cyclone (Indian) Wind Speeds Tropical Depression < 61 km/hr winds Tropical Storm between 61 & 120- km/h winds Saffir-Simpson Intensity Scale 1: km/hr winds (74-95 mph), minor bldg. damage km/hr winds ( mph), damage to roofs, windows, trees downed km/hr winds ( mph), some structural damage, wall failures, large trees downed km/hr winds ( mph), extensive structural damage, most signs and trees downed 5. > 250 km/hr (>150 mph), catastrophic damage to entire buildings common Tropical Cyclones Atmospheric disturbance (in Atlantic, usually a wave from the Sahara) Low pressure cell along tropical convergence zone breaks away and flows into N or S hemisphere Warm surface water (>27ºC, ~81ºF) evaporates readily and in great quantities. Warm, moist air rises, expands, cools & Precipitates rain releasing huge amounts of E to the atm (latent heat of vaporization) and supplies additional energy for WIND Near surface flow rushes in to replace rising air - deflected by Coriolis resulting in spinning storm Wind speed > 119 km/h (74 m/h) Tropical Cyclone Structure In one day, a large cyclone releases 2.4 trillion kilowatthours of power Equal to the annual electrical power needs of the US Why do they spin? We need to draw this. 7

8 Draw N hemisphere cyclonic spin Hurricanes + land = devastation Damage from Wind (exceeding 120 mph) Heavy rain (flooding) Storm surge along the coast (up to 40 feet high) Low pressure of the storm causes a bulge or hill to form on the surface of the ocean. When the storm comes ashore, this hill does too (storm surge) 8

9 Hurricanes produce huge waves: Hurricane Ivan (09/04) Track (green path) of storm directly over wave buoy (red dot) Measured increasing average storm wave height, H (red circles) and maximum height, H max (blue x s) as eye approaches (green square). H max = 27.7 m 24 waves with H >15 m The measured H max is not true H max of storm. NE quad of the storm passed E of buoy, thus the wave with the true H max past NE of buoy. Rita Hurricanes = Major source of sediment to the coast [sufficient to account for all (in-org) sediment in healthy saltmarsh wetlands] Katrina Damage From Katrina: Much of it avoidable Nov. 2009, Federal Judge Ruled Army Corps of Engineers culpable in flooding of Lower 9th Ward and neighboring St. Bernard Parish Corps failed to maintain the Mississippi River-Gulf Outlet navigation channel linking river to the gulf Corp failed to maintain banks of channel, allowing widening of channel and a wave of flood water to impact the levee causing it to fail and flood. Corps inaction cited as "monumental negligence by the judge and a clear case of talk is cheep. First time Corps ever found liable for disaster. Government now open to 100,000 s lawsuits 9

10 Review Questions What causes the circulation patterns in the atmosphere? How are these patterns affected by the Coriolis effect? Describe the Coriolis effect and how it works. What causes sea breezes and monsoons? Tropical Cyclones What conditions are necessary for their formation? Why do cyclones in the northern hemisphere spin counter clockwise? How do cyclones do damage and why are they beneficial? 10