Announcements First assignment (deep-sea sediments) I expect to be finished grading exams in two weeks. Second problem set due November 7th New topic: Physics of atmospheric and oceanic circulation Atmospheric circulation (Ch. 7) Learning goals: Understand the roles of pressure gradients and Coriolis in atmospheric circulation. Learn causes of large-scale patterns of atmospheric flow. Why does the wind blow? What makes the ocean flow? Pressure gradients and Coriolis 1: Forces Pressure gradients, Coriolis, Solar heating and latitude 2: Large-scale circulation patterns Hadley cells, etc., Surface winds 3: Meso-scale circulation patterns Monsoons, Effects of continents on atmospheric pressure 4: Small-scale patterns Storms and Fronts in the U.S. Mountains and weather Onshore Breezes Hurricanes 1
Pressure gradients Formation: hot air rises (lower density) low pressure area cold air sinks (higher density) high pressure area Pressure gradient: Area of changing atmospheric pressure High Low: Down the pressure gradient Resulting air motion (no coriolis): Air moves down the pressure gradient (from high pressure to low pressure) Pressure Force Distance Coriolis effect (acceleration) 90 to right of direction of motion in N.H. earth s rotation velocity latitude 90 to left of direction of motion in S.H. C F = 2 w v sin f Coriolis effect 1 0.8 0.6 0.4 Equator 0.2 Coriolis effect as a function of latitude Pole 0 0 15 30 45 60 75 90 Latitude (f) Large-scale patterns of atmospheric circulation Atmospheric pressure distribution Polar Cell Ferrel Cell Hadley Cell 2
Mesoscale patterns of atmospheric circulation: Changes in location of the intertropical convergence zone due to differential seasonal heating and cooling of continents and ocean surface Winter: High pressure over Asia, forces ITCZ south Summer: Low pressure over Asia, forces ITCZ north Monsoon season in India and Africa 2010 Pakistan floods Seasonal variation in pressure across the globe. Differences in heat capacity of land and sea create the complex patterns. http://marymackillop.org 21 million injured or homeless 1/5 th of Pakistan under water Associated press 3
Corresponding winds in winter Isobar: Equal P Winds blow along isobars (mostly geostrophic) Corresponding winds in summer L Weather.com maps classic weather maps 4
L Weather map symbols 1 -- Cold Front 2 -- Warm Front 3 -- Stationary Front 4 -- Occluded Front (Cold front overtakes warm front) 5 -- Trough (Elongated region with low pressure) 6 -- Squall Line (Narrow thunderstorm band) 7 -- Dry Line (Separates moist air from dry air) 8 -- Tropical Wave (Low pressure zone in trade wind region moving west) Causes of precipitation Source of moisture (humidity) Drop in temperature (raises relative humidity) Airborne condensation nuclei (allows droplets to form) At fronts: Steeply sloping front: Vigorous precipitation Gradual slope: Sustained precipitation 5
Mountains and weather: 1: Rising air cools, water vapor condenses to form rain 2: Windward sides of mountain ranges receive more rainfall, and deserts are typical of the leeward sides Effects of Washington State Mtn. ranges on precipitation levels Prevailing wind direction Olympic Mts. Puget Lowland Cascade Mtns. Eastern, WA Annual Rainfall Totals Sequim: 16.5 Wenatchee: 8 Forks: 10 feet Bellingham: 36 Spokane: 18 Rain forest Rain shadow Windward Leeward 3: Washington State Convergence Zone: Air masses flow around Olympics converge near Seattle and Everett. Convergence forces air masses to rise, resulting in a little more rain 6
Small-scale patterns of atmospheric circulation: Coastal onshore breeze Current hurricanes: Typhoon Lan (85 mph), tropical depression 26 10 Atlantic hurricanes this year. 6 greater than category 3 (>111 mph) Western Hemisphere s Deadliest Hurricanes Hurricane Date Areas Struck Deaths "The Great Hurricane" October 10-16, 1780 Martinique St. Eustatius 22,000 Barbados Ships Hurricane Mitch Oct 26 - Nov 4, 1998 Central America: Honduras Nicaragua 11,000+ Galveston, Texas September 8, 1900 Galveston Island 8,000 Hurricane Fifi September 14-19, 1974 Honduras 8,000 Dominican Republic September 1-6, 1930 Dominican Republic 8,000 Hurricane Flora Sep 30, 1963 - Oct 8, 1963 Haiti Cuba 7,200 Martinique September 6, 1776 Point Petre Bay 6,000 Hurricane Katrina August 29, 2005 New Orleans 1,800 $80 billion Hurricane Katrina 7
Hurricanes: low pressure centers energized by latent heat Hurricane Categories Cat. Winds Storm surge 1 74-95 mph 4-5 feet 2 96-110 mph 6-8 ft 3 111-130 mph 9-12 ft 4 131-155 mph 13-18 ft 5 155 mph > 18 ft Current weather Image from http://cliffmass.blogspot.com/ 8