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Cloud types and processes Lifting Expansion Cooling Stability Local winds Speed, force & direction Thermal circulation Land & sea breeze Monsoon Mountain & valley breeze Katabatic wind • Mistral • Santa Anna Rain shadow winds • Chinook • Foehn Desert winds Determining wind speed and direction Wind characterized by direction, speed, and gustiness Wind direction describes the direction from which it is blowing Northerly: from north Seabreeze / onshore: from sea Determining wind speed and direction Force in the wind As wind blows against an object it exerts force (V2 or P x A) Determining wind speed and direction Topic: Wind Power Turbines need moderate, steady winds Only a few percent of nation’s total energy needs generated by wind power in the first half of the century Winds speeds Fig. 4, p. 235 Local Winds Thermal circulation Heating and cooling of the atmosphere above the ground create cold-core, high and warm-core, low pressure cells. Wind travels from high to low and rises until it cools and begins to sink. Fig. 9-23, p. 239 Local winds Land & sea breeze Uneven heating of land and water Day: land hot, water cold = sea breeze Night: water hot, land cold = land breeze Sea breeze convergence Stepped Art Fig. 9-25, p. 241 Sea (lake) breeze front Local winds Local Winds and Water Local winds will change speed and direction as they cross a large body of water due to less friction, greater speed and greater Coriolis effect Divergence draws air down Convergence sends air up Local Winds Mountain and Valley Breeze On mountain slopes, warm air rises during the day creating a valley breeze During night nocturnal drainage of cool air creating a mountain breeze Associated with cumulus clouds in the afternoon Local and regional winds Seasonally Changing Winds: The Monsoon Arabic-derived word for seasonal Winds change direction seasonably causing extreme dry and wet season • Summer ~ day (sea breeze) • Winter ~ night (land breeze) Eastern and southern Asia North America Africa Southeast Asia Monsoon Winter Land mass cools faster than ocean Cool air flows down to sea • Upper level air descends Summer Land mass heats faster than ocean Warm air ascends • Draws in air from ocean Monsoon Summer Convergence (L) Instability • Warm, moist ocean air Rainfall Winter Divergence Dry upper level air Clear North American Monsoon Heating in NW Mexico & SW US replaces that in Himalayas Lower elevation Not as strong Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Total 1.07 0.82 1.69 0.65 0.7 0.65 0.23 0.09 0.24 2.56 3.23 1.68 13.61 Local and large scale winds Katabatic winds Cold wind rushes down elevated slopes, usually 10 kts or less but can reach hurricane strength Katabatic wind High altitude Air cools Density increases Loses buoyancy Sinks Heats adiabatically • May result in hot or still-cold wind Katabatic winds Mistral S France, winter Up to 90 km/hr winds Cool, clear Santa Anna S California, Fall-Winter Hot, dry From east or northeast down canyons Very fast, desiccates vegetation, providing fuel for fires Katabatic winds Cold dry air Pulled down by gravity (negative buoyancy) Compression heating Expansion Pressure driven, fast, hot, dry wind Local Winds Orographic (rain shadow) winds Chinook (East slope of Rockies) Foehn (Europe) Dry warm descending on the leeward side of a orographic barrier Local Winds Desert winds Dust storms, sand storms, haboobs, dust devils Iraq haboob Fig. 9-42, p. 250 Dust devil formation Fig. 9-43, p. 251 Dust devils Fig. 9-44, p. 251 Martian dust storms Readings Questions for review (9): 1-9, 11-15, 18-23, 25-29 (esp. 29) Friday (Winds) Chapter 9 (238-254) Monday (Global wind systems) Chapter 10 (259-273)
