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Chapter 5 Atmospheric Pressure and Wind Atmospheric Pressure Definition: The pressure exerted by the weight of all the air(atmosphere) above Approximately 14.7 lbs/sq in Air pressure is exerted on us and on everything in all directions at once Gas molecules freely move about – bounce around They are contained in our atmosphere by the earth’s surface and by gravity Atmospheric Processes Pressure, Density, and Temperature – they are all related – dependent on one another Pressure and Density Mapping Pressure Inches: Barometers express atm pressure in inches (or millimeters) – an expression of height of mercury in a barometer – Example: 29.06 in Millibars: express atm pressure in terms of force per surface area – Example: 1013.25 millibars average sea level pressure = 1013.25 millibars Terms Mapping Pressure Highs: areas of higher pressure relative to surrounding air masses Lows: areas of lower pressure relative to surrounding air masses Ridges: elongated areas of relatively high pressure Troughs: elongated areas of relatively lower pressure Mapping with Isobars Rotation High/Low Pressure Systems High = Clockwise Low = Counterclockwise Pressure Gradient Drawing isolines demonstrates or shows how quickly the pressure is rising or falling This is called the pressure gradient The closer the isolines the steeper the slope and the faster the rate of pressure change Movement along the pressure gradient generates movement of air masses i.e., wind Winds Remember: Insolation is the ultimate cause of wind – Unequal heating of earth’s surface – Temp gradients generate pressure gradients – Pressure gradient sets winds in motion The Nature of Wind Direction of Movement – Pressure Gradient – Coriolis Effect – Friction Cyclones and Anticyclones Wind Speed Pressure Gradient Friction Cyclones and Anticyclones Cyclones and Anticyclones Wind Speed Determined by the Pressure Gradient Farther apart = slower General Circulation of the Atmosphere Hadley Cells Components of General Circulation Jet Streams Hadley Cells occur only in tropical latitudes Cyclonic System Equator General Circulation of Atmosphere 0°N Easterlies Westerlies Easterlies 0° Easterlies Westerlies Easterlies 0°S ITCZ = Inter-tropical convergence zone Components of General Circulation Subtropical Highs Trade Winds Intertropical Convergence Zone The Westerlies Polar Highs Polar Easterlies Subpolar Lows Trade Winds 25°N to 25°S Easterlies – named for the direction they come from Most dominant winds Westerlies 30° to 60° N & S Can be influenced by surface Jet Stream Polar to temperate latitudes Rossby wave influence Global Atmospheric Circulation Jet Streams The term "jet stream" is often used to refer to the rivers of wind high in the atmosphere - above about 20,000 feet - that steer storms. They also help determine locations of areas of high and low air pressure at the Earth's surface. The Jet Stream and Rossby Waves Modifications of General Circulation Seasonal Variations in Latitude Monsoons Seasonal Variations in Latitude - Monsoons Seasonal variations are less likely to affect the equatorial region and the poles South Asian Monsoon Seasonal Pressure and Precipitation Patterns Localized Wind Systems Sea and Land Breezes Valley and Mountain Breezes Katabatic Winds Foehn/Chinook Winds Sea and Land Breezes Valley Breezes Cold air sinks Hot air rises Mountain Breezes Katabatic - Chinook – Foehn – Santa Ana Downslope Winds Katabatic: powerful, simple spilling of air downslope – air drainage, most common in Greenland and Antarctica, most famous is the “mistral” in southern France Chinook: Rocky Mountains Foehn: Alps Santa Anas: California, high speed, high temp, and very dry