Transcript ch_5

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
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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
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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