1	Lecture 4: Pressure and Wind Pressure, Measurement, Distribution Forces Affect Wind Geostrophic Balance Winds in Upper Atmosphere Near-Surface Winds Hydrostatic Balance (why the sky isn’t falling!) Thermal Wind Balance
2	"Wind is moving air" Wind is moving air.
3	Force that Determines Wind Pressure gradient force Coriolis force Friction Centrifugal force
4	Thermal Energy to Kinetic Energy
5	Global Atmospheric Circulation Model
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9	Pressure Gradient Force PG = (pressure difference) / distance Pressure gradient force force goes from high pressure to low pressure. Closely spaced isobars on a weather map indicate steep pressure gradient.
10	Thermal Energy to Kinetic Energy
11	Balance of Force in the Horizontal
12	Coriolis Force
13	Coriolis Force Coriolis force causes the wind to deflect to the right of its intent path in the Northern Hemisphere and to the left in the Southern Hemisphere. The magnitude of Coriolis force depends on (1) the rotation of the Earth, (2) the speed of the moving object, and (3) its latitudinal location. The stronger the speed (such as wind speed), the stronger the Coriolis force. The higher the latitude, the stronger the Coriolis force. The Corioils force is zero at the equator. Coriolis force is one major factor that determine weather pattern.
14	Another Kind of Coriolis Force The Coriolis force also causes the east-west wind to deflect to the right of its intent path in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflections are caused by the centrifugal force associated with the east-west motion, and , therefore, related to rotation of the Earth, and are also considered as a kind of Coriolis force. Although the description of the deflection effect for north-south and east-west motions are very different, their mathematical expressions are the same.
15	Coriolis Force Change with latitudes
16	How Does Coriolis Force Affect Wind Motion?
17	Geostrophic Balance
18	Scales of Motions in the Atmosphere
19	Frictional Effect on Surface Flow
20	Surface Friction Friction Force = c * V c = friction coefficient V = wind speed
21	Surface Geostrophic Flow
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24	Centrifugal Force
25	Gradient Wind Balance The three-way balance of horizontal pressure gradient, Coriolis force, and the centrifugal force is call the gradient wind balance. The gradient wind is an excellent approximation to the actual wind observed above the Earth’s surface, especially at the middle latitudes.
26	Super- and Sub-Geostrophic Wind
27	Pressure Gradients Pressure Gradients The pressure gradient force initiates movement of atmospheric mass, wind, from areas of higher to areas of lower pressure Horizontal Pressure Gradients Typically only small gradients exist across large spatial scales (1mb/100km) Smaller scale weather features, such as hurricanes and tornadoes, display larger pressure gradients across small areas (1mb/6km) Vertical Pressure Gradients Average vertical pressure gradients are usually greater than extreme examples of horizontal pressure gradients as pressure always decreases with altitude (1mb/10m)
28	"Why didn’t the strong vertical..." Why didn’t the strong vertical pressure gradient push the air rise?
29	Hydrostatic Balance in the Vertical
30	What Does Hydrostatic Balance Tell Us? The hydrostatic equation tells us how quickly air pressure drops wit height. èThe rate at which air pressure decreases with height (DP/ Dz) is equal to the air density (r) times the acceleration of gravity (g)
31	The Ideal Gas Law An equation of state describes the relationship among pressure, temperature, and density of any material. All gases are found to follow approximately the same equation of state, which is referred to as the “ideal gas law (equation)”. Atmospheric gases, whether considered individually or as a mixture, obey the following ideal gas equation:
32	Hydrostatic Balance and Atmospheric Vertical Structure Since P= rRT (the ideal gas law), the hydrostatic equation becomes: dP = -P/RT x gdz è dP/P = -g/RT x dz P = P_s exp(-gz/RT) P = P_sexp(-z/H) The atmospheric pressure decreases exponentially with height
33	Temperature and Pressure
34	Thermal Wind Relation
35	Thermal Wind Equation ¶U/¶z µ ¶T/¶y The vertical shear of zonal wind is related to the latitudinal gradient of temperature. Jet streams usually are formed above baroclinic zone (such as the polar front).
36	Subtropical and Polar Jet Streams
37	Jet Streams Near the Western US
38	Scales of Motions in the Atmosphere
39	Cold and Warm Fronts
40	Tropical Hurricane The hurricane is characterized by a strong thermally direct circulation with the rising of warm air near the center of the storm and the sinking of cooler air outside.
41	Naming Convention
42	Ocean Temperature And Hurricane
43	Monsoon: Sea/Land-Related Circulation
44	How Many Monsoons Worldwide?
45	Sea/Land Breeze
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