1	Chapter 7: Forces and Force Balances
2	Forces that Affect Atmospheric Motion Newton’s second law of motion states that the rate of change of momentum (i.e., the acceleration) of an object, as measured relative to coordinates fixed in space, equals the sum of all the forces acting. For atmospheric motions of meteorological interest, the forces that are of primary concern are the pressure gradient force, the gravitational force, and friction. These are the fundamental forces. For a coordinate system rotating with the earth, Newton’s second law may still be applied provided that certain apparent forces, the centrifugal force and the Coriolis force, are included among the forces acting.
3	Pressure Gradient Force PG = (pressure difference) / distance Pressure gradient force goes from high pressure to low pressure. Closely spaced isobars on a weather map indicate steep pressure gradient.
4	Examples of Pressure Gradient
5	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)
6	Gravitational Force
7	Frictional Force Frictional force (drag) is strongest near the Earth’s surface and decreases rapidly with height. The atmospheric layer in which frictional force is important is call thed boundary layer, whose depth can vary from a few hundred meters to a few thousand meters. There are three sources to generate turbulence eddies to give rise to the frictional force: (1) mechanical turbulence (airs encounter surface roughness), (2) thermal turbulence (air near Earth’s surface get heated, and (3) wind-shear induced turbulence.
8	Example on a Merry-Go-Around
9	Coriolis Force
10	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.
11	Coriolis Force Change with latitudes
12	How Does Coriolis Force Affect Wind Motion?
13	Geostrophic Balance
14	Scales of Motions in the Atmosphere
15	Example: Winds and Height on 500mb
16	Frictional Effect on Surface Flow
17	Surface Friction Friction Force = c * V c = friction coefficient V = wind speed
18	Surface Geostrophic Flow
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21	Hydrostatic Balance in the Vertical
22	Thermal Wind Relation
23	Jetstream and Front
24	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).
25	Jetstream
26	Three Different Jetstreams
27	Jet Streams Near the Western US