Chapter 6 Atmospheric Forces and Wind Key Terms Acceleration Anticyclones Buys Ballot s law Centrifugal force Centripetal acceleration Coriolis force Cyclones Force Frictional force Geostrophic balance Geostrophic wind Gradient balance Gradient wind Summary Gravitational force Guldberg Mohn balance Hydrostatic balance Isobaric chart Knots Land breeze Law of momentum Mesoscale Microscale Momentum Newton s second law of motion Planetary scale systems Pressure gradient Pressure gradient force PGF Sea breeze Speed Subgeotrophic flow Subsynoptic scale Supergeostrophic flow Synoptic scale systems Thermal wind Velocity Wind gust The wind is air in motion Newton s Second Law relates the acceleration of air to forces The important forces at work besides gravity in the atmosphere are as follows PGF directed from higher pressure toward lower pressure at right angles to lines of constant pressure or constant height on an isobaric chart The greater the change in pressure over a given distance the stronger the PGF Coriolis force an apparent force cause by the rotation of the Earth The Coriolis force acts to the right of the motion in the Northern Hemisphere and to the left of motion in the Southern Hemisphere The Coriolis force is zero at the equator and increases in magnitude toward the poles This force is zero if the velocity of the air parcel is zero and increases as the air parcel s velocity increases Centrifugal force directed outward from the center of curving motion It increases as the speed of the object increases the radius of curvature decreases or both Frictional force acts in the direction opposite to movement The magnitude of this force depends on the type of surface and the wind speed Friction is weaker over a lake and stronger over a forest or mountain Winds arise as a result of different balanced combinations of these forces These balances are as follows Hydrostatic Balance a balance of gravitational force and the vertical PGF This balance explains the lack of strong vertical winds in most of the atmosphere Geostrophic balance a balance between the horizontal pressure gradient and Coriolis forces Geostrophic winds blow parallel to isoheights Buys Ballot s Law states that in the Northern Hemisphere lower pressure is to the left when your back is to the wind In addition the wind in geostrophic balance is stronger when the isobars or isoheights are slow together and weaker when they are far apart Because of this balance winds blow counterclockwise around lows and clockwise around highs in the Northern Hemisphere The reverse is true in the Southern Hemisphere Gradient balance a three way balance of horizontal pressure gradient Coriolis and frictional forces This balance explains why winds blow at angle toward a lower pressure rise upward at the centers and cause cloud wet weather in lows It also explains why highs are sunny regions of sinking dry air The thermal wind a combination of hydrostatic balance and geostrophic balance explains why winds in the middle latitudes generally become more westerly with increasing altitude This is because the surface temperature usually decreases toward the poles The sea breeze is a similar although much smaller scale circulation that is driven initially by pressure gradients created by unequal heating of land and ocean Winds and wind patterns occur at all scales in the atmosphere
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