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Newton’s Laws of Motion An object at rest will remain at rest and an object in motion will remain in motion as long as no force is applied to the object. The force exerted on an object equals its mass times the acceleration produced. Acceleration: speeding up, slowing down, change of direction of an object. Newton’s Laws of Motion Newton’s Law of Motion Forces affecting speed and direction of wind Pressure gradient force Coriolis effect Friction Forces that Influence Winds Pressure Gradient Force Difference in pressure over distance • Horizontal gradient is smaller than vertical one Directed perpendicular to isobars from high to low • Close isobars = strong gradient Large (small) change in pressure over short distance is a strong (weak) pressure gradient The force that causes the wind to blow • Wind speed increase with gradient Pressure gradient force Pressure gradient force Pressure gradient force Forces that Influence Winds Coriolis Force Apparent deflection due to rotation of the Earth • Right turn in northern hemisphere and left turn in southern hemisphere Only influence direction, not speed • Stronger wind = greater deflection • Closer to pole = greater deflection Only has significant impact over long distances Coriolis effect Coriolis effect Coriolis effect Coriolis effect Coriolis effect Forces that Influence Winds Winds in the Upper Atmosphere No friction Upper air motions undergo Coriolis deflection Pressure gradient force balances with the Coriolis force • Geostrophic flow (wind) results Temporary imbalance • Gradient wind results Winds in the upper atmosphere Geostrophic Winds Balance between PGF, Coriolis • No other significant forces “Earth turning” winds • Caused by only Earth’s rotation and pressure gradients • Move in direction of rotation (W to E) Travel parallel to isobars • Spacing of isobars indicates speed – Close = fast – Spread out = slow Geostrophic winds Free Atmosphere Pressure Gradient Geostrophic winds Geostrophic wind Fig. 6, p. 215 Geostrophic & Gradient Flow Winds in the upper atmosphere Gradient Winds Aloft Gradient wind parallel to curved isobars • PGF ≠ coriolis Æ curving wind Cyclonic • Counterclockwise • Around Low Anticyclonic • Clockwise • Around High Gradient winds Gradient wind Gradient winds Winds on Upper-level Charts Winds parallel to contour lines and flow west to east Heights decrease from north to south Forces that Influence Winds Friction Surface effect Friction reduces the wind speed which in turn decrease the Coriolis effect. Winds cross the isobars at about 30° into low pressure and out of high pressure Friction A force of opposition Slows air (accelerates opposite motion) Maximum near surface • Planetary boundary layer Minimum aloft • Negligible above 1.5 km = the free atmosphere Affects Coriolis deflection Friction Friction Friction Fig. 8-30, p. 216 Friction Down under Fig. 8-31, p. 216 Winds and Vertical Motion Air rises over low Air sinks (subsides) over high Ridges & troughs Ridges and troughs in the northern hemisphere Cyclones & anticyclones High pressure areas (anticyclones) Clockwise motion in northern hemisphere Descending air Clear skies Low pressure areas (cyclones) Counterclockwise motion in northern hemisphere Ascending air Clouds Upper atmosphere Ridges = surface anticyclones Troughs = surface cyclones