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Meteorology 10 - Weather and Climate Fall 2008 CHAPTER 6 FOCUS: forces of air motion Air Pressure and Winds Which way will the wind blow and how strong will it be? 1. Pressure Gradient Force 2. Coriolis Force (Earth’s Rotation) 3. Centrifugal Force 4. Friction We are going to focus on one of these at a time…here goes Which way will the wind blow and how strong will it be? 1. Pressure Gradient Force 2. Coriolis Force (Earth’s Rotation) 3. Centrifugal Force 4. Friction Pressure Gradient Force (PGF) The source of all wind…. Define PGF, word for word. First: Pressure is the force exerted by the air per unit area from the weight of the column of air above it. 99% of mass 1013.25 millibars 70% of mass 14.7 lbs/sq. inch 1 atmosphere Pressure Gradient Force (PGF): The source of all wind…. Two ways to further describe pressure: Either as HIGH (H) or LOW (L) Look at each set in the figure and see how the pressure is defined relative to the other column of air. Fig. 6-2, p. 143 Pressure Gradient Force Pressure Gradient = change in pressure distance In this simplified figure, there is no pressure “gradient” in the horizontal direction Height 200mb 400mb 600mb 800mb 1000mb Ocean Land Pressure Gradient Force Newton’s First Law of Motion “A force is required to make something move” Pressure Gradient Force (PGF) The source of all wind…. 200mb Cool the air in Column 1 and the air gets dense P= RT When P is constant T are together a constant such that the following must be true T 200mb 1000mb 1000mb Pressure Gradient Force (PGF) The source of all wind…. Pressure Gradient Force driven by temperature: •Two primary examples follow. •But here are the basic physical principals Afternoon Height L 200mb H 400mb Horizontal PGF 600mb H Colder (Ocean) 800mb 1000mb L Warmer (Land) Thermal Circulation - Pressure Gradient Force Sea Breeze Onshore Wind Thermal Circulation - Pressure Gradient Force Land Breeze Offshore Wind Take home message: air flows from H to L pressure Fig. 6-11, p. 151 Surface and Upper Air Charts • • • • isobaric maps contour lines ridges troughs • Color-filled contour maps are the same as ordinary contour maps, except that the area between adjacent lines is filled in with color. Surface of constant pressure Warm Cold Constant pressure chart 5520 m 5580 m 5640 m Editable Text Figure 3, p. 150 Figure 2, p. 150 Which way will the wind blow and how strong will it be? 1. Pressure Gradient Force 2. Coriolis Force (Earth’s Rotation) 3. Centrifugal Force 4. Friction Other forces on the atmosphere: “Coriolis Force” • Air moves in the reference frame of the earth. • But, the earth is rotating... The atmosphere is not in a “fixed” frame of reference Coriolis Force • • • • real and apparent forces Coriolis force strength and direction of the Coriolis force factors that affect the Coriolis force • It is sometimes claimed that “water swirls down a bathtub drain in opposite directions in the northern and southern hemispheres”. This is not true. Deflection of ball’s path in rotating reference frame Is the ball moving in a straight or curved path??? Depends on your frame of reference!! Fig. 6-14, p. 153 Coriolis Force in a rotating reference frame http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/gifs/coriolis .mov Coriolis Force due to twisting of North-South reference frame as Earth turns Fig. 6-15, p. 153 Coriolis Force due to rotation of North-South reference frame on spherical Earth Objects are deflected to the right of their path in the Northern Hemisphere Greatest near poles No change at equator Fig. 6-15, p. 153 Earth reference frame changes orientation due to Earth rotation A moving object appears to be deflected from its original course The earth rotates, so All moving objects are deflected by the Coriolis force • Coriolis force is ‘noticable’ for objects that are in motion for a significant fraction of the time it takes for the earth to rotate once (a day). - an airplane flying to Paris? - air moving across the U.S.? - water spinning down a drain? Coriolis Force depends on •Earth’s rotation •Speed of object •Latitude of object Coriolis Force = 2Vsin = Earth’s rotation rate V = object’s velocity = latitude where object is located Coriolis Force = 2Vsin Northern Hemisphere V CF = 0o Equator Southern Hemisphere = 30o CF V = - 30o Moving objects “deflected” to the right of their path in the Northern Hemisphere (to the left in Southern Hemisphere) Straight-line Flow Aloft • combination of the pressure gradient and Coriolis forces • geostrophic wind • Geostrophic winds can be observed by watching the movement of clouds. Colorado-sized area in the Northern Hemisphere • Motionless air • Uniform pressure NO FORCES, so NO WIND Colorado-sized area in the Northern Hemisphere Low Pressure PGF High Pressure Colorado-sized area in the Northern Hemisphere Which way will the wind blow and how strong will it be? 1. Pressure Gradient Force 2. Coriolis Effect (Earth’s Rotation) 3. Centrifugal Force 4. Friction Curved Winds Around Lows and Highs Aloft • cyclonic and anticyclonic flow – not in geostrophic balance curved • centripetal force – force ‘felt’ by the air parcels • gradient wind Gradient Wind: General case of wind along curved path Centrifugal Force (CE) Acts away from center of curvature PGF CF CE PGF = CF + CE • Pressure gradient is stronger around a Low e.g., hurricanes PGF CF Centrifugal Force Gradient Wind: General case of wind along curved path Centrifugal Force (CE) Acts away from center of curvature CF PGF CF = PGF + CE CE • Pressure gradient is weaker around a High PGF CF Centrifugal Force Winds on Upper-level Charts • gradients in contour lines • meridional and zonal winds • Height contours on upper-level charts are interpreted in the same way as isobars on surface charts. Figure 6.19: An upper-level 500-mb map showing wind direction, as indicated by lines that parallel the wind. Wind speeds are indicated by barbs and flags. (See the blue insert.) Solid gray lines are contours in meters above sea level. Dashed red lines are isotherms in °C. Fig. 6-19, p. 158 Stepped Art Which way will the wind blow and how strong will it be? 1. Pressure Gradient Force 2. Coriolis Effect (Earth’s Rotation) 3. Centrifugal Force 4. Friction Surface Winds • planetary boundary layer • friction • frictional effects on the wind • Most people rarely venture out of the planetary boundary layer. Winds and Vertical Motions • divergence and convergence • hydrostatic equilibrium – gravity balanced with vertically directed PGF