5 Basics about WindWind direction is the direction from which the wind is blowingA north wind blows from the north to the southIt is reported according to compass directionsPrevailing wind direction is the most frequent directionWind speedReported on U.S. weather maps in knots1 knot = 1.15 miles/hour = 0.5 meter/secondIf wind > 15 knots and highly variable, the weather report will include the wind gust, the maximum speed
6 Figure 01: Wind directions in angles, compass headings.
7 Forces Have magnitude (or strength) and direction Multiple forces can act on the same pointThe resultant force is the net forceIf two forces act in opposite directions, the net force will have the direction of the stronger force and a strength equal to the difference of the two forcesIf two forces act at an angle to each other, the resultant force is along a diagonal and away from where the two forces are applied
10 Forces and MovementA force applied to an object often results in movementAn object’s velocity is the magnitude and direction of its motionThe speed of the object, the distance traveled in a given amount of time, is the magnitude of the motionAcceleration is a change in an object’s velocity—magnitude, direction or both
11 Forces cause the wind to blow Forces that act on air create horizontal windA force acting through a distance does workWork is equivalent to energyUltimately, the sun provides the energy that allows the winds to blowRadiation causes temperature imbalances, which lead to pressure imbalances and a force
12 Newton’s second law of motion Says thatthe sum of the forces = mass x accelerationOr that acceleration = sum of forces/massHelps scientist forecast changes in the wind direction and speed, or its accelerationRequires that we specify which forces are acting and how strong they areIs also called the Law of momentumMomentum of an object is its mass x its velocity
13 Gravity, the strongest force Does not act horizontally, so does not influence the horizontal winds.Does influence vertical air motionsIs directed downward toward the center of EarthIs a very strong forceKeeps our atmosphere from escapingEquals the mass x 9.8 m/s2
14 The Pressure Gradient Force (PGF) The force that results from pressure differences over distances in a fluidA pressure gradient is a change in pressure over a distancePGF always directed from high to low pressureIs stronger when isobars more closely spacedIs stronger when the difference in pressure is greater over a particular distanceDetermines the way air moves only if no other forces are acting
16 Figure B01B: Air over plane wing, with lift and drag
17 Figure 04: Pressure gradient force in highs and lows.
18 The horizontal pressure gradient force Is always directed from high to low pressureIs stronger where the density is less—higher in the troposphereWhen stronger, causes stronger windsIs always important in horizontal windsIs not generally in the same direction the wind blows, because other forces can act
19 Figure 05: Surface weather map From Plymouth State University Weather Center, [http://vortex.plymouth.edu/make.html.].
20 Isobaric Charts Plot the altitude of a given pressure surface Units of altitude are called geopotential metersAlso called a constant-pressure chartCommon levels are 850, 700, 500, 250, and 200 mbAre useful for portraying horizontal pressure gradients above the groundThe spacing between the lines of constant height is proportional to the PGFThe winds in general blow parallel to the height contours, at right angles to the PGF
21 Figure 06: 500-mb isobaric chart From Plymouth State University Weather Center, [http://vortex.plymouth.edu/make.html.].
22 Figure 07A: Isolines of constant height are proportional to the PGF
23 Figure 07B: Isolines of constant height are proportional to the PGF
24 Figure 07C: Isolines of constant height are proportional to the PGF
25 Centrifugal Force/Centripetal Acceleration Centripetal acceleration is a change in direction even if the speed does not changeFrom the point of view of an observer experiencing the centripetal acceleration, there is an apparent force called the centrifugal forceThe faster the speed and the tighter the curve, the larger is the centripetal accelerationThe sign of the centripetal acceleration is positive for cyclones, negative for anticyclones, and always directed inward to the center
27 The Coriolis Force Deflects the wind to the right in the NH Deflects the wind to the left in the SHIs strongest at the polesIs zero at the equatorIs stronger for stronger windsIs weaker for weaker windsIs zero for calm. It cannot start a wind
28 Figure 09A: Curving path of ocean buoy Adapted from Joseph et al., Current Science, 92 (2007).
29 Figure 6.10: The centrifugal (CENTF) and Coriolis forces acting on an air parcel moving with respect to the rotating EarthModified from A. Persson, Bull. Amer. Meteor. Soc., 79 : 1378.).
30 Figure 11A: Coriolis force at different latitudes.
31 Figure 11B: variation of Coriolis force with latitude and wind speed
32 Figure B02B: Carl-Gustaf Rossby Courtesy of University of Chicago News Office
33 The Friction ForceActs in the direction opposite to the direction the wind is blowingActs to slow down the windIs most important at Earth’s surfaceGets stronger when the winds are strongerIs not important above the boundary layer (the lowest 1 km in the atmosphere)The rougher the surface and the stronger the wind the greater is the friction force
35 Why force-balances are important Force-balances simplify Newton’s second law of motion by limiting the number of forcesForce-balances describe winds that come close to describing the observed windsEven though the forces are balanced, the wind need not be calmThe PGF is important in every force balanceOnly the PGF can set calm air into motion
37 Hydrostatic BalanceGravity (downward) balances the Vertical Pressure Gradient Force (upward)Does not apply inside cumulus clouds, because buoyancy is important thereDoes apply generally in the atmosphereLimits vertical motions to be much weaker than horizontal winds
39 More on Hydrostatic Balance The pressure gradient force is stronger when the air is less denseThe density of air is less when the air Temperature is higherPressure decreases upward less rapidly when the air has a higher temperatureHydrostatic balance helps explain the sea breeze and other thermal circulations
40 Pressure levels on weather maps The atmosphere is very close to hydrostatic balanceThis means that the height of a particular pressure level is roughly equivalent to the pressure at a related height levelAn altimeter is a barometer with a height scaleUpper-level weather maps are labeled in mWinds on a weather map are strong when the height contours are close together, weak where they are farther apart
41 Geostrophic BalanceIs a balance between the horizontal pressure gradient force and the Coriolis forceIgnores the friction forceHas isobars that are straight linesDoes not mean that the wind is calmHas a wind called the geostrophic windWinds on weather maps above the surface are close to the geostrophic windBlows with lower pressure (height) on the left (NH)
43 The Geostrophic Wind Is a wind in geostrophic balance Is parallel to the isobarsIn the NH has low pressure on the leftIn the SH has low pressure on the rightIn the NH the wind blows clockwise around high pressure centers and counterclockwise around low pressure centersIn the SH CW flow around lows and CCW flow around highs
45 Gradient Balance and the Gradient Wind Gradient balance is between the PGF, the Coriolis force and the centrifugal forceGradient balance allows curving wind patterns called the gradient windThe centrifugal force is always outwardAround a low the centrifugal force opposes the PGF and the resulting flow is subgeostrophicAround a high the centrifugal force adds to the PGF and the resulting flow is supergeostrophic
46 Figure 16: As in Figure 6-15, except now we also include the centrifugal force, leading to gradient balance.
47 Adjustment to Geostrophic Balance Initially there is an imbalance of forcesAir parcels move toward lower pressure (PGF)As soon as there is a wind, the Coriolis force actsParcels oscillate towards a balance between the PGF and the Coriolis forceAdjustment takes minutes to hoursAdjustment is temporary and incomplete
48 Figure 17: Wavy path of parcel adjusting to balance
49 Guldberg-Mohn Balance Is a balance between the PGF, the Coriolis force, and frictionFriction slows the wind and the Coriolis force weakensThe winds blow across the isobars at an angle toward low pressure (away from high pressure)Between 15° and 30° over waterBetween 25° and 50° over landFriction damps oscillations during adjustment to balance
51 Figure 19: A numerical simulation of how varying amounts of friction affect the adjustment to Guldberg–Mohn balance.Modified from Knox, J., and Borenstein, S., J. Geoscience Ed., 46 : 190–192.
52 Figure B03A: Chart of wind speeds and max wave heights
53 Figure 21: The isobars at the surface drawn over a satellite image of a cyclone Image created by Prof. Joshua Durkee, Western Kentucky University, using GREarth software.
54 The Thermal WindThe thermal wind relates temperature and winds to each otherThe winds are more westerly as you go up wherever it’s colder toward the poles
55 Putting horizontal and vertical winds together At the surface, the wind blows across the isobars into low-pressure areasAt the center of the low-pressure area the air must riseLow-pressure areas are usually cloudy and wetAt the surface, the wind blows across the isobars out of high-pressure areasAt the center of the high-pressure area the air must sinkHigh-pressure areas are usually clear and dryThese patterns are the result of Guldberg-Mohn balance
56 Figure 22: Schematic of pressure levels when air is heated
57 Figure 23: Cross-section of winds at various pressure levels
61 The thermal circulation The sea breeze is a thermal circulationA thermal circulation has both horizontal and vertical air motionsThe horizontal pressure gradient force is most important in a thermal circulationUpward air motions occur in the warmer air column of the circulation; downward air motions occur in the cooler air column
62 The sea breeze Is a daytime circulation Depends on differential heating at the surface between land and waterHas the warmer, rising air column over the land, which absorbs more incoming solar radiationHas the cooler, sinking air column over the water, which absorbs less radiationAir flows from warmer to cooler column aloftAir flows from cooler to warmer column at the surface
64 Figure 26A: Satellite image of sea breeze Courtesy of SSEC, University of Wisconsin-Madison
65 Figure 26B: Satellite image of sea breeze Courtesy of SSEC, University of Wisconsin-Madison
66 Figure 26C: Satellite image of sea breeze Courtesy of SSEC, University of Wisconsin-Madison
67 Figure 26D: Satellite image of sea breeze Courtesy of SSEC, University of Wisconsin-Madison
68 The sea breeze and the land breeze As solar heating diminishes in the late afternoon, the sea breeze weakensAt night, differential cooling occursThe cooler, sinking air column is over land, where radiational cooling is more rapid than over the waterThe warmer, rising air column is over the waterThe land breeze develops at nightAir flows towards the land aloftAir flows towards the water at the surface
70 Scales of motion in the atmosphere Describe the size and lifetime of wind patterns in the atmosphereDetermine which forces are most important to forming the wind patternsAre largest when the lifetimes are longestAre smaller when the lifetime is shorterHave a variety of names and definitions
71 More on scales of motion Microscale: <1 km in diameterPGF, centrifugal, friction forces are importantMesoscale: Between 1 and 1000 km in sizePGF, centrifugal, friction, and Coriolis Force for largest sizesSynoptic scale: At least 1000 km in sizeBalance between PGF and Coriolis Force dominatesPlanetary scale: Roughly 10,000 km in size
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