Presentation on theme: "Outline for Lecture 13 Factors Affecting Wind"— Presentation transcript:
1 Outline for Lecture 13 Factors Affecting Wind Winds Aloft and Geostrophic FlowCurved Flow and the Gradient WindSurface windsHow Winds Generate Vertical Air Motion3/13/03
2 Formation of A Sea Breeze No pressure gradient,no wind.(b) Unequal heatingcreates pressuredifferences aloft whichcauses air to flow.The transfer of air aloft(from the land to the sea)creates a surface high overthe sea which results in aflow of air from sea to land(a.k.a. a sea breeze).Returning airSea Breeze
4 The Coriolis ForceNamed after the French Scientist Gaspard Gustave Coriolisfree moving objectsare deflected to the rightof their path in the NorthernHemisphere (to the left oftheir path in the SouthernHemisphere) because of theEarth’s rotation.It depends on an object’sspeed—higher speed meansstronger Coriolis Force.
5 The Coriolis ForceCoriolis deflection of winds blowing eastward at different latitudesCoriolis force alsoincreases with increasingLatitude…strong over poles,no effect over equator.StrongMiddlingWeakNone
6 Friction Altitude Friction acts at the surface. winds at the surface aren’t as strong as those at higher altitudesAir is a little viscous, so the layer next to the surface is also affected, but not as much.Altitude
7 Winds Aloft and Geostrophic Flow Balance pressure gradient force Coriolis Force: Geostrophic Flow.Geostrophic winds:go in a straight linego parallel to the isobarshave speeds proportional to the pressure gradient force.
8 Geostrophic Flow WHAT!?!? pressure difference starts wind wind gets going a little, starts being deflected by Coriolis forcewind goes faster in response to pressure difference, gets deflected more by Coriolis ForceEventually, the two balance
9 Newton’s Laws of Motion (condensed version) An object in motion tends to stay in motion(unless acted upon by an external force)II. F = ma(that’s pretty much it.)
10 Put down the pencilsLet’s break down the forces
11 Just a Pressure Gradient Start with just a pressure gradientThen wind blows straight from High P to Low PAnd accelerates as it goesView from topHLPressurereturn
12 Just a Pressure Gradient Wind blows straight from High to Low, eventually evening out the pressure and stopping the wind.HLPressure
13 Just a Pressure Gradient The wind would blow at the same speed regardless of altitude.HLPressurereturn
14 Now add FrictionThen wind still blows straight from High P to Low P, but it doesn’t get moving as fast as soon, especially near the groundView from topHLPressureback up
15 Friction and Pressure Gradient The pressure difference also evens out eventually, though it might take a bit longer.HLPressure
16 Friction and Pressure Gradient Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere.HLPressure
17 Friction and Pressure Gradient Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere.HLPressureback up
18 Pressure Gradient and Coriolis Forget friction.The wind starts out straight, but as soon as it starts building up speed, the Coriolis force turns it a bit to the right.HLThe wind can’t accelerate any more over herebecause it’s going parallel to the isobarsThis is the Geostrophic Flow
19 Pressure Gradient and Coriolis The Coriolis force limits the wind speed by redirecting it AND it prevents wind from blowing straight from H to LHL
20 Pressure Gradient and Coriolis Since the wind never really reaches the low, the pressure difference is maintained, and the low never fills!HL
21 Reality: Pressure Gradient, Coriolis Force, and Friction Coriolis Force turns the wind some, friction slows the wind some, and the result is roughly a 30º angle between isobars and wind.HL30º
22 Reality: Pressure Gradient, Coriolis Force, and Friction The wind doesn’t blow straight from High to Low, but it does eventually get in there and even out the pressure difference, so H and L don’t last forever without a source of energyHL30º
23 Reality: Pressure Gradient, Coriolis Force, and Friction Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere.HLPressure
24 Reality: Pressure Gradient, Coriolis Force, and Friction Coriolis Force is turning the wind toward us in the right part of the picture.HLPressure
25 Reality: Pressure Gradient, Coriolis Force, and Friction Since the Coriolis Force depends on wind speed, its effect decreases toward the ground where the wind speed is slower.CoriolisHLFrictionPressure
27 Winds Aloft and Geostrophic Flow Geostrophic winds are up high and go straight:only Coriolis and Pressure Gradient Forces are important.Friction is important down low:below about 1500 meters.How do the Coriolis and Pressure Gradient forces change?Coriolis ForceWind speedLatitudePressure GradientWind flows fromhigh to low pressure.“Isobaric packing”P.G.F.
28 Winds Aloft and Geostrophic Flow Wind direction is directly linked to the prevailingpressure pattern.Dutch meteorologist Buys Ballott, 1857Buys Ballott’s Law states: In the Northern Hemisphere if youstand with your back to the wind, lower pressure will be found toyour left and higher pressure will be found to the right.Best when there are no frictional forcesor topography involved .
29 Curved Flow and the Gradient Wind Cyclonic FlowAnticyclonic FlowActual flow around pressure systems are never this regular (because of small changes in the pressure field).
34 Troughs and RidgesAn elongated region of low pressure (trough) or high pressure (ridge)Tend to be quite common at higher altitudesAt the surface, a trough is usually a fairly weak feature
35 How Winds Generate Vertical Air Motion Around a surface low pressure center, a net inward transport of aircauses a shrinking of the area occupied by the mass. This is knownas horizontal convergence.
36 Airflow Associated with Cyclones and Anticyclones “Upper level support” is important in cyclone development
37 Wind speeds and isobars STRONG WINDSSlack windsSlack windsThe tighter they’re packed, the stronger the wind
38 Factors that promote vertical airflow Friction:air flow from ocean to land (upward motion)air flow from land to ocean (downward motion)Mountain ranges
39 Wind MeasurementWind roses provide a method of representing prevailing winds byindicating the percentage of time the wind blows from variousdirections