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Air Pressure and Wind Chapter 6. Pressure zHear this term often in weather forecasts but what does it mean in the atmosphere? -From earlier, it’s the.

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Presentation on theme: "Air Pressure and Wind Chapter 6. Pressure zHear this term often in weather forecasts but what does it mean in the atmosphere? -From earlier, it’s the."— Presentation transcript:

1 Air Pressure and Wind Chapter 6

2 Pressure zHear this term often in weather forecasts but what does it mean in the atmosphere? -From earlier, it’s the weight of the air above -How about weather? zHigh pressure? -Usually nice weather zLow pressure? -Associated with stormy weather

3 Wind zAnother weather element we deal with on a regular basis -Anyone know why the wind blows? -Turns out that wind and pressure are related -In fact, wind blows due to horizontal differences in pressure zIf there is high pressure over one part of the country and low pressure over another part: -The atmosphere is out of balance and the wind blows in an attempt to restore balance

4 Air Pressure and Wind zA couple of chapters ago we talked about temperature zBefore that, pressure and density zIn the atmosphere, these variables are all related such that a change in one will cause changes in the others -Ex. If temperature changes, there will be a corresponding change in pressure and/or density

5 Air Pressure and Wind zThe “Ideal Gas Law” or “Equation of State” illustrates this: zPressure is approximately equal to density x a constant x temperature -We can ignore the constant and just go with…..

6 Air Pressure and Wind zIf the pressure doesn’t change: -An increase in T results in a decrease in density zWarm air is less dense and therefore rises -A decrease in T results in an increase in density zCold air is dense and sinks zJust like we’ve been saying all along

7 Air Pressure and Wind zIf the temperature doesn’t change: -An increase in pressure results in an increase in density -A decrease in pressure results in a decrease in density zAt the same temperature, air at a higher pressure is more dense than air at a lower pressure

8 Atmospheric Pressure zHow does all of this stuff relate to the atmosphere? zWe’ve said from the beginning that pressure is basically the weight the air above us. -Like at the right -Pressure at the surface is due to the weight of all air molecules in the colum

9 Atmospheric Pressure zSimplified model -Assumes air can’t leave the column zColumns of air have the same # of molecules and are at the same temperature -The pressure at the surface is the same zWhat would happen if the temperatures of the air changed? -Cool #1, Warm #2

10 Atmospheric Pressure zCity 1, temp decreased so density increased zCity 2, temp increased causing density to decrease -Just like the gas law said zPressure stayed the same zBottom line: It takes a shorter column of cold air to exert the same amount of pressure as a taller column of warm air

11 Atmospheric Pressure zNow let’s go up to a certain height in the atmosphere -At this level, in which column is the pressure greatest? -So, relatively speaking, the pressure is high at this level in column 1 and low in column 2 HL

12 Atmospheric Pressure zPoints: -Pressure changes more rapidly w/ height in cold air masses -Warm air aloft is associated w/ high pressure aloft -Cold air aloft is associated w/ low pressure aloft -Differences in temperature cause differences in pressure HL

13 Atmospheric Pressure zFinally, notice that the heights of pressure surfaces (500 mb in this example) are lower in the cold air column 500 mb

14 Atmospheric Pressure zThe difference in pressure establishes a force we call the “pressure gradient force” (directed from H to L) zNow, if we remove the side barriers of the columns, air will rush from high to low pressure in order to equalize things …. -WIND!!

15 Atmospheric Pressure zPressures at the surface will also change due to molecules moving -Pressure will rise at City 1 and fall at City 2 zTo make a long story short: -Differences in temp from place to place can cause differences in pressure resulting in the movement of air.

16 Sea-level Pressure

17 Measuring Air Pressure zEven though pressure is exerted on us at all times, it’s hard to detect small changes -Can you tell difference between high and low? zWe can detect big changes in pressure though -Like popping of ears in the mountains or in planes zAir pressure equalizing inside/outside ears

18 Measuring Air Pressure zMercury Barometer -Just a large, hollow glass tube immersed in mercury -As air pressure changes, mercury is forced up or down the tube…pretty simple right zOn average, the height of the mercury would be inches (avg. sea level pressure) -Or millibars

19 Measuring Air Pressure zAneroid Barometer -Has a hollow metal “cell” which expands or contracts as pressure changes -Same type as in the 3-dial weather instruments people hang on walls

20 Measuring Air Pressure

21 Altimeters zJust an aneroid barometer -Calibrated to equate pressure to height -Must be corrected constantly by pilots!

22 Altimeters zOr this might happen w/ poor visibility

23 Atmospheric Pressure zSeen something like this on TV right? zLines are called “isobars” -These are lines of equal pressure (in millibars) zQuestion: -If elevation varies across the US (and it does), and we know pressure changes quickly w/ height, then why are these types of maps nice and neat? -Shouldn’t they be really screwy looking?

24 Atmospheric Pressure zThis kind of map depicts “sea-level pressure”, not surface pressure -We wouldn’t always be able to tell where high and low pressure systems were otherwise zHow do we figure out what sea-level pressure is at each location where measurements are taken?

25 Sea-level Pressure zTo get a sea-level pressure chart: -1) Measure surface pressure -2) Correct for instrument error zTemperature, gravity, materials of barometer, etc. -3) Correct for altitude -4) Draw isobars (usually at 4 mb increments) zConnect the dots essentially z1) and 2) are easy. What about 3) and 4)?

26 Altitude Correction zNear the earth’s surface, pressure changes at 10mb per 100m zSo, if a station is 300m altitude, 30mb needs to be added to the surface pressure to get sea-level pressure zOnce the altitude correction is done everywhere, we can draw the isobars

27 Isobars

28 Surface Chart zEnd result is a “sea-level pressure chart” or “surface chart” z“Closed” highs and lows show where centers of pressure systems are

29 Pressure and Wind zNorthern Hemisphere: surface winds blow clockwise and outward from high pressure (anticyclones) zcounter clockwise and inward around low pressure systems (cyclones) zNote: -Winds cross isobars slightly -Tightly packed - stronger winds -Reversed flow in Southern Hemisphere

30 Isobaric Map (Upper Air Chart) zShows the height of a pressure surface - constant pressure chart -In meters (60m intervals) zThis one is 500mb zFrom Monday - pressure surfaces are higher up in warm air -So, the 500mb heights are higher toward the south

31 Isobaric Map (Upper Air Chart) zWhere heights are low - cold air zHigh heights - warm air zElongated areas of low heights/pressure are called troughs -cold air zElongated areas of high heights/pressure are called ridges -warm air

32 Isobaric Map (Upper Air Chart) zOther uses?? zWind - shows us direction and speed -Like surface map except winds tend to blow parallel to height lines -Closer lines - stronger wind speeds zSteering - upper level winds determine where surface systems go and whether or not they strengthen (more later)

33 Surface and Upper Air Charts zBoth surface and upper air charts are extremely valuable to meteorologists -Surface charts identify where pressure systems are located and their intensities -Upper air charts indicate where these systems will move and how they will strengthen/weaken in time

34 Wind zNow with all this background, we can determine why the wind blows zMore specifically, what the direction and speed will be zAnything that moves does so due to the forces acting upon it -Throwing a ball - pushing away by the hand, friction from the air, gravity, etc. -Same thing for the wind

35 Wind zActually 4 forces acting to influence wind speed and direction -1) Pressure gradient force -2) Coriolis force -3) Friction -4) Centripetal force

36 Pressure Gradient Force zDue to the difference in pressure over a distance zGreater pressure gradients lead to a greater PGF -like at the right zHurricanes are a good example -Very low pressures at the center -Pressure increases rapidly as you move away from the center -Strong PGF

37 Pressure Gradient Force zALWAYS directed from high to low pressure zDirection is at right angles to the isobars zThis does not mean that wind blows directly from high to low though…….other forces…..

38 Pressure Gradient Force zWait just a second. Since pressure changes much faster w/height than horizontally, isn’t the PGF incredibly strong in the vertical? zI’ve already said that vertical air motions are very small (usually) compared to horizontal winds…..what’s up with that?

39 Pressure Gradient Force zGravity almost exactly balances the upward directed PGF - “Hydrostatic balance”

40 High or Low Pressure?

41 Coriolis Force (Effect) zTricky subject zAn “apparent” force due to the rotation and curvature of the earth -Causes wind to deflect to the right in the Northern Hemisphere zLeft in SH -Force is at a right angle to the wind -Things drain differently in SH??? Umm..no

42 Coriolis Force

43 zMaximum at poles, zero at the equator zFaster speeds - stronger Coriolis force zIt’s why aircraft fly in “circular” paths

44 Coriolis Force zSummary: -Causes objects to deflect to the right of a straight path in the NH (left in SH) -Amount of deflection depends on z1) Rotation of the earth z2) Latitude z3) Speed of object (wind, airplane, etc.)

45 PGF - Coriolis Balance zAbove the friction layer near the surface, the PGF and CF roughly balance each other -That’s why air aloft flows parallel to isobars zWind which flows at a constant speed parallel to evenly spaced isobars is called -Geostrophic Wind

46 Geostrophic Wind zAlways low pressure to the left and high pressure to the right (Northern Hemisphere) zSpeed depends on the “packing” or “tightness” of isobars -loosely packed = weak wind : tightly packed = strong wind

47 Geostrophic Wind zOnly a theoretical wind but still, a good approximation of winds above the surface zWhy only theoretical? -Isobars are rarely evenly spaced OR straight

48 Gradient Wind zIn this case, the wind is called a gradient wind -Basically the same as geostrophic wind except that it blows parallel to curved isobars -Note: both geostrophic and gradient winds refer to air flow well above the surface

49 Wind Review z4 forces (talked about 2 so far) -1) Pressure gradient force zDirected from High to Low pressure zStronger PGF = stronger wind -2) Coriolis force zDeflects wind to right in NH zFaster wind = stronger CF zZero at equator, max at poles -These two forces are roughly in balance above the surface zCauses upper level winds to generally flow west to east in mid-latitudes (parallel to isobars or height contours)

50 Friction zFriction affects air flow near the surface (lowest 1 km or so) zSlows down the wind (drag) zIf wind slows, what happens to the Coriolis Force?? -Weaker zSo, the PGF is now greater than the CF and flow is across isobars -~ 30º angle

51 Centripetal Force zA little confusing so just remember it is: -The force required to keep an object (wind) moving in a circular path -Directed inward toward the center in both high and low pressure systems

52 Convergence and Divergence zNow that we know a little about surface and upper level winds…. -How do they affect vertical air motions? zBy convergence and divergence patterns zex. ConvergenceDivergence

53 Convergence and Divergence zRemember what winds are like around high and low pressures? -Winds diverge from high pressure and converge at low pressure zIf air converges at low pressure (at the surface for ex.), what must it do? -Must rise (can’t go into the ground right?)

54 Convergence and Divergence zWhat about air diverging from a high pressure center (again at the surface)? -Some air will have to sink from above to replace it -This explains why we have clear weather w/ highs and cloudy weather w/ lows zSo far we’re just talking about the surface. But what’s going on above the surface high and low pressure systems?

55 Convergence and Divergence zAir that is forced to rise due to convergence at the surface low eventually diverges aloft. zAir converges aloft above the surface high and sinks to replace the diverging air at the surface. -Think of it like columns of air above the surface pressure systems

56 Convergence and Divergence zIn either of these examples, if convergence = divergence, what happens to the surface pressure. zHint: -This means the # of air molecules over the surface does not change. zPressure stays the same!

57 Convergence and Divergence zWhat if convergence and divergence are not equal?? zOver the low: zDivergence aloft > Convergence at the surface? -Net loss of air over low - pressure gets even lower -Hurricanes?? (not Miami) zDiv < Conv? -Net gain of air - pressure increases

58 Convergence and Divergence zOver the high: zConvergence aloft > Divergence at the surface? -Net gain of air over high - pressure gets even higher zConv < Div? -Net loss of air - pressure decreases

59 Convergence and Divergence zIn summary: -Pressure at the surface is largely dependent on wind patterns at both the surface and aloft zThis is why meteorologist actually care about what is happening above the surface of the earth -If this seems a little fuzzy to you, look at figure 6.21 and convince yourself of it.

60 Measuring Wind zDescribed by: -1) Direction zAlways direction it’s coming FROM zCan be N/S/E/W or in degrees on a compass -2) Strength zUsually mph, m/s (meters per second), knots (nautical miles per hour) -NOTE: Nautical mile > statute mile

61 Measuring Wind zWind directions: zReal easy, just think of a 360° circle -East wind - 90° -South wind - 180° -West wind - 270° -North wind - 360° zAgain, always described in terms of direction from -ex. NW wind is out of the northwest, not toward the northwest

62 Measuring Wind zWind vane -Simple instrument which measures direction only zAnemometers -Measures speed only -Example is of a “cup” anemometer

63 Measuring Wind zAerovane -Measures both wind speed and direction -Will face into the wind giving direction -Propellers rotate to yield wind speed -Info is transmitted electronically -One on top of the Love Building

64 Measuring Wind zRadiosonde -Balloon is tracked from the surface -Simple calculations to determine its speed (wind) zRADAR -Doppler in particular -Can determine wind speed and direction by frequency changes in the emitted RADAR pulse zSatellites -Cloud drifts


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