2PressureHear this term often in weather forecasts but what does it mean in the atmosphere?From earlier, it’s the weight of the air aboveHow about weather?High pressure?Usually nice weatherLow pressure?Associated with stormy weather
3Wind Another weather element we deal with on a regular basis Anyone know why the wind blows?Turns out that wind and pressure are relatedIn fact, wind blows due to horizontal differences in pressureIf 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
4Air Pressure and WindA couple of chapters ago we talked about temperatureBefore that, pressure and densityIn the atmosphere, these variables are all related such that a change in one will cause changes in the othersEx. If temperature changes, there will be a corresponding change in pressure and/or density
5Air Pressure and WindThe “Ideal Gas Law” or “Equation of State” illustrates this:Pressure is approximately equal to density x a constant x temperatureWe can ignore the constant and just go with…..
6Air Pressure and Wind If the pressure doesn’t change: An increase in T results in a decrease in densityWarm air is less dense and therefore risesA decrease in T results in an increase in densityCold air is dense and sinksJust like we’ve been saying all along
7Air Pressure and Wind If the temperature doesn’t change: An increase in pressure results in an increase in densityA decrease in pressure results in a decrease in densityAt the same temperature, air at a higher pressure is more dense than air at a lower pressure
8Atmospheric PressureHow does all of this stuff relate to the atmosphere?We’ve said from the beginning that pressure is basically the weight the air above us.Like at the rightPressure at the surface is due to the weight of all air molecules in the colum
9Atmospheric Pressure Simplified model Assumes air can’t leave the columnColumns of air have the same # of molecules and are at the same temperatureThe pressure at the surface is the sameWhat would happen if the temperatures of the air changed?Cool #1, Warm #2
10Atmospheric Pressure City 1, temp decreased so density increased City 2, temp increased causing density to decreaseJust like the gas law saidPressure stayed the sameBottom line: It takes a shorter column of cold air to exert the same amount of pressure as a taller column of warm air
11Atmospheric PressureNow let’s go up to a certain height in the atmosphereAt 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 2LH
12Atmospheric Pressure Points: L H Pressure changes more rapidly w/ height in cold air massesWarm air aloft is associated w/ high pressure aloftCold air aloft is associated w/ low pressure aloftDifferences in temperature cause differences in pressureLH
13Atmospheric PressureFinally, notice that the heights of pressure surfaces (500 mb in this example) are lower in the cold air column500 mb500 mb
14Atmospheric PressureThe difference in pressure establishes a force we call the “pressure gradient force” (directed from H to L)Now, if we remove the side barriers of the columns, air will rush from high to low pressure in order to equalize things ….WIND!!
15Atmospheric PressurePressures at the surface will also change due to molecules movingPressure will rise at City 1 and fall at City 2To make a long story short:Differences in temp from place to place can cause differences in pressure resulting in the movement of air.
17Measuring Air Pressure Even though pressure is exerted on us at all times, it’s hard to detect small changesCan you tell difference between high and low?We can detect big changes in pressure thoughLike popping of ears in the mountains or in planesAir pressure equalizing inside/outside ears
18Measuring Air Pressure Mercury BarometerJust a large, hollow glass tube immersed in mercuryAs air pressure changes, mercury is forced up or down the tube…pretty simple rightOn average, the height of the mercury would be inches (avg. sea level pressure)Or millibars
19Measuring Air Pressure Aneroid BarometerHas a hollow metal “cell” which expands or contracts as pressure changesSame type as in the 3-dial weather instruments people hang on walls
21Altimeters Just an aneroid barometer Calibrated to equate pressure to heightMust be corrected constantly by pilots!
22AltimetersOr this might happen w/ poor visibility
23Atmospheric Pressure Seen something like this on TV right? Lines are called “isobars”These are lines of equal pressure (in millibars)Question: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?
24Atmospheric PressureThis kind of map depicts “sea-level pressure”, not surface pressureWe wouldn’t always be able to tell where high and low pressure systems were otherwiseHow do we figure out what sea-level pressure is at each location where measurements are taken?
25Sea-level Pressure To get a sea-level pressure chart: 1) Measure surface pressure2) Correct for instrument errorTemperature, gravity, materials of barometer, etc.3) Correct for altitude4) Draw isobars (usually at 4 mb increments)Connect the dots essentially1) and 2) are easy. What about 3) and 4)?
26Altitude CorrectionNear the earth’s surface, pressure changes at 10mb per 100mSo, if a station is 300m altitude, 30mb needs to be added to the surface pressure to get sea-level pressureOnce the altitude correction is done everywhere, we can draw the isobars
28Surface ChartEnd result is a “sea-level pressure chart” or “surface chart”“Closed” highs and lows show where centers of pressure systems are
29Pressure and WindNorthern Hemisphere: surface winds blow clockwise and outward from high pressure (anticyclones)counter clockwise and inward around low pressure systems (cyclones)Note:Winds cross isobars slightlyTightly packed - stronger windsReversed flow in Southern Hemisphere
30Isobaric Map (Upper Air Chart) Shows the height of a pressure surface - constant pressure chartIn meters (60m intervals)This one is 500mbFrom Monday - pressure surfaces are higher up in warm airSo, the 500mb heights are higher toward the south
31Isobaric Map (Upper Air Chart) Where heights are low - cold airHigh heights - warm airElongated areas of low heights/pressure are called troughscold airElongated areas of high heights/pressure are called ridgeswarm air
32Isobaric Map (Upper Air Chart) Other uses??Wind - shows us direction and speedLike surface map except winds tend to blow parallel to height linesCloser lines - stronger wind speedsSteering - upper level winds determine where surface systems go and whether or not they strengthen (more later)
33Surface and Upper Air Charts Both surface and upper air charts are extremely valuable to meteorologistsSurface charts identify where pressure systems are located and their intensitiesUpper air charts indicate where these systems will move and how they will strengthen/weaken in time
34Wind Now with all this background, we can determine why the wind blows More specifically, what the direction and speed will beAnything that moves does so due to the forces acting upon itThrowing a ball - pushing away by the hand, friction from the air, gravity, etc.Same thing for the wind
35Wind Actually 4 forces acting to influence wind speed and direction 1) Pressure gradient force2) Coriolis force3) Friction4) Centripetal force
36Pressure Gradient Force Due to the difference in pressure over a distanceGreater pressure gradients lead to a greater PGFlike at the rightHurricanes are a good exampleVery low pressures at the centerPressure increases rapidly as you move away from the centerStrong PGF
37Pressure Gradient Force ALWAYS directed from high to low pressureDirection is at right angles to the isobarsThis does not mean that wind blows directly from high to low though…….other forces…..
38Pressure Gradient Force Wait just a second. Since pressure changes much faster w/height than horizontally, isn’t the PGF incredibly strong in the vertical?I’ve already said that vertical air motions are very small (usually) compared to horizontal winds…..what’s up with that?
39Pressure Gradient Force Gravity almost exactly balances the upward directed PGF - “Hydrostatic balance”
41Coriolis Force (Effect) Tricky subjectAn “apparent” force due to the rotation and curvature of the earthCauses wind to deflect to the right in the Northern HemisphereLeft in SHForce is at a right angle to the windThings drain differently in SH??? Umm..no
43Coriolis Force Maximum at poles, zero at the equator Faster speeds - stronger Coriolis forceIt’s why aircraft fly in “circular” paths
44Coriolis Force Summary: Causes objects to deflect to the right of a straight path in the NH (left in SH)Amount of deflection depends on1) Rotation of the earth2) Latitude3) Speed of object (wind, airplane, etc.)
45PGF - Coriolis BalanceAbove the friction layer near the surface, the PGF and CF roughly balance each otherThat’s why air aloft flows parallel to isobarsWind which flows at a constant speed parallel to evenly spaced isobars is calledGeostrophic Wind
46Geostrophic WindAlways low pressure to the left and high pressure to the right (Northern Hemisphere)Speed depends on the “packing” or “tightness” of isobarsloosely packed = weak wind : tightly packed = strong wind
47Geostrophic WindOnly a theoretical wind but still, a good approximation of winds above the surfaceWhy only theoretical?Isobars are rarely evenly spaced OR straight
48Gradient Wind In this case, the wind is called a gradient wind Basically the same as geostrophic wind except that it blows parallel to curved isobarsNote: both geostrophic and gradient winds refer to air flow well above the surface
49Wind Review 4 forces (talked about 2 so far) 1) Pressure gradient forceDirected from High to Low pressureStronger PGF = stronger wind2) Coriolis forceDeflects wind to right in NHFaster wind = stronger CFZero at equator, max at polesThese two forces are roughly in balance above the surfaceCauses upper level winds to generally flow west to east in mid-latitudes (parallel to isobars or height contours)
50FrictionFriction affects air flow near the surface (lowest 1 km or so)Slows down the wind (drag)If wind slows, what happens to the Coriolis Force??WeakerSo, the PGF is now greater than the CF and flow is across isobars~ 30º angle
51Centripetal Force A little confusing so just remember it is: The force required to keep an object (wind) moving in a circular pathDirected inward toward the center in both high and low pressure systems
52Convergence and Divergence Now that we know a little about surface and upper level winds….How do they affect vertical air motions?By convergence and divergence patternsex. Convergence Divergence
53Convergence and Divergence Remember what winds are like around high and low pressures?Winds diverge from high pressure and converge at low pressureIf air converges at low pressure (at the surface for ex.), what must it do?Must rise (can’t go into the ground right?)
54Convergence and Divergence What about air diverging from a high pressure center (again at the surface)?Some air will have to sink from above to replace itThis explains why we have clear weather w/ highs and cloudy weather w/ lowsSo far we’re just talking about the surface. But what’s going on above the surface high and low pressure systems?
55Convergence and Divergence Air that is forced to rise due to convergence at the surface low eventually diverges aloft.Air 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
56Convergence and Divergence In either of these examples, if convergence = divergence, what happens to the surface pressure.Hint:This means the # of air molecules over the surface does not change.Pressure stays the same!
57Convergence and Divergence What if convergence and divergence are not equal??Over the low:Divergence aloft > Convergence at the surface?Net loss of air over low - pressure gets even lowerHurricanes?? (not Miami)Div < Conv?Net gain of air - pressure increases
58Convergence and Divergence Over the high:Convergence aloft > Divergence at the surface?Net gain of air over high - pressure gets even higherConv < Div?Net loss of air - pressure decreases
59Convergence and Divergence In summary:Pressure at the surface is largely dependent on wind patterns at both the surface and aloftThis is why meteorologist actually care about what is happening above the surface of the earthIf this seems a little fuzzy to you, look at figure 6.21 and convince yourself of it.
60Measuring Wind Described by: 1) Direction 2) Strength Always direction it’s coming FROMCan be N/S/E/W or in degrees on a compass2) StrengthUsually mph, m/s (meters per second), knots (nautical miles per hour)NOTE: Nautical mile > statute mile
61Measuring Wind Wind directions: Real easy, just think of a 360° circle East wind - 90°South wind - 180°West wind - 270°North wind - 360°Again, always described in terms of direction fromex. NW wind is out of the northwest, not toward the northwest
62Measuring Wind Wind vane Anemometers Simple instrument which measures direction onlyAnemometersMeasures speed onlyExample is of a “cup” anemometer
63Measuring Wind Aerovane Measures both wind speed and direction Will face into the wind giving directionPropellers rotate to yield wind speedInfo is transmitted electronicallyOne on top of the Love Building
64Measuring Wind Radiosonde RADAR Satellites Balloon is tracked from the surfaceSimple calculations to determine its speed (wind)RADARDoppler in particularCan determine wind speed and direction by frequency changes in the emitted RADAR pulseSatellitesCloud drifts