Presentation on theme: "ESCI 106 – Weather and Climate Lecture 6"— Presentation transcript:
1ESCI 106 – Weather and Climate Lecture 6 Jennifer D. Small
2Weather Fact of the Day: September 8 1994: A Nor’Easter wreaked havoc on costal MD.50 mph winds (gusts to 79 mph) destroyed 100s of tents/vending areas at the end-of-summer Sunfest in Ocean City.Windblown fires burned several shops along the boardwalk9 foot waves flooded other areas.Damage up to $5 million!!
5Understanding Pressure AIR PRESSURE is the pressure exerted by the weight of the air above.Is DEFINED as: the FORCE exerted against a surface by the continuous collision of gas molecules
6Measuring Air Pressure Unit: Newton (N)At Sea Level one “atmosphere” exerts14.7 pounds per square inch101,325 N per square m (N/m2)Meteorologist use millibars (mb)1 mb = 100 N/m2Standard Sea Level Pressure~ mb** This is a number you MUST memorize!!!!
7Understanding Pressure Example: Why aren’t we crushed by the weight of the air above us?1) We developed under this pressure.2) Pressure force of air is exerted in all directions3) If you lower the pressure drastically the cells of our bodies would burst!!Balloon SHRINKS in all directions and dimensions equally!!
8Understanding Pressure Example: Why aren’t we crushed by the weight of the air above us?1) We developed under this pressure.2) Pressure force of air is exerted in all directions3) If you lower the pressure drastically the cells of our bodies would burst!!Force is only in one direction.Just the weight of an aquarium on top, not equally in all dimensionsPOP!!!!
9Measuring Air Pressure Besides mb you may also have heard “inches of mercury” or in of Hg.Refers to Mercury BarometersBarometer = instrument to measure pressure.
11Pressure and Weather - Intro Aneroid BarometersOften found in homesNo Mercury (safer!!)Typically you find the following relationships:LOW Pressure = “rain”HIGH Pressure = “fair weather”Not ALWAYS trueNO LIQUID!! An air chamber changes shape as pressure changes.
12Pressure and Weather - Intro CHANGE in pressure is a better predictor of the weatherDecreasing PressureIncreasing cloudinessIncreasing PressureClearing conditions
13Pressure Changes with Altitude FACT: The pressure at any given altitude in the atmosphere is equal to the weight of the air directly above that point!!!Air becomes less dense because the weight of the air above it decreases.Why air is “thin” higher in the atmospherePressure reduces by ½ for each 5 kilometers
14Pressure Changes with Altitude Upper Atmosphere(Mesosphere)Middle Atmosphere(Stratosphere)Sea Level(Troposphere)Canister of air fitted with a movable pistonWeight is added…. Pressure increasesMore weight is added…. Pressure increases further
15Horizontal Variations in Air Pressure Adjustments need to be made for elevationEverything is converted to SEA-LEVEL equivalentsA) = 1008B) = 1014C) = 1020
16Influence of Temp and Water Vapor (A) Warm AirFast moving moleculesTypically less denseLOW PRESSURE(B) Cold AirSlow moving moleculesTypically more denseHIGH PRESSURE**Factors other then Temp can affect Pressure… you can have “warm” high pressure
17Influence of Temp and Water Vapor The addition of water vapor actually makes air LIGHTER (less Dense)!!!!Molecular weights of N2 (14) and O2 (16) are greater than H2O (10)If you “substitute” some of the N2 and O2 with H20 the overall weight of air will be less!N2: 4 * 14 = 56O2: 2 * 16 =32H2O: 5 * 10 = 50Total = 138N2: 7 * 14 = 98O2: 3 * 16 =48Total = 146
18Influence of Temp and Water Vapor HIGH PRESSURESUMMARYCold, dry air masses produce High Surface PressuresCold, humid air masses are less “high” than cold, dryWarm, dry air masses are less “low” than warm, humidWarm, humid air masses produce Low Surface PressuresLOW PRESSURE
19Airflow and Pressure Movement of air can cause variations in pressure Net flow of air into a region = CONVERGENCENet flow of air out of a region = DIVERGENCE
20What is Wind?Wind is the result of horizontal differences in air pressure!Air flows from areas of HIGH pressure to areas of LOW pressureHIGHLOW
21What is Wind?Wind is nature’s attempt at balancing inequalities in pressureFACT: Unequal heating of the Earth’s surface generates these inequalities.FACT: Solar radiation is the ultimate source of energy for Wind
22Factors Affecting Wind If the Earth did NOT rotate and if there was NO friction wind would flow in a straight line from High to Low pressureThree main forces that affect windYOU NEED TO MEMORIZE THESE!!!Pressure Gradient ForceCoriolis ForceFriction
23Basic Rules for Winds: Horizontal differences in pressure causes winds Horizontal differences in pressure are caused by differences in heatingWinds flow from regions of high pressure to regions of low pressureHorizontal differences in P lead to the PRESSURE GRADIENT FORCE
24NO TEMPERATURE DIFFERENCE TEMPERATURE DIFFERENCE Basic Rules for Winds:NO TEMPERATURE DIFFERENCETEMPERATURE DIFFERENCEWINDNO WIND600 mb700 mb1000 mbT = 20T = 20T = 20T = 30
25Pressure Gradient Force Horizontal Pressure Differences (HPD)Winds flow from High pressure to Low pressure if only affected by HPDHigher PLower P500 mb500 mb700 mb700 mbSea Breeze1000 mb1000 mbWARM NighttimeCOOL
26ISOBARS Isobars or contours (lines or curves) of constant Pressure Just like your isotherms for temperatureThey are corrected for altitude to equivalent Sea Level Pressure (SLP)
28PGF – Change over Horizontal Difference STRONGER when isobars are closer togetherSame CHANGE in Pressure (ΔP)When given Pressure Heights, the PGF points from regions of High Pressure to regions of Low PressureΔPΔPT = 20T = 30T = 20T = 30LARGE DISTANCESMALL DISTANCE
29The STEAPER the SLOPE the FASTER the ball will roll!!! ISOBARS & PGFIf all we had was the PGF wind would act like a Ball rolling down a slope… rolling at 90 Degrees to the slope!100 m500 m200 m300 m400 m400 m500 m300 m200 m100 m500 m300 m100 mThe STEAPER the SLOPE the FASTER the ball will roll!!!
30ISOBARS & PGF - More Examples 1020 mb1000 mb1016 mb1004 mb1008mb1012 mb1012 mb1008 mb1016 mb1004 mb1020 mb1000 mbFor a conical hill, the PGF points in all directionPGFPGFPGF, perfectly down hill at right angles to the isobars
31ISOBARS & PGF - More Examles Winds if we ONLY knew the PGF.WINDISSLOWWINDISFASTIf the isobars are further or closer together…1004 mb992 mb996 mb1008 mb1000 mb1012 mb1004 mb1008 mb1016 mb1012 mb1016 mb1020 mb1020 mbPGFPGFChange in P over large distance:SMALL PGFChange in P over small distance:LARGE PGF
32Pressure Gradient Force Summary: Change in P over large distance = small PGFChange in P over small distance = large PGFPGF is at right angles to isobarsCauses wind to START MOVINGHowever… two forces cause wind speed and direction to be different than predicted by the PGFCoriolis (rotation of the Earth)Friction
34Vertical Pressure Gradient In general higher pressures closer to the surface.Hydrostatic EquilibriumThe balance maintained between the force of gravity and the vertical pressure gradient that does not allow air to escape to space.If we combine the effects of vertical and horizontal pressure gradients we get circulation.SEA BREEZE is a great example
36Coriolis Force Results from the rotation of the Earth Causes the PGF to cross isobars NOT at right angles.Winds curve to the RIGHT in the Northern HemisphereWinds curve to the LEFT in the Southern Hemisphere
37Coriolis Force - Example On a non-rotating Earth, the rocket would travel straight to it’s target.Earth rotates 15 deg per hour….Even though the rock travels in STRAIGHT line, when we plot it’s path on the surface it follows a path that CURVES to the RIGHT!
38Coriolis Force – Earth’s Rotation Rotation is Clockwise in SHRotation is Counter Clockwise in NH
39Coriolis Force – Summary Always Deflects a moving body (wind) to the rightOnly affect wind direction, not speedIs affected by wind speed (the stronger the wind, the greater the deflecting force)Is strongest at the poles and nonexistent at the equator… latitude dependentThese two determine the MAGNITUDE of the Coriolis Force
41Friction Applied to wind within ~1.5 km of the surface Friction ALWAYS acts in the direction OPPOSITE the direction of motion!!!!Friction affect air at the surface more than air aloft.
42Winds Aloft and Geostrophic Flow Where friction doesn’t play a role!!When only the PGF and Coriolis Forces (Fc) affect an air parcel1020 mb1016 mb1012 mb1008mb1004 mb1000 mbPGFFcFcFcWINDFcDirection of MOTION!
43Winds Aloft and Geostrophic Flow An air parcel is at equilibrium only if PGF acts in the opposite direction to the Coriolis force (no net force).Therefore in Geostrophic Flow, winds run parallel to isobars in a straight pathDirection of MOTION!Coriolis, FcPGFWIND900 mb904 mb908 mb912 mb
44Curved Flow and Gradient Wind Gradient Wind – winds that follow curved paths around high and low pressure cells.Speed of the wind depends on how close the isobars areLHPGFCoriolisWind
45Adding in Friction to Coriolis and PGF Geostrophic Flow and FrictionCauses parcel to slow downCoriolis decreases in strengthFriction cases wind to lean towards the direction of the PGFDirection of MOTION!Coriolis, FcPGFFriction
46Adding in Friction to Coriolis and PGF The addition of friction causes the wind to lean toward the PGF force (or in the direction of the low pressure) in both hemispheres.Because the Coriolis Force pulls wind to the right in the NH and to the left in the SH we see opposite wind directions when comparing the NH to the SH.
47Surface Winds - Friction + Coriolis + PGF The addition of friction causes the wind to lean toward the PGF force (or in the direction of the low pressure) in both hemispheres.Because the Coriolis Force pulls wind to the right in the NH and to the left in the SH we see opposite wind directions when comparing the NH to the SH.
50Factors that Promote Vertical Airflow Friction – can cause convergence and divergenceWhen air moved from the smooth ocean to the “rough” land, the wind slows downResults convergence as air “pile up” upstream (like on a highway with construction).When air goes from land to ocean you see divergence and subsidence
51Factors that Promote Vertical Airflow Mountains – hinder the flow of airAs air passes over it is compressed vertically, causing divergence aloftAfter going over, onto the lee side, air experiences vertical expansion… causing horizontal convergence.
53Scales of Atmospheric Motion Time ScaleDistance ScaleExamplesMacroscalePlanetaryWeeks or longerkmWesterlies, trade windsSynopticDays to weekskmMid-latitude cyclones, anticyclones, hurricanesMesoscaleMinutes to hours1-100 kmThunderstorms, tornadoes, and land-sea breezeMicroscaleSeconds to minutes<1 kmTurbulence, dust devils and gusts
54Large and Small Scale Winds Macroscale WindsPlanetary: Westerlies, trade windsSynoptic: Cyclones and anti-cyclones, Hurricanes (weather map size)Mesoscale WindsThunder storms, tornadoes, etcPart of larger macroscale wind systems.Microscale WindsChatoic motions including gusts and dust devils
55Local Winds (mesoscale) True local winds are caused by topographic effects or variations in local surface compositionLand and Sea BreezesMountain and Valley BreezesChinook (Foehn Winds)Katabatic (Fall Winds)Country Breezes
56Land and Sea BreezesMost intense ones form along tropical coastlines adjacent to cool ocean currents.
58Lee side air is heated by compression Chinook (Foehn Winds)Warm Dry air moving down the east slopes of the Rockies (Chinook) or Alps (Foehn).Lee side air is heated by compression
59Local Chinook-like Wind Santa Ana WindsHot and dry winds increase the threat of fire in Southern California.Typically September to March but can happen at any time the desert is cooler than SoCal.
60Katabatic (Fall) Winds Originate when cold air, situated over a highland area (like an ice sheet) is set in motion.Gravity carries the cold air over the rim like a waterfall.The air is heated like a Chinook, but because it start so cold it stays cold.
61Country Breezes Associated with large urban areas Light wind blowing in from the countrysideClear, calm nightsCity is warmer (urban heat island)
62Global Circulation Single-Cell Model Three-Cell Model First idea George Hadley in 1735Solar energy drives the windsDoesn’t account for rotationThree-Cell ModelProposed in1920sEquator and 30 N (S)30 N (S) and 60 N (S)60 N (S) and 90 N (S)
63Single-Cell Model The equator is heated Rises Travels toward cold PolesAir cools and sinksTravels back to the equator
64Three-Cell Model – Hadley Cell Air rises at the equatorAir travels north and subsides between N (S) (Horse latitudes)From the center of the Horse Latitudes the surface flow splitsTrade Winds: equator-ward due to CoriolisWesterlies: Go towards the polesWhere the trade winds (N and S) meet is called the Doldrums. Light winds and humid conditions.
65Three-Cell Model – Ferrell Cell 30-60 N (S)More complicated than the Hadley cell.Net surface flow is toward the polesCoriolis bends them to the west….called Westerlies!More sporadic and less reliable than the trade windsMigration of cyclones and anti-cyclones disrupts the general westerly flow.
66Three-Cell Model – Polar Cell 60-90 N (S)Relatively little is known about the circulation at high (polar) latitudesSubsidence at the poles produces a surface flow that moves equatorward and is deflected by Coriolis into the Polar Easterlies.As cold air moves equatorward it meets with the warmer westerly flow and clashes forming the Polar Front.
67Observed distribution of Pressure and Winds Equatorial LowNear the equator the warm rising branch of the Hadley cells is associated with a low pressure zone.Ascending moist, hot air with lots of precipitationAlso referred to as the Intertropical Convergence Zone (ITCZ)
68Observed distribution of Pressure and Winds Subtropical HighsAt about N(S) where westerlies and trade winds originate (subsidence from aloft)Caused mainly by the Coriolis deflectionGenerally the rate at which air accumulates in the upper troposphere exceeds the rate at which the air descends to the surfaceThus they are called semi-permanent highs.
69Observed distribution of Pressure and Winds Subpolar LowAnother low-pressure region between corresponding to the polar frontResponsible for much of the stormy weather in the mid-latitudes
70Observed distribution of Pressure and Winds Polar HighsAt the poles, where the polar easterlies originateHigh pressure develops over the cold polar areas due to extreme surface cooling.Because the air near the poles is cold and dense it exerts a higher than average pressure.
71Down sloping air = No clouds MonsoonsA seasonal reversal in weather patternsAn alternation between two types of weather pattersEx: India – Wet hot summer, dry cool(ish) winterA seasonal reversal of wind alsoSUMMER MONSOONWINTER MONSOONHLHLDown sloping air = No cloudsHCOLDLLHHot Indian ContinentWarm OceanWarm Ocean