The Geostrophic WindPoint 1: Air parcel initially motionless. Flip switch to “turn on” laws of physics. Parcel begins to accelerate toward lower pressure due to PGFPoint 2: As PGF accelerates air parcel, the parcel begins to be deflected to its right by Coriolis forcePoints 3 and 4: As air parcel continues to speed up, Coriolis force grows in magnitude. PGF is constant (determined by spacing between isobars)Point 5: Eventually air parcel deflected enough so it is moving parallel to height contours. At this point, PGF and Coriolis forces will be equal and oppositeWind called the geostrophic windWinds aloft blow parallel to height contours (or isobars)Pressure gradient force balances Coriolis forceSame magnitude, acts in opposite directionNet force (acceleration) on air is zeroAssumes straight isobars (or height contours)Stronger PGF = Faster windsTighter contour lines = Faster windsDetermining Wind Direction (in the Northern Hemisphere)If the wind hits you in the back, low heights (low pressures) are on your leftUpper-level flow onlyNorthern Hemisphere onlyAnalyzing Upper-Air MapsGeostrophic wind valid for straight flow aloft:First, determine areas of high heights and areas of low heightsAt each point, the PGF acts perpendicular to the height lines, toward low heightsGeostrophic wind blows parallel to height lines such that if the wind hits you in back, low heights are on your LEFTCoriolis force acts perpendicular and to right of wind direction (should point in opposite direction of PGF)Circular MotionNewton’s second law: Force = mass x accelerationAcceleration is any change in speed or directionScientifically, breaking a car is acceleration even though it is slowing downSpeed and direction together are called velocityFor circular motion, object is constantly changing direction (accelerating), even if speed is constantNet force required to maintain circular motion called centripetal forceDirected inward toward center of circleFor curved flow, PGF and Coriolis forces cannot balance (add to zero)Balanced forces à No accelerationOne force must be greater than the other because there must be a net inward force to change directionCurved Flow Around Lows AloftPGF directed inward toward center of low (red arrow)Coriolis force directed to right of winds; outward (blue arrow)For curved flow, must have net force directed inward toward center of lowCentripetal forceKeeps flow in circlePGF must be greater than Coriolis force around lowsCurved Flow Around Highs AloftPGF directed outward from center of high (red arrow)Coriolis force directed to right of winds; inward (blue arrow)For curved flow, must have net force directed inward toward center of highCentripetal forceKeeps flow in circleCoriolis force must be greater than PGF around highsCurved Flow Aloft: Gradient Wind BalanceCalled gradient wind balance (although PGF, Coriolis forces not balanced!)Lows: PGF > CoriolisHighs: Coriolis > PGFMust have these force imbalances or flow would not go in a circleNeed net force inward toward center of circleRecall that PGF determined by height gradientCoriolis force determined by wind speedCan conclude that, for a given PGF, wind speed will be faster around highs and slower around lowsUpper-Level MapsWinds blow parallel to contour linesContour lines typically wavy, oriented generally west-to-eastZonal flow – Westerly flowMeridional flow – Northerly or southerly flowWarmer air (high heights) typically found in TropicsColder air (low heights) typically found near polesNomenclatureCyclones: Low pressure systemsNorthern Hemisphere: Flow counter-clockwiseSouthern Hemisphere: Flow clockwiseCyclonic flowAnticyclones: High pressure systemsNH: Flow clockwiseSH: Flow counter-clockwiseAnticyclonic flowSurface WindsGenerally slower than upper-level windsCross isobars at an angleWind does NOT flow parallel to isobars like upper-level windsWhy?? FRICTION!Friction opposes motion, slowing the winds, and weakening the Coriolis forceDisrupts force balances seen with upper-level flowInitially, assume winds in geostrophic balanceAdd frictionSlows windsWeakens Coriolis forceNet force in direction of PGFWind deflected in direction of PGFToward low pressureCoriolis force always perpendicular and to right of windFriction opposes windResults in three way balance between PGF, Coriolis, frictional forcesNorthern HemisphereLows: Winds spiral inward, counter-clockwise rotationHighs: Winds spiral outward, clockwise rotationSouthern HemisphereLows: Winds spiral inward, clockwise rotationHighs: Winds spiral outward, counter-clockwise rotationSurface winds ALWAYS cross isobars at an angle from high to low pressureSURFACE WINDS DO NOT BLOW PARALLEL TO ISOBARS!!!!Upper-Level and Surface Winds 10/08/2012The Geostrophic Wind-Point 1: Air parcel initially motionless. Flip switch to “turn on” laws of physics. Parcel begins to accelerate toward lower pressure due to PGF-Point 2: As PGF accelerates air parcel, the parcel begins to be deflected to its right by Coriolis force-Points 3 and 4: As air parcel continues to speed up, Coriolis force grows in magnitude. PGF is constant (determined by spacing between isobars)-Point 5: Eventually air parcel deflected enough so it is moving parallel to height contours. At this point, PGF and Coriolis forces will be equal and opposite-Wind called the geostrophic wind-Winds aloft blow parallel to height contours (or isobars)-Pressure gradient force balances Coriolis forceoSame magnitude, acts in opposite direction oNet force (acceleration) on air is zerooAssumes straight isobars (or height contours)-Stronger PGF = Faster winds-Tighter contour lines = Faster windsDetermining Wind Direction (in the Northern Hemisphere)If the wind hits you in the back, low heights (low pressures) are on your left-Upper-level flow only-Northern Hemisphere onlyAnalyzing Upper-Air MapsGeostrophic wind valid for straight flow aloft:-First, determine areas of high heights and areas of low heights-At each point, the PGF acts perpendicular to the height lines, toward low heights-Geostrophic wind blows parallel to height lines such that if the wind hits you in back, low heights are on your LEFT-Coriolis force acts perpendicular and to right of wind direction (should point in opposite direction of PGF)Circular Motion-Newton’s second law: Force = mass x acceleration-Acceleration is any change in speed or directionoScientifically, breaking a car is