What is the Jet StreamThe jet stream is a river of fast moving air in the upper atmosphereExists near tropopause levelAbout 10 km (6 miles) above sea level300-200 mbFlow is geostrophic (parallel to height contours)Flow is not the same speed everywhereReview of PressureRecall that pressure is force (weight) per unit areaMore force over same area yields greater pressureWeight is a forceMore air molecules over a point yield higher pressureLess air molecules over a point yield lower pressureThere are more air molecules above a surface high than above a surface lowChanging the Surface PressureTo change the surface pressure, a column of air must add or lose air moleculesThis changes the weight of the column and hence the pressureAdd molecules à Convergence of airIncreases pressureLose molecules à Divergence of airDecreases pressureConvergence and DivergenceUsually opposite between surface and aloftUpper-air divergence generally above surface convergence and vice versaIn, up, and outIn, down, and outNet divergence:DIV > CONVNet divergence decreases surface pressureSubtracts molecules from air columnLows get lower (strengthen)Highs get lower (weaken)Net convergence:CONV > DIVNet convergence increases surface pressureAdds weight to air columnLows get higher (weaken)Highs get higher (strengthen)Recall that at the surface, air flows counter-clockwise and inward toward the center of lowsSurface convergenceIncreases pressureLows strengthen when their surface pressure decreasesRequires divergenceDivergence required to develop a surface low cannot be caused by surface low; must look aloftHow to diagnose regions of convergence and divergence aloft???Wind speeds matter!!Can regions of slower and faster flow aloft cause convergence or divergence?YES!Traffic jam example: http://www.youtube.com/watch?v=DKJIkYarp5EWhen faster flow slows down, convergence resultsWhen slower flow speeds up, divergence resultsRemember gradient wind balance!Recall that flow aloft is faster than geostrophic around ridges aloftFlow aloft slower than geostrophic around troughs aloftWhen air flows from a ridge to a trough, it slows downGenerates convergence aloftWhen air flows from a trough to a ridge, it speeds upGenerates divergence aloftLook downstream (EAST) of troughs in jet stream to find upper-level divergence!!This is where surface lows will form!Surface Low Pressure SystemsDivergence aloft causes surface pressures to fallAir converges into low pressure systems at the surfaceCounter-clockwise and inward flow in NHAir rises, cools, water vapor condenses, forming clouds and precipitationExpect surface low to be EAST of upper-air trough axisSurface High Pressure SystemsConvergence aloft causes surface pressures to riseAir diverges from high pressure systems at the surfaceClockwise and outward flow in NHAir sinks, warms, dries out, inhibiting cloud formationMid-Latitude CyclonesHow do mid-latitude cyclones (low pressure systems) form?Convergence and divergence are key!Net divergence – Net decrease of air molecules over a point on the surfaceSurface pressure decreasesHow Does This Relate to the Surface PatternWhere would you expect to find a surface low pressure system?East (downstream) of an upper-level troughDivergence aloft, surface pressure falls (decreases)A surface high pressure system?East of an upper-level ridgeConvergence aloft, surface pressure rises (increases)Surface lows cannot strengthen directly beneath upper-level troughsNo divergence aloftStrengthening and WeakeningStrengthening Low: Pressure dropsNet divergence (DIV Aloft > CONV Sfc)Weakening Low: Pressure risesNet convergence (CONV Sfc > DIV Aloft)Strengthening High: Pressure risesNet convergence (CONV Aloft > DIV Sfc)Weakening High: Pressure dropsNet divergence (DIV Sfc > CONV Aloft)Vertical MotionVertical motions usually smallSteady rain: cm/s (about 1 mile/day)Faster rising air = More interesting weatherStrong upward-directed PGF balanced by gravityCalled hydrostatic balanceWorks well most of the timeDoes not apply to thunderstorms, tornadoesHydrostatic BalanceMid-Latitude Cyclones: Development 10/17/2012What is the Jet Stream-The jet stream is a river of fast moving air in the upper atmosphere-Exists near tropopause leveloAbout 10 km (6 miles) above sea levelo300-200 mb-Flow is geostrophic (parallel to height contours)-Flow is not the same speed everywhereReview of Pressure -Recall that pressure is force (weight) per unit area-More force over same area yields greater pressureoWeight is a force-More air molecules over a point yield higher pressure-Less air molecules over a point yield lower pressure-There are more air molecules above a surface high than above a surface lowChanging the Surface Pressure-To change the surface pressure, a column of air must add or lose air moleculesoThis changes the weight of the column and hence the pressure-Add molecules à Convergence of airoIncreases pressure-Lose molecules à Divergence of airoDecreases pressureConvergence and Divergence-Usually opposite between surface and aloftoUpper-air divergence generally above surface convergence and viceversaoIn, up, and outoIn, down, and out-Net divergence:oDIV > CONVoNet divergence decreases surface pressureSubtracts molecules from air columnLows get lower (strengthen)Highs get lower (weaken)-Net convergence:oCONV > DIVoNet convergence increases surface pressureAdds weight to air columnLows get higher (weaken)Highs get higher (strengthen)-Recall that at the surface, air flows counter-clockwise and inward toward the center of lowsoSurface convergenceoIncreases pressure-Lows strengthen when their surface pressure decreasesoRequires divergenceoDivergence required to develop a surface low cannot be caused by surface low; must look aloft-How to diagnose regions of convergence and divergence aloft???oWind speeds matter!!-Can regions of slower and faster flow aloft cause convergence or divergence?oYES!oTraffic jam example: http://www.youtube.com/watch?v=DKJIkYarp5E-When faster flow slows down, convergence results-When slower flow speeds up, divergence results-Remember gradient wind balance!-Recall that flow aloft is faster than geostrophic around ridges aloft-Flow aloft slower than geostrophic around troughs aloft-When air flows from a ridge to a trough, it slows downoGenerates convergence aloft-When air flows from a trough to a ridge, it speeds upoGenerates divergence aloft-Look