WIND SPEED (VELOCITY) SHEARConvergence is occurring when wind speeds aredecreasing downstream; that is, mass is accumulatingupstream. Conversely, divergence is occurring whenwind speeds are increasing downstream; that is, mass isbeing depleted upstream.DIRECTIONAL AND SPEED WIND SHEARWind speed in relation to the wind direction is alsoa valuable indicator. For example, on a streamlineanalysis chart we can analyze both wind direction andwind speed, variations in wind speed along thestreamlines, or the convergence or divergence of thestreamlines.The following are some of the combinations orvariations of wind speed and direction:. In a field of parallel streamlines (wind flow), ifthe wind speed is decreasing downstream (producing anet inflow of air for the layer), convergence is takingplace. If the flow is increasing downstream (a netoutflow of air from the layer), divergence is occurring.. In an area of uniform wind speed along thestreamlines, if the streamlines diverge (fan out),divergence is occurring; if the streamlines converge(come together), convergence is taking place.. Normally, the convergence and divergencecomponents are combined. The fact that streamlinesconverge or diverge does not necessarily indicateconvergence or divergence. We must also consider thewind speeds— whether they are increasing ordecreasing downstream in relation to whether thestreamlines are spreading out or coming together.. If, when looking downstream on the streamlines,the wind speed increases and the streamlines diverge,divergence is taking place. On the other hand, if thewind speed decreases downstream and the streamlinescome together, convergence is taking place.There are other situations where it is more difficultto determine whether divergence or convergence isoccurring, such as when the wind speed decreasesdownstream and the wind flow diverges, as well as whenwind speed increases downstream and the wind flowconverges. A special evaluation then must be made todetermine the net inflow or outflow.DIVERGENCE AND CONVERGENCE(COMPLEX MOTIONS)In this section we will be discussing high-levelconvergence and divergence in relation to downstreamcontour patterns and the associated advection patterns.Low tropospheric advection (and also stratosphericadvection) certainly play a large role in pressure changemechanisms.Since the term divergence is meant to denotedepletion of mass, while convergence is meant to denoteaccumulation of mass, the forecaster is concerned withthe mass divergence or mass convergence in estimatingpressure or height changes. Mass divergence in theentire column of air produces pressure or height falls,while mass convergence in the entire column of airproduces pressure or height rises at the base of thecolumn.Mass Divergence and Mass ConvergenceMass divergence and mass convergence involve thedensity field as well as the velocity field. However, themass divergence and mass convergence of theatmosphere are believed to be largely stratified into twolayers as follows:l Below about 600 hPa, velocity divergence andconvergence occur chiefly in the friction layer, which isabout one-eighth of the weight of the 1,000-to 600-hPaadvection stratum, and may be disregarded incomparison with density transport in estimating thecontribution to the pressure change by the advectionstratum.. Above 600 hPa, mass divergence andconvergence largely result from horizontal divergenceand convergence of velocity. However, on occasion,stratospheric advection of density may be a modifyingfactor.The stratum below the 400-hPa level may beregarded as the ADVECTION stratum, while thestratum above the 400-mb level maybe regarded as thesignificant horizontal divergence or convergencestratum. Also, the advection stratum maybe thought ofas the zone in which compensation of the dynamiceffects of the upper stratum occurs.The Isopycnic LevelAt about 8km (26,000 ft) the density is nearlyconstant. This level, which is near the 350-hPa pressuresurface, is called the isopycnic level. This level is the1-2