Thus, an eastbound aircraft may encounter the CAT as it enters a cirrus shield and a westbound flight conversely feels the turbulence on moving out of the cirrus. North and south bound flights may stay in this zone at a longer duration. Turbulence associated with jet level winds is a fleeting thing and over the well traveled lanes across the Pacific or Atlantic 5 aircraft may transverse a potent area without hardly a jolt while then all of the sudden, the sixth aircraft may experience quite a shaking. We will probably never be able to forecast with great certainty when and where CAT will occur, however we will know the areas of high probability of CAT, and therefore will be able to advise the captain of the aircraft when to put on the seat belt sign and perhaps avoid serving food or hot coffee during the crossing of these turbulence prone areas. The significant weather charts for upper levels attempt to delineate these areas but these areas are likely made larger than necessary and also may often miss areas prone to turbulence.
Most turbulence occurs with the frontal zones or zones of strong surface convergence.
If it was that simple, there would be less of a problem. However fronts are often not analyzed in the right location as the analyst may pay much more attention to curvature of flow than to horizontal shear in trying to locate frontal zones of maximum convergence in the boundary layer flow. This is particularly so when we compare satellite cloud features with analyzed fronts. Finding the right location of these lines of maximum convergence and cyclonic vorticity and deformation in the surface flow is therefore important in predicting where the CAT at upper levels will occur. Looking at the flow aloft near the level of non-divergence, about 500 mb, we also find values of zero vorticity and zero deformation and zero divergence in a deep middle level layer of the troposphere where intense rising motion and convection may occur.