Calculations For Horizontal Wind Shear. The calculations for horizontal shear may be done relatively quickly on plotted constant pressure charts using a set of dividers. Set your dividers for a one-and-a-half-degree-of-latitude spread at the latitude you are investigating. From the plotted wind reports or the isotach lines (20-knot interval), estimate the change in wind speed across the spread of the dividers and relate the change to the turbulence intensity in table 6-1-2. A second method of delineating an area of tur-bulence uses the spacing between the analyzed isotachs (20-knot interval). A spacing between isotachs of more than 1.2 degrees of latitude means that the wind speed change is less than 25 knots per 90 nautical miles, or that the turbulence is light. A spacing between 1.2 degrees of latitude and 0.9 degree of latitude means the wind speed

change is from 25 knots to 50 knots per 90 nautical miles, or that the turbulence is moderate. A spacing between 0.9 and 0.6 degree of latitude indicates that the horizontal shear is from 50 to 90 knots per 90 nautical miles, or that the turbulence in that area is severe. Spacing of less than 0.6 degree of latitude indicates extreme tur-bulence. While these methods work well with the automated NWS Upper Air Analysis charts and most manually plotted upper air analysis charts, they do not work well on the computer-derived upper air products from FNOC, because of the 5-degree spacing between wind plots and the lack of isotachs.

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