Once you have calculated the gust speed, use the plotted wind speeds and height scale to find the mean wind direction in the 10,000 foot to 14,000 foot AGL layer. Use this direction as the direction for the maximum gust. Remember, a thunderstorm downrush, with its associated gusts, spreads out in all directions under the thunderstorm base. Your maximum gust direction should generally be in the same direction the storm is moving. When calculating a maximum gust direction in a frontal thunderstorm, you should consider the 10,000- to 14,000-foot wind as representative even if the thunderstorm is occur-ring on the cool side of the surface position of the front. The slope of both warm and cold fronts allows either a warm sector sounding or a pre-warm frontal sounding to be representative for the 10,000- to 14,000-foot winds.

The following is the modified Eastern Airlines method of calculating expected turbulence within

a convective cloud, using a plotted Skew T, Log P Diagram. The resulting turbulence criteria are based on a medium-size, twin-engine, fixed-wing aircraft (DC-3/C-117/C-47, length 65 feet, wing span 95 feet) and must be adjusted subjectively for other aircraft. You may wish to follow along on figure 6-2-17 as you read through the computa-tion procedure.

1. Divide the plotted atmosphere into two layers at the 9,000-foot MSL height.

2. Plot the forecast maximum temperature (FMT) on the surface level.

3. From the convective condensation level (CCL), descend dry adiabatically to the surface to locate the convective temperature (CT), and have the forecaster adjust CT according to local-objective techniques and expected insolation.

4. Subtract 11C from FMT to give a new temperature, which we will designate T₃

5. Find the intersection of the T₃ isotherm and the dry adiabat projected upward from FMT.

If the intersection occurs above 9,000 feet, no turbulence is expected below 9,000 feet. Go to step 7.

6. Draw a moist adiabat from the intersection upward to the 9,000-foot level. The temperature difference between this moist adiabat and the ambient temperature determines the severity of the turbulence that should be expected within convective clouds. Use the following guideline:

7. For the layer above 9,000 feet, project a moist adiabat upward from the CCL to the 400-millibar level. The maximum temperature difference between the moist adiabat and the free air temperature is the most turbulent layer. turbulence will be classified as follows:

8. If two criteria overlap near the 9,000-foot level, use the greater turbulence.

9. If the CCL is above 9,000 feet, evaluate turbulence from CCL upwards only.

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