Calculations for Vertical Wind Shear. Calculations for vertical shear are best done on a plotted Skew T, Log P Diagram. First, calculate the change in height (in feet) between plotted wind reports, using your height scale. You may enter these in pencil between your wind plots. Then, calculate the actual change in wind speed between your plotted reports and enter these values between the plotted reports. Multiplying your calculated change in wind speed by 1,000 (feet) and dividing the result by your calculated change in height will result in the vertical wind shear per 1,000 feet. This result may be compared with table 6-1-2 to determine the expected turbulence.

For flights outside your local area, you will not often have the convenience of a plotted Skew T or, in many cases, even an upper wind report. Comparison of the winds between standard constant pressure chart surfaces will result in a vertical shear value, but this value can be lower than may actually exist, because of averaging. Considering that you are evaluating layers 4,000 to 8,000 feet thick between standard constant pressure surfaces, a layer of turbulence may be easily hidden. If your calculation yields a shear value indicating turbulence may be present, say 8 knots per 1,000 feet (or moderate turbulence), then you can be certain that you have a layer of at least moderate turbulence, possibly greater, somewhere within the thick layer you have just evaluated.

FRONTAL TURBULENCE. Frontal tur-bulence is caused by both wind shear between the two air masses separated by the frontal zone and by the lift and mixing of air in the warmer air mass being forced aloft. Figure 6-1-4 depicts a typical frontal turbulence situation; in this case, a cold front.

Keep in mind that the vertical cross section greatly exaggerates the height, so the frontal slope appears very steep. A strong cold front would actually have a slope of about 1:50, or would slope toward the colder air 50 feet for every foot gain in altitude. Warm frontal slopes maybe as shallow as 1:300. With the wind in the warm air mass flowing directly into the page and the wind in the underlying cold air mass flowing directly out of the page, we have a pronounced vertical directional shear and a pronounced horizontal directional shear between the air masses.

Turbulence due to wind shears will be limited to the frontal transition zone, where the air from the two different air masses is mixing. Generally, faster-moving fronts and fronts separating air masses of great temperature differences will have strong temperature inversions marking the mixing zone and narrower mixing zones. Turbulence within the mixing zone can be calculated by taking the vector difference of the vertical winds between the warm air mass and the cooler air mass, and comparing this difference to the turbulence criteria in the vertical wind shear column of table 6-1-2.

When the transition zone is extremely narrow, as in a fast-moving cold front, the turbulence through the transition zone will be very brief for any aircraft transiting the front. However, the abrupt change in wind direction will be more of a hazard to the aircraft than the turbulence. This wind shear hazard will be discussed in more detail in the next section. The remainder of the turbulence experienced in the vicinity of frontal zones will be associated with convective activity or be the result of high winds producing low-level turbulence.

LOW-LEVEL TURBULENCE. The lowest 50 millibars of the atmosphere is topped by the boundary layer. Within this layer, the geostrophic winds are affected by friction as they interact with Earths surface. Progressing downward from the boundary layer to the surface, wind speeds decrease logarithmically and back directionally. Overland, we generally expect a 50-percent decrease in speed and a 30-degree backing in direction; while over water, we can expect a 70-percent reduction in speed and a 15-degree backing in direction from the boundary layer to the surface. The frictional increase when nearing the surface causes a mixing of air, as numerous vertical eddies are formed. These eddies produce wind shear, which is felt as turbulence by aircraft. The strength of the turbulence maybe calculated based on the type of terrain and the strength of the wind speed. See table 6-1-3 for the low-level turbulence threshold values.

MOUNTAIN WAVE TURBULENCE. The most severe type of terrain-induced wind shear turbulence is mountain wave turbulence. This turbulence occurs when the wind flow across a mountain is disturbed, creating eddy currents. Turbulence from mountain waves has been experienced at altitudes up to 40,000 feet. Even low mountains may create turbulence that can extend to a height 25 times that of the mountain. The intensity of the wave is a function of height, degree of slope of the mountain, and the strength of the wind.

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