Wind shear (speed shear) is found by comparing the wind speed at two pressure levels. Shear vorticity is found by comparing the wind at two locations at the same pressure level. Both produce an eddy of rotating air. The units for wind shear and shear vorticity are seconds to the negative 1. These units result from dividing the change in wind speed by the distance between the two locations. Below are two example problems for finding shear. The wind speed is 20 knots at 850 mb while the wind speed is 30 knots at 700 mb. The distance between 850 and 700 mb is 1,600 meters. What is the value of wind shear? The change in wind speed is 30 - 20 = 10 knots = 5 m/s The change is distance is 1,600 m The change in wind speed over distance = 5/1,600 s^-1 = 0.003125 s^-1 The calculation above is multiplied by 1000 to simplify the number. This will give a speed shear value of about 3 units. An operational interpretation of speed shear is given below. 0 to 3 weak, 4 to 5 moderate, 6 to 7 large, 8+ extreme Shear vorticity is found in a similar same way and has the same units. Shear vorticity is determined about a vertical axis. Here is an example problem: At location A at 500 mb the wind speed is 30 knots, at location B at 500 mb the wind speed is 50 knots. The distance between the two locations is 400 km. How many units of shear vorticity are produced between the two locations? The change in wind speed is 50 - 30 = 20 knots = 10 m/s The change in distance is 400,000 m The change is wind speed over distance = 10/400,000 = 0.000025 s^-1 One unit of vorticity is equal to 0.00001 s-1 or 1*10^-5 s^-1 This produces a shear vorticity value of 2.5 units. |