Turbulence — Page 118, Lesson 103

Let’s start with the topic of low altitude windshear, which is a critical hazard during takeoff and landing. I want to walk you through the two main types: vertical windshear and horizontal windshear. First, vertical windshear is defined as the change in wind velocity with height. In other words, as you climb or descend through a layer of air, the wind speed and/or direction can change. This is typically measured in knots per 100 feet — so if the wind speed increases by, say, 10 knots over a 200-foot climb, that would be a vertical windshear of 5 knots per 100 feet. Now, horizontal windshear is the change in wind velocity with horizontal distance. This means as you fly forward across a given area, the wind speed or direction changes. It is typically measured in knots per 1000 feet — so a change of 15 knots over a 3,000-foot horizontal distance would be 5 knots per 1000 feet. The excerpt then tells us that the remainder of this chapter consists of a UK Civil Aviation Authority Aeronautical Information Circular covering low altitude windshear. Some of that material has already been covered earlier in the chapter, but the parts that are new should be highlighted. So we are essentially at the start of that supplementary material. To summarise: vertical windshear is measured in knots per 100 feet, horizontal windshear in knots per 1000 feet, and both describe a change in wind velocity — the vector quantity that includes both speed and direction. These are the two fundamental ways we classify windshear at low altitude.