First, the definition: wind is air in horizontal motion — Page 151, Lesson 128

I want to walk you through the opening of Chapter 10, which is all about winds. This is a fresh topic, so let's start from the very beginning. First, the definition: wind is air in horizontal motion. That's the core idea — we're talking about air moving sideways, not up or down. When we describe wind, we use something called Wind Velocity, abbreviated as W/V, and it has two components: direction and speed. Now, wind direction is always given as the direction from which the wind is blowing. That's a critical convention. For example, if the wind is coming from the north and heading south, we call it a north wind. It is normally given in degrees true — that is, measured clockwise from true north. However, when a pilot receives wind direction from Air Traffic Control, or ATC, it will be given in degrees magnetic, which is referenced to magnetic north instead. So you need to be aware of which reference is being used depending on the source. Let's look at Figure 10.1, which illustrates this concept of wind direction. Wind speed is usually given in knots. But some countries give the speed in metres per second, written as ms⁻¹. The Met Office, the Meteorological Office, often works internally in kilometres per hour, and on reports and forecasts you'll see that shown as KMH. So you may encounter any of these units depending on the context. Now, there's a standard way to represent wind on a chart using a wind vector. On this vector, the wind direction is from the feathers to the point, and that point indicates the location where the wind is being measured. Figure 10.2 shows an example: a wind of 240° true at 125 knots. So the feathers are at the tail end, and the arrow points in the direction the wind is blowing toward. And by convention, the feathers always point towards the low pressure. That's a useful rule — if you see a wind barb on a chart, the side with the feathers is pointing toward lower pressure. Next, we have two important terms for how wind direction changes. Veering is a change of wind direction in a clockwise direction. Backing is a change of wind direction in an anticlockwise direction. And this applies in both hemispheres — north and south. So if you're watching the wind shift from west to northwest, that's a veer. If it shifts from west to southwest, that's a back. Figure 10.3 illustrates both veering and backing. That covers the fundamental definitions for wind direction, speed, and how we describe changes. We've set the stage for the rest of the chapter, which will go into gradient wind, antitriptic wind, and many other specific wind types.