Wind is generated by pressure differences between high and low pressure… — Page 151, Lesson 131

I want to walk you through the topic of winds, starting with the fundamental idea of what wind actually is. Wind is generated by pressure differences between high and low pressure systems. Those differences give rise to what we call the pressure gradient force, or PGF — and that force is simply the change of pressure over distance. The PGF acts directly from high pressure toward low pressure. Now, the spacing of the isobars on a weather chart determines the magnitude of that force. The closer together the isobars are, the greater the pressure difference over that distance, and therefore the greater the PGF and the stronger the wind speed. Widely spaced isobars mean a smaller pressure gradient and lighter winds. There's an important observational rule called Buys Ballot's Law. It tells us that if we stand with our back to the wind in the Northern Hemisphere, low pressure is on our left — and in the Southern Hemisphere, low pressure is on our right. This law implies something crucial: the wind does not flow directly from high pressure to low pressure. Instead, it flows nearly parallel to the isobars. If you examine an analysis chart, you'll see that surface wind does indeed flow nearly parallel to the isobars, and above the friction layer, the wind generally flows exactly parallel to the isobars. That brings us to two model winds we need to consider: the geostrophic wind and the gradient wind. Let's focus on the geostrophic wind first. Like any theorized or model wind, the geostrophic wind requires a number of assumptions to reduce the complexity of reality and make the model simpler. The assumptions are these: the geostrophic wind is said to have only two forces, and those two forces must be working opposite to each other and in balance. Those two forces are the pressure gradient force and the Coriolis force. Let's look at the pressure gradient force in more detail. The PGF acts from high pressure to low pressure. We can see the strength of this force by studying the spacing between isobars. Closely spaced isobars indicate a large pressure gradient force — this is common in low pressure systems. Widely spaced isobars indicate a small pressure gradient force — this is common in high pressure systems. The PGF controls the wind speed. A large pressure gradient force creates strong winds, whereas a small pressure gradient force creates light winds. Wind speed is directly proportional to the pressure gradient force — that's a direct relationship. There's a practical tool called the Geostrophic Wind Scale, or GWS. Using it, you take the distance between two isobars, and reading from left to right, you measure the geostrophic wind speed using the scale shown at the bottom of the diagram. You'll notice that the wider the spacing of the isobars, the lighter the wind. Now let's look at the second force: the Coriolis force, or CF. The Coriolis force is caused by the rotation of the Earth. It acts at 90 degrees to the wind direction, causing air to turn to the right — or veer — in the Northern Hemisphere, and to the left — or back — in the Southern Hemisphere. The Coriolis force is maximum at the poles and minimum at the Equator. So in the geostrophic wind model, these two forces — the pressure gradient force pushing from high to low, and the Coriolis force deflecting the air at a right angle — work opposite each other and reach a balance. That balance is what produces a wind that flows parallel to the isobars, not directly across them.