So what happens — Page 159, Lesson 137

I want to walk you through how the geostrophic wind is actually constructed, and then we'll move into the gradient wind. Let's start with the Northern Hemisphere, because that's what the diagram shows us. Imagine a parcel of air that starts being accelerated from high pressure toward low pressure. As soon as that air begins to move, the Earth's rotation introduces a force called the Coriolis force. Now, the key here is that as the wind speed increases—because it's being pulled toward the low pressure—the strength of the Coriolis force also increases. The Coriolis force always acts perpendicular to the direction of motion, and in the Northern Hemisphere it deflects the wind to the right. So what happens? The wind doesn't just keep going straight into the low. Instead, it gets deflected more and more to the right as it speeds up. Eventually, the pressure gradient force (PGF) pushing toward low pressure and the Coriolis force pushing in the opposite direction become exactly balanced. At that point, the two forces are acting opposite each other, and the wind is no longer accelerating across the isobars—it now blows parallel to the isobars. That's the geostrophic wind. There's a handy rule for the Northern Hemisphere: if you stand with your back to the wind, the low pressure is on your left. That's a quick way to check the relationship between wind direction and pressure distribution. Now, for the wind to actually be geostrophic, several conditions must be met. First, it has to occur above the friction layer—that's the layer near the Earth's surface where terrain and obstacles slow the wind down. Second, the latitude must be greater than 15 degrees; closer to the equator, the Coriolis force is too weak to balance the pressure gradient. Third, the pressure situation must not be changing rapidly—we need a steady state. And fourth, the isobars must be straight and parallel; if they're curved, we get a different wind, which we'll talk about next. One more important point about the geostrophic wind: it can apply at all heights above the friction layer. But as you go higher, the wind speed should increase. Why? Because the density of the air decreases with height. Assuming all other factors like the pressure gradient remain unchanged, lower density means the same pressure difference produces a faster wind. Now let's move to the gradient wind. The gradient wind is what happens when the isobars are curved instead of straight. When the isobars curve, a third force comes into play—centrifugal force—which makes the wind follow a curved path while still blowing parallel to the isobars. So the gradient wind is the wind that blows parallel to curved isobars, and it results from a combination of three forces: the pressure gradient force (PGF), the Coriolis force (CF), and centrifugal force. Let me define centrifugal force precisely. Centrifugal force is the force acting perpendicular to the direction of rotation and away from the centre of rotation. So if you're going around a curve, this force tries to pull you outward. Now consider a depression—that's a low-pressure system. If air is moving steadily around a depression, the centrifugal force opposes the pressure gradient force. Think about it: the PGF is trying to pull air inward toward the low centre, but centrifugal force is trying to fling it outward. Because these two forces oppose each other, the net inward force is reduced, and therefore the wind speed is lower than it would be if the isobars were straight. Here's the practical consequence: the gradient wind speed around a depression is less than the geostrophic wind for the same isobar spacing. So if you take a Geostrophic Wind Scale—that's a tool used to estimate wind speed from isobar spacing on a weather chart—and apply it to curved isobars around a depression, it will overread. It will give you a wind speed that's too high, because it doesn't account for the centrifugal force slowing things down.