In a high, both forces point outward, so they add together — Page 159, Lesson 139

I want to walk you through the gradient wind in a high-pressure system, then the antitriptic wind, and finally how friction affects surface winds. Let's start with the gradient wind in an anticyclone — that's a high-pressure system. In an anticyclone, the centrifugal force acts in the same direction as the Pressure Gradient Force, or PGF. Remember, the Pressure Gradient Force is the force that pushes air from high pressure toward low pressure. Centrifugal force is the apparent outward force you feel when moving along a curved path. In a high, both forces point outward, so they add together. That means the centrifugal force increases the magnitude of the PGF. As a result, the wind speed in a high-pressure system will be greater than the equivalent geostrophic wind speed. Let me be precise about that comparison. The gradient wind speed around an anticyclone is greater than the geostrophic wind for the same isobar interval. So if you take the Geostrophic Wind Scale — often abbreviated GWS — and use it to estimate wind speed in a high, it will underread. It will give you a value that's too low. Here's a concrete example from the book. In a system where the radius of curvature of the isobars is 500 nautical miles, and the geostrophic wind speed is 40 knots, the actual gradient wind speed in a cyclonic system — that's a low-pressure system — will be 34 knots. But in an anticyclonic system — a high — it will be 58 knots. So the same geostrophic reference of 40 knots gives you 34 knots in a low and 58 knots in a high. That's a significant difference. Now, I want to make sure you don't confuse this. When we discuss the gradient wind, we are making two separate comparisons. First, we compare the wind in a low-pressure system to the equivalent geostrophic wind. Second, as a separate argument, we compare the wind in a high-pressure system to the equivalent geostrophic wind. We are not comparing the wind speed in a low-pressure system directly with the wind speed in a high-pressure system. Each is compared to its own geostrophic reference. Next, let's look at the antitriptic wind. The antitriptic wind is the wind that blows in low latitudes where the Coriolis Force, or CF, is very small. Coriolis Force is the apparent deflection caused by the Earth's rotation, and it's weak near the equator. So in those low-latitude regions, the wind behaves differently. That's the antitriptic wind. Now, let's move to winds below about 2000 to 3000 feet — that's roughly 1 kilometre. Friction between the moving air and the land surface reduces wind speed near the ground. This reduction in wind speed also reduces the Coriolis Force. Why? Because CF depends on wind speed — slower wind means weaker CF. This causes the two forces that balance in the geostrophic wind — the Pressure Gradient Force and the Coriolis Force — to become out of balance. Now, CF is less than PGF. The wind in this situation is called the surface wind. Since surface friction has reduced the wind velocity, resulting in a reduction in the Coriolis Force, the PGF is now more dominant. This causes the wind to blow across the isobars towards the low pressure. So instead of flowing parallel to the isobars like the geostrophic wind does above the friction layer, the surface wind crosses the isobars at an angle, always turning toward the low. Let me recap the key points. In a high-pressure system, the gradient wind is faster than the geostrophic wind for the same isobar spacing, so the Geostrophic Wind Scale underreads. The antitriptic wind occurs in low latitudes where Coriolis Force is very small. And surface winds, below about 2000 to 3000 feet, are slowed by friction, which reduces the Coriolis Force, allowing the wind to blow across the isobars toward low pressure.