Now, in the Southern Hemisphere, the Coriolis force acts in the opposite… — Page 159, Lesson 140

I want to walk you through some practical rules that meteorologists and pilots use to estimate surface wind from the upper wind, and then we'll look at how the wind changes throughout the day. Let's start with what I call the "rough rules" for surface wind over land in the Northern Hemisphere. On average, the surface wind over land is backed by 30 degrees from the geostrophic or gradient wind direction. Let me explain what "backed" means: if you stand facing the direction the wind is coming from, a backed wind has turned counterclockwise relative to the upper wind. So the surface wind direction is rotated 30 degrees to the left of the geostrophic wind direction. At the same time, its speed is reduced by 50% — so if the geostrophic wind above is 40 knots, the surface wind over land would be about 20 knots. Now, in the Southern Hemisphere, the Coriolis force acts in the opposite direction. So instead of backing, the surface wind is veered from the 2000-foot wind — that means it's turned clockwise relative to the upper wind. But the numerical values are the same: still a 30-degree change and a 50% speed reduction. Over the sea, friction is very much less than over land, so the surface winds are much closer to the geostrophic values. In the Northern Hemisphere, surface wind over the sea is backed by only 10 degrees from the geostrophic or gradient wind direction, and its speed is reduced to 70% of the upper wind speed. So if the geostrophic wind is 40 knots, the surface wind over the sea would be about 28 knots. Again, in the Southern Hemisphere, the surface wind will veer instead of back, but the same numerical values apply. Now let's move to a very important topic for flight planning: the diurnal variation of the surface wind. This is a regular change in the surface wind that occurs over each 24-hour period. During the day, the wind veers and increases, reaching its maximum strength around 1500 hours — that's 3 PM local time. Then it backs and decreases thereafter, with minimum strength occurring around 30 minutes after sunrise. What causes this daily cycle? It's due to thermal turbulence. During the day, the sun heats the ground, which warms the air near the surface. This warm air rises and mixes with the air moving freely above. So the surface air gets mixed with the stronger, faster-moving air from higher up, which increases the surface wind speed and changes its direction. This effect is most marked on clear sunny days, and particularly in unstable air masses with sunny days and clear nights. Now let's look at how the wind at 1500 feet and the surface wind change together throughout the day. I want you to picture two levels: the surface and 1500 feet. By day — especially on sunny days around 1500 hours — thermal currents are strongest. These thermal currents cause interaction between the surface and the top of the friction layer. As the 2000-foot wind descends toward the surface, it becomes increasingly affected by surface friction. So it steadily reduces in speed and turns toward the low pressure. In the Northern Hemisphere, that means it backs; in the Southern Hemisphere, it veers. By night, thermal currents cease. The top of the friction layer effectively drops below 1500 feet. So at 1500 feet, the wind now assumes the 2000-foot direction and speed — it becomes faster and, in the Northern Hemisphere, it veers. Meanwhile, the surface wind no longer has interaction with the stronger wind above, so it will decrease and, in the Northern Hemisphere, it backs. This creates a very important situation: a marked windshear can occur between 1500 feet and the surface. This windshear can significantly affect aircraft handling, for example during an approach to land. So to summarize the key terms: backing means the wind direction turns counterclockwise (to the left) over time or with decreasing height in the Northern Hemisphere; veering means it turns clockwise (to the right). The diurnal cycle gives us a predictable pattern of stronger, veered winds by day and weaker, backed winds at night, with the risk of windshear during the transition.