Upper Winds — Page 177, Lesson 155

I want to walk you through the topic of upper winds. These are the winds that blow at higher altitudes, well above the surface, and they behave a little differently from what you might be used to near the ground. Let’s start with the charts we use. Upper-wind charts are constant-pressure charts — they show a specific pressure level, like 500 hPa, and the heights at which that pressure is found. The heights shown on these contour charts are heights AMSL — that means Above Mean Sea Level. So when you see a contour line labelled 5,400 metres on a 500 hPa chart, that tells you the 500 hPa pressure surface is at 5,400 metres above sea level at that location. These charts give us valuable information about how pressure is changing over a distance. If the contour lines are closely spaced, we can assume a high pressure gradient exists — that means pressure changes rapidly over a short distance. If we can identify where the low pressure is, we can then discover what the strength and direction of the resulting wind will be. To find wind direction, we can use Buys Ballot’s Law. You may remember this from surface winds: with your back to the wind in the Northern Hemisphere, lower pressure is on your left. For upper winds, the same principle applies — the wind will blow parallel to the contour lines, just like surface winds blow parallel to isobars. So if you stand with your back to the upper wind in the Northern Hemisphere, the lower-value contour — effectively lower pressure — is on your left. Now, wind speed is proportional to the distance between the contour lines. Closer spacing means stronger wind. The wind we find from these charts is for the height of that particular constant pressure chart. For example, a 500 hPa chart corresponds to about 18,000 feet in ISA — the International Standard Atmosphere. Let me give you the standard pressure levels and their approximate flight levels. Charts are provided for: - 850 hPa — FL050 - 700 hPa — FL100 - 500 hPa — FL180 - 400 hPa — FL240 - 300 hPa — FL300 - 250 hPa — FL340 - 200 hPa — FL390 - 150 hPa — FL450 - 100 hPa — FL530 - 50 hPa — FL610 These are produced as spot wind and temperature charts, which we cover in Chapter 27. Next, let’s talk about isotachs. Isotachs are lines joining places of equal wind speed. On Figure 11.2, they are shown as red dashed lines. So if you see a red dashed line labelled 80 knots, every point along that line has a wind speed of 80 knots at that pressure level. Now, here’s a really important concept: thermal wind. The pressure changes that exist in the upper atmosphere — the ones that control our upper winds — are directly related to the temperature differences between air masses. Figure 11.3 shows this clearly. The temperature difference between two air masses dictates the pressure we find in the upper atmosphere. Because the pressure differences that produce the upper winds are created by surface temperature differences, the upper winds are referred to as thermal winds. Let me explain that mechanism. Cold air is denser, so the pressure drops off more quickly with height over cold air compared to warm air. This means that at a given pressure level — say 500 hPa — the height of that pressure surface is lower over cold air and higher over warm air. That creates a slope in the pressure surface, and that slope drives the wind. Let’s revisit Buys Ballot’s Law with this in mind. In Figure 11.3, assuming we are in the Northern Hemisphere, from Buys Ballot’s law the wind will be blowing into the diagram. We can now modify Buys Ballot’s law: If we stand with our back to the wind in the Northern Hemisphere, then low pressure, low temperature, or low altitude are on the left. In the Southern Hemisphere, they are on the right. We can deduce from this that upper winds will generally be westerly in both hemispheres. The exceptions to this, and the reasons, will be discussed later in jet streams and global climatology. To summarise: low surface temperatures lead to comparatively low pressure at altitude compared to high surface temperatures. This gives low altitude for a specified pressure level over cold air and higher altitudes over warm air. That temperature contrast is what drives the thermal wind.