Upper Winds — Page 186, Lesson 159

I want to walk you through the topic of upper winds, starting with a look at the average upper winds at the 300 to 200 hectopascal level. Figure 11.9 in your materials shows this average pattern. There's an important seasonal note here: this general disposition of winds shifts about 15 degrees south in January and about 15 degrees north in July. So the whole wind pattern migrates with the seasons. Now, within this general flow, we can get local jets. These are narrow, fast currents of air that arise due to local thermal or dynamic circumstances. A specific example is the Somali jet, also called the Findlater jet, which occurs off East Africa. These are distinct from the larger jet streams we'll discuss. Speaking of other jets, the term 'jets' as opposed to 'jet streams' can refer to narrow, fast currents of air that exist at low level. So not all fast-moving air currents are the high-altitude jet streams—some can be found much closer to the ground. Let's talk about the direction and speed of jet streams. Their direction is generally westerly—meaning they flow from west to east. Maximum speeds occur near the tropopause, which is the boundary between the troposphere and the stratosphere. In Europe and the North Atlantic, speeds of 200 knots have been recorded. In Southeast Asia, speeds can reach 300 knots. So these are very powerful currents of air. Now, a critical operational concern for pilots is Clear Air Turbulence, abbreviated as TURB. Clear air turbulence occurs around the boundaries of jet streams. Why? Because of the large horizontal and vertical windshears present there. Windshear is a change in wind speed or direction over a short distance, and when it's large, it creates turbulence. Where is this turbulence strongest? It is strongest near to, or just below, the jet axis on the cold air side—that's the low pressure side. There is also a secondary area of turbulence above the axis. So if you're flying near a jet stream, the most turbulent area is typically just below the core on the cold side, with another turbulent zone above the core. Let's look at the movement of jet streams. Like most other weather phenomena, jet streams move with the sun. More specifically, subtropical jets are based on Hadley cells—these are large-scale atmospheric circulation patterns. The subtropical jet will move north during the northern summer as the heat equator moves north, and then it moves south during the northern winter. Polar front jets in the Northern Hemisphere behave differently. They move north as the polar front moves north in summer, and during this time their speed decreases. In winter, the polar front moves south, and because of the greater temperature difference between the air masses, the speed of the polar front jet increases. So winter brings stronger polar front jets. Now, how do we recognize jet streams? From the ground, when cloud amounts allow, jets may be recognized by wind-blown wisps of cirrus cloud blowing at right angles to the clouds at lower levels. So you might see high cirrus clouds streaming in a different direction than the lower clouds. In the air, the presence of a jet is difficult to see visually. But there are clues: temperature differences, increases in wind speed, drift, and clear air turbulence are all evidence that you're near a jet stream. On charts, jets can be picked out quite easily by inspecting upper wind charts. More graphically, you can look at a Significant Weather Chart, often called a SigWx chart. Figure 11.12 shows jet streams on a SigWx chart. Finally, let's talk about forecasting jet streams. Forecasting is largely a matter of producing charts from upper air soundings by radiosonde. A radiosonde is an instrument package carried aloft by a weather balloon that measures temperature, pressure, and humidity. Thickness charts were mentioned earlier as a means of establishing thermal wind patterns, but for forecasting, meteorologists use contour charts. Contour charts show lines of constant height of a pressure surface, which helps identify the wind patterns. In-flight reports of temperature and wind velocities from aircraft are a useful confirmation of the upper air soundings. Over oceans and deserts, where ground-based observations are sparse, these in-flight reports are vital supplements to the data.