Aviation Weather: Jet Stream It is one of the most dynamic of weather phenomenon on Earth and one of the most misunderstood. The Jet Stream is normally described as a river of high speed winds, far above the Earth's surface, which play a major factors in determining whether you will play golf, sing in the rain, or fly. So what is the jet stream and how does it effect aviation? Contrary to what some television graphics would have you believe, the jet stream is not a continuous river of air meandering across the globe. Instead, [Sticking with the river analogy] it is a series of rapids, comprised of 50 knot and greater winds, connected by stretches of calmer waters, as the entire mass flows from west to east through the upper atmosphere. Each of the areas of strong winds align themselves with weaknesses in the atmosphere just as we saw with some of the surface features. And rarely, these jets can merge to form winding streams thousands of miles in length.  Let's take a look at the April 3, 2000 jet stream chart above. North America actually has two major jet features. In the northwest we see a jet with winds from 50 to 125 knots. It is pulling in moist cool air from the northern pacific and some dry polar air from the polar regions. This would be an example of the northern or Polar Jet Stream often referred to by television meteorologists. Strong jets plunging from Canada southward into the states are often the harbinger of cold weather outbreaks in southern states. The strongest jet on this chart starts over the tropical Pacific waters off Mexico before sweeping across the Gulf States, the Ohio Valley and the Northeast. The highest winds are over 170 knots passing over the Chicago area. Jets pulling from these warmer waters are normally referred to as the Sub-Tropical Jet Stream. But how do these rivers of high winds form? If you remember from our review of general upper-level wind patterns, we saw that pressure maps use the height of a said pressure level, say 200 mb, to show the difference between high and low pressure areas. If we were to look at the height of the tropopause, that boundary between the troposphere and stratosphere, from the equator to the poles, we would see a similar pattern. The height of the tropopause at the equator may be near 60,000 feet with the height at the poles only 20,000 feet. If we could look down upon the tropopause from space we would not see a slide-like curved downward slope to the poles. Instead we would see a stairway like tropopause. Each of these steps, mark a major break in the tropopause. ; Over North America, we tend to find two such breaks as quasi-permanent features. These are the polar and sub-tropical jets mentioned above. The mechanism which drives these breaks is a major temperature difference over a relatively short area. The photo above illustrates this break in the tropopause and the oval nature of the jets themselves. Basically, the break in the tropopause allows the differing air masses to mix. This leads to a venturi effect which accelerates the air in the core of the jet to the speeds noted on jet stream charts. As you move out from the core of the jet, the winds diminish in intensity. The reason we tend to hear more about the jet stream in winter is that this is the season which provides the greatest contrast in temperatures between the pole and the equator. Obviously, jet stream winds and their formative environs can have a significant effect on aviation. These effects run the gamut from a welcomed west to east speed boost to clear air turbulence severe enough to do major damage to an aircraft. These factors and others will be elaborated on as we begin our study of specific weather impacts on aviation in the next section. Here are two Jet Stream map sites centered on North America. The first is the source of the top figure on this page. The second is similar but overlays the jet stream information on a contemporary satellite photo. If you are looking for the
locations of various North American jets today, click North American Jets. For a look at the
jets overlayed on the North American Satellite image,
click North American Jets with Satellite. (For Flight Simulation Use Only) |
