So far we have covered the specific weather conditions at the airport we plan to leave from, the same for the airport we plan to arrive at and any airports in between. All to begin to paint a picture of conditions enroute. Then we looked at wide-scale surface conditions to get a better understanding of the weather conditions over the entire flight region. So we should be ready to crank up the engine? Right? No way. You see we have only looked at the surface conditions. To try and draw a complete understanding of the weather along our route, based on this limited knowledge, would be like walking into the ground floor of the World Trade Center, taking a look around, and then claiming to know what is happening on every floor above. METARs and surface conditions might be all you need if you plan to taxi between the two airports. But to fly there, we need to know what is happening on all those other floors above us.  This diagram shows you the basic layout of the atmosphere and as you can see it is a fairly complex layer of gases. Luckily, we only have to deal with the lowest layer, the Troposphere, the layer which provides the habitat for life to exist on this planet. When we think of Mount Everest at over 29,000 feet or the Concorde cruising at 50,000 feet, we tend to think how high that is. But in truth, everything we consider the sky lies in that small bottom region. Ninety-percent of the precious oxygen and other gases we need for survival lies in the troposphere and very low regions of the Stratosphere. The area between those two adjoining layers is called the Tropopause. Our study deals with only the atmosphere from the surface to the tropopause. If we wished to, we could draw another chart, just like the one above, but this time only dealing with the troposphere. While we could use height measurements once again, it would be better to use pressure readings in millibars(mb). While Americans tend to still cling to inches of mercury for barometric pressure readings, the scientific and aviation communities now rely on millibars, as does the rest of the world. If we were to look at overall averages, the barometric pressure at sea level would be about 1013 mb. By the time we reach the tropopause the pressure has dropped to about 142 millibars. Okay, but just this once: That is 29.91 inHg at the surface and 4.19 inHg at the tropopause. By the way, you need to start using temperatures in degrees Celsius as well ;-) It must be remembered that sea level pressure fluctuates daily and even hourly. All of us have seen nasty weather accompany a strong low pressure area, only to have beautiful blue skies the very next day as high pressure builds in. It is not unusual to see significant differences in pressure across relatively short distances. On April 3, 2000, a 1001 mb low was approaching the western Great Lakes, as a 1032 mb high settled in over eastern Wyoming. Viewed in profile, you could imagine an upward bulge in the atmosphere where the high is and a deep tough in the area of low pressure. It is this property, this variation in pressure, which is used to prepare upper-level charts. Say we want to look at the 850 mb chart, that is roughly at 5,000 feet. This portion of the April 3 1200Z chart shows the wide difference in pressure heights we discussed. You can click on the chart if you need a wider view.  As indicated at the top of the chart, we are looking at 850 mb height, temperatures and winds as indicated by the wind barbs. At the lower left you can pick out a light gray "H" next to a green number, 571 in this case. That is the height you have to climbing meters to reach 850 mb. The first digit, which would always be a "1" is left off. If you convert 1,571 meters to feet you get 5,154 feet. If look down from the "12Z" at the top of the chart, you will eventually come across a green 367 in the trough. That equates to only 4,485 feet. So the 850 mb height is 669 feet lower in the trough than it is near the high pressure ridge. Being able to identify upper-level lows can definitely help you avoid some nasty weather, but there is other important information on upper-level charts. Say you had a flight planned between Phoenix, Arizona, and Bismarck, North Dakota, in one of the new high performance singles. After making the checks noted on previous pages, you take a look at the 850 mb chart above. Sure you plan on flying higher than that, but it is the first in the list. What would be the first thing to catch your eye? How about those 35 and 45 knot north winds toward the end of the flight? But this is only 5,000 feet. We will fly at FL190 and only have to deal with those winds when landing. A 40 knot headwind on landing could be a bit tough, but let's take a look at FL190, or 500 mb.  Once again, you can click on the image for a wider version. So let's see, our 35-40 knots winds at 5,000 feet are now running 45-80 knots at FL190. That could play havoc with fuel predictions, as well as your time of arrival. It will also probably be a bumpy ride. But that is for a later lesson. There are also some great references on how to glean tons of information out of these upper-level charts. Check the Bibliography and links pages. In addition to the 850 mb (~FL 050) chart, you will also find 700 mb(~FL 100), 500 mb (~FL 180), 300 mb (~FL 290) and 200 mb (~FL 390). Click on any of the links above to see the latest chart for that pressure level. (For Flight Simulation Use Only) |
