Chapter 2 - The Lower Part of the Three-dimensional Atmosphere

The plan for the day was to depart from Montgomery Airpark at Gaithersburg, Maryland (GAI), and fly to Oneonta Municipal in New York. There I had scheduled a rental car and would see my old college town. After a lunch, Muriel and I would drive over to visit my father-in-law at the farm, spend Saturday and then fly back late Sunday. Winter in the Catskills can be as sleepy or as active as you want it to be. The cross-country skis were there because that's where the action is. Ice generally coats the waterfall turning it into a winter sculpture. And the pond provides skating with no one hot-dogging in and around the crowd.

I watched the TV weather as a matter of habit. To be sure, there was winter weather, but most of it was south of us. The speeds of the low pressure areas, which for the last two weeks were moving rapidly across the country, had begun at the start of the week to slow down. The latest storm was only moving at about half the speed as one traveling approximately the same path a week earlier. The weather was forecast to be good for the entire weekend so the decision was to go, at least as far as scheduling the plane and pre-launch procedures. One can change one's mind anytime before tie-down on the other end. The professional forecasts were for clear with a slight southwesterly wind at 6,000 feet up and I figured on some high level cloudiness on the return trip on Sunday. I had scheduled the club plane for Monday as well as things do happen.

Figure 2-1 is the surface map of the day of the trip. The fronts, highs and lows were moving very slowly that day. Both the pros and I agreed that the storm associated with the low over Arkansas would probably move eastward and develop but would be too far out to sea to be a factor. One numerical simulation moved the low with a slight curvature off the coast; another numerical simulation had moved it off the coast with a strong curvature to the North. The European Center for Medium-range Weather Forecasts (ECMWF) simulations agreed with the U.S. simulations on the movement of the storm. I wasn't particularly worried that the storm would move north and be a big winter storm inland. The beaches would probably feel the effects. They did.

I checked in with Flight Service before going out to the plane to get a briefing and to file my flight plan. I know electronic systems can do the job for me, but I still like to deal with people. Departure from GAI was scheduled for 10 AM EST. The briefer brought up no surprises so the plan was still to go.

In the pilot lounge, I turned up the loudspeaker connected to the AWOS2 at Montgomery County Airpark (GAI). It intoned almost the same report as Dulles, temperature of 45, dew point 29, "Clear below 12,000 feet", and a visibility of 10 miles which the AWOS interpolated from its three foot measurement over in the cage. Since the winds were 310 at 5 knots at GAI, we would be able to use runway 32 and depart the pattern on a heading directly to the EMI VOR.

During the preflight, I could see no surprises with the wind conditions, and the sky was a wintertime pale blue with some cirrus present, consistent with the large scale flow. The winds were not particularly gusty as the sock seemed reasonably steady. Two students took off in 152s, followed by a light twin. Only one of the planes appeared to experience turbulence as they gained altitude, one of the 152s; I could hear a faint echo of my instructor chiding me to keep it aligned to the runway after lifting off. A jet somewhere near the tropopause left a contrail which was drifting gently to the southeast.

Muriel and I did the preflight, performed the runup and launched into a gentle sky. Once Muriel had settled in with the VFR charts in her hand, we settled down to a cruise climb. Don't ask me to explain it but she once spent an hour and a half wandering around a shopping center parking lot looking for our car. But, given a VFR chart and a nice day, she knows exactly where we are and can do a perfect how-goes-it. So we turned northeasterly towards the EMI VOR, passing over Davis Airport and the bustling town of Damascus. As Sugarloaf Mountain slid past the left wing, we emerged from under the controlled airspace. I listen to Dulles Air Traffic Control because of the traffic; this morning it was light and I started a cruise climb.

We emerged from the haze layer at about 3,500 feet northeast of Frederick, Maryland. There were a few widely spaced cumulus humilis or fair weather cumuli which had bases about that height. Climbing over the EMI VOR and being extremely careful about traffic, we topped the cumuli at 5,000 feet. It seems like every nice Saturday you need to "take a number" at the EMI VOR between 2,500 and 3,500 feet. I think every student in the world converges there either tracking to or from the VOR or setting up for a practice IFR approach to Frederick or BWI.

Climbing near Clearfield Airport, I noticed that I was nearing 5,000 feet, the realm of the 850 millibar (mb) upper air chart. The view was nice on the clear blue day. The Appalachians were standing up to 3,000 feet to the west and the farmland of Maryland and Pennsylvania was rolling beneath. Winds were light and, according to the DME, were from 210 at 6 knots. I reached over and pulled out the 850 mb weather map, Figure 2-2, which was made from the 7 AM (12Z) data. The solid lines on the 850 mb chart are labeled in tens of meters (decameters) so the 150 decameter line that is over the Pennsylvania - New York border is really 1,500 meters above mean sea level, or 4,920 feet. The lines are really a contour map of the surface where the pressure is 850 mbs. The nearest contour line to the south crosses over the Carolinas and is the 153 decameter line which is 5,019 feet above mean sea level. The difference isn't much for flying but is significant for meteorology. A check of my outside air temperature (OAT) gauge showed it was close.

The 850 mb chart depicts the flow around 5,000 feet MSL, so there is considerable interchange between the weather on this chart and the surface chart. Notice the temperatures on the 850 mb chart, they are pretty much right above the cold temperatures on the surface chart. The troughs are generally found near the cold air simply because the cold air is dense. The molecules in the cold air are closer together or dense here. The lower 15 percent of the molecules are closer to the ground than the ones in regions where the air near the ground is less dense.

Between the surface and the 850 mb charts are where you normally find the cool or cold pools of air in winter. These are the air masses sometimes referred to on TV or in a briefing. Usually, the two surfaces are quite well attached. Low pressure areas on the surface chart will almost always have a corresponding 850 mb low height area just to the west. As a consequence, where you see a Low pressure area on the TV weather show, you can almost always guess that there will be an 850 mb low height just to the west of the Low pressure area on the surface.

As I looked at this map (figure 2-2), I was reminded of a discussion I had with a young pilot who took my meteorology class. I thought he was convinced that pressure was something magic. Sometimes it seems to be so. But the definition is quite simple. For most weather applications, which is to say weather which occurs which is larger than a thunderstorm, atmospheric pressure is simply the weight of the air above a unit area. Chemists and physicists alike define pressure as force per unit area. The force is the weight of the air above.

Imagine you had an extruded aluminum pipe one square inch in cross-section area as in figure 2-3. Now imagine you fitted the bottom with a weightless plug which had no friction with the sides of the pipe, but let no air through. You attach it to a set of bathroom scales with a weightless rod. Evacuate the air from all around the pipe (hold your breath) while leaving the air in the pipe untouched. Now the scales will read just the weight of the air molecules in the pipe. The scales should read around 14.6 pounds depending on the weather today. (You can let the atmosphere back now and breathe.)

What you were measuring was the weight of just the molecules of the air which were inside the pipe. Of course, the individual molecules didn't collapse on the bottom of the pipe and lie on the plug. The ones bouncing on the plug were driven that way by their bouncy neighbors above them. But the combined bounces of the molecules resulted in the force which pushed down the plug and was read on the scales.

I think it finally became clear to that young fellow when I had him calculate the weight of the air above the Earth. Since the Earth has a lot of square inches of surface, there's a lot of air up there. Working the problem out on a calculator, we found that there is about 1,500 trillion tons (1.45 x 1015 tons) of nitrogen, oxygen, water, and other assorted gas being held to this old Earth by gravity. The 14.6 pounds of air is just the little portion of the whole atmosphere that happens to be above the square inch at sea level. Denver, a mile above sea level, the scales would read about 12 pounds.

Since pressure is simply the weight of the air above a unit area, why the confusion on the part of some pilots? I asked a fellow flying club member and he started talking about pitot tubes, Bernoulli flow over a wing, and other things like that. I began to see. In the ensuing discussion we finally realized we had to look at what was going on at the molecular level. Take lift, for example. Lift is caused by a small difference in atmospheric pressure between the top and the bottom of the wing when the airplane is moving through the air. Slow down to below stall speed, and there is still lift, but not enough to balance the weight of the aircraft, so it simply descends. How that pressure difference got there is a function of the art and science of wing design and the seven degree angle of attack.

At sea level there is a ton of air above each square foot of wing. Since most light singles have around 150 square feet of lifting surface, that's 150 tons of air to work with. A difference of one half of a percent of the average atmospheric pressure between top and bottom of the wings (some say the fuselage is also a lifting surface) is enough to lift 2,000 pounds of aircraft into the blue. At Aspen, Colorado, on a hot summer's day, you're up past the lowest part of the atmosphere. With only two thirds of a ton of air above you, you need all the speed you can muster to generate a one percent pressure difference to produce the lift. Or you can wait until the cool of the morning where the air is most dense near the ground.

Meteorologists use mbs as the preferred unit of measure. It has some advantages beyond being a simple multiple of the fundamental metric pressure unit, the Pascal. One of the reasons for using mbs over pounds per square inch is that the upper level pressures are a little easier to visualize. At sea level, the pressure is usually near 1,000 mbs if you round off a little. If you were sunning yourself and your significant other on one of the beaches in the Caribbean and noted that the sea level pressure is approximately 1,000 mbs, with a little reflection you would probably agree that 100 percent of the air molecules are above you. Suppose, now, that you got bored with the area and decided to fly over to another island to see what is over there. As you take off, you start putting air molecules below you.

If you were to climb to around 5,000 feet, you would be above 15 percent of the air molecules; the air above would contain only 85 percent of the molecules. If you had a barometer calibrated in mbs and had set it to 1,000 mbs at sea level, it would read 850 mbs. A little higher at 700 mbs, roughly 70 percent of air is above you with 30 percent below. The rule is: simply knock off the right most digit of the pressure in mbs and you have an approximate percentage of air molecules above you.

The magic 12,000 need-oxygen level is at about 650 mbs where you're above 45 percent of the air molecules. By the same reasoning the 500 mb point has roughly 50 percent of the air molecules above and 50 percent below. I will often refer to the 500 mb point as the middle of the atmosphere by weight. It is - as long as the ground below is near mean sea level and the MSL pressure is nearly 1,000 mbs. The air really is getting thin at the 500 mb level, around 17,000 feet. It is not surprising that a piston engine has trouble getting enough oxygen to burn and needs a turbo boost. People do too.

The top of the weather producing atmosphere, the tropopause, is usually around 100 to 300 mbs. Of course that means that 10 percent to 30 percent of the molecules of air are above that level and 70 or 90 percent below. Small wonder those aircraft that get into these altitudes need fast planes and high lift wings to stay up there. The troposphere contains almost all of the weather which affects us. Ninety nine percent of the clouds you will ever see will be in the troposphere. The other clouds are the extremely rare noctilucent clouds which form about 85 kilometers above the sea. Figure 2-4 is a graph of temperature with height which shows the various layers of the atmosphere. The troposphere is the first 8 to 12 miles where most of us fly. The tropopause is the top of the layer where the temperature stops decreasing with increasing height and the stratosphere begins. The constant temperature with height in the stratosphere indicates stable air and, for the most part, people flying in the flight levels above the troposphere do experience smooth air most of the time as well as reasonably fast wind speeds from west to east around the globe.

You probably saw this graph in ground school but it’s worth taking another look at it. The graph shows two features of the sun's heat. Most of the sunlight which reaches the earth heats the ground or water areas which in turn heat the air just above it. The basking turtle along the river, our cars sitting in the sun along the curb, the asphalt pavement in between, the green trees, the oceans, tarred roofs and everything else on the earth’s surface is warmed by the sunlight. As it warms its temperature rises; just ask any cat. And it warms the air in contact with it. The newly warmed air becomes less dense than it was and rises to be replaced by cooler air from above. As the process continues hour by hour and day by day, the temperature of the troposphere approaches the line in the graph. The ultraviolet component of light heats the ozone in the stratosphere and above, warming it and making the air in the stratosphere stable.



As we approached the Harrisburg Class B airspace, I checked in and was cleared to 7,500 with VFR advisories. Checking the DME, I found we indeed had a quartering tail wind of about 10 knots. Probably the little highs and lows on the 850 mb chart were responsible for the boost from the south. I estimated that we were just above 800 mbs, probably around 790 mb.

Northeast of Harrisburg the puffy cumuli of the Piedmont gave way to clear skies below. Near Hometown, Pennsylvania, I found it increasingly difficult to hold altitude. Since it isn't macho to deviate from altitude on a VFR contact flight, practice for IFR I suppose, I found myself trimming every few miles. Of course, the southwest winds flowing gently over the mountains were producing waves in the air above. After a while, I decided to let the plane ride the waves as long as the deviations weren't significant.

Setting the autopilot to intercept the AVP VOR, I pulled up the weather chart for the next level up, the 700 mb chart. Figure 2-5 is a contour map of that level, where 70 percent of the atmosphere is above you and 30 percent is below. Normally the contours (in tens of meters) are around 7,000 feet. This is the last weather map made below the service ceilings of a piston engine aircraft without a turbocharger because it ranges between 10,000 feet (690 mb.) and 12,000 feet (650 mb.).

There is often, but not always, a correlation between the atmospheric flows on the 850 and 700 mb charts. Sometimes however, the cold air mass is entirely below the 700 mb level. Features which have a size of the state of West Virginia or Ohio and are detectable from the many surface observations and satellite pictures will be missed by the widely spaced locations of where we take data above the ground. The converse is also true. Upper level disturbances will sometimes travel along independent of what's going on below. The large features will be tied together. The troughs on both the charts are some 2,000 miles across and are linked.

The contours on the upper air charts are the same kind of contours as those of the topography on the VFR charts or on the USGS topographic maps. The lines are contours of MSL altitude of the place where the barometric pressure is 700 mb, enabling you to see the hills and valleys of the surface of constant pressure. Meteorologists don't call them hills and valleys. A valley usually has a stream in it, and the troughs don't imply a stream. Similarly a hill sort of implies one goes up or down. Since the winds blow along the contour lines, ridges and troughs are preferred names.

It is worth some time and effort to try to visualize the ridges and troughs (often spelled trof) of figure 2-5. Visualizing is somewhat easier if we assume the Earth is flat like the map and you use your hand to follow the contours. Notice the trough over the Western Appalachians which extends through the Great Lakes. The place with the most slope in that trough, where the contours are closest together, is to the east of the trough. The contours spread apart over the Virginia and Maryland and continue to spread apart over New York and New England.



Nearing the Class B airspace at Scranton Pennsylvania, I tuned in ATIS and received an update on the pleasant surface conditions. After a winter of very cold weather and near record snowfall, they were basking in a 35 degree (F) heat wave. With permission of ATC, we descended to 4,000 feet to watch the city stirring. Skiers were out in droves. The slopes looked like a river which had been sprinkled with paint. Even the East Branch of the Susquehanna River was beginning to show signs of opening the zipper of its jacket of ice to the bright morning sun. Once north of William B. Scranton International, we climbed back up to avoid the turbulence of the northern Poconos.

Leaving Scranton, we climbed to give the Poconos and the Southern Catskill Mountains a good bit of altitude. At 5,500 feet, I wondered briefly about going to 11,000 feet and catching some of the stronger southwesterly tail wind at 700 mbs. Off to the right I could see some cumulus lenticularis clouds, those lens shaped clouds which mean turbulence. I called Flight Watch and inquired about them and if there were any pilot reports. A commuter had reported moderate turbulence at 14,000 feet an hour ago over Poukeepsee (POU), so I leveled off at 7,500 feet, thankful for the tailwind at that level. We could still feel the waves but there didn't seem to be any turbulence. In fact, it was smoother than driving the secondary roads through the hills below. I reported the smooth, wavy air to Flight Watch, thanked them, and went back to ATC to report level at 7,500.

As White Birch field at Hancock slid past the right wing I called ATC. Throttling back to ease the burden on the plane as one always expects some turbulence in this area, I started a gradual descent to end up at Rockdale VOR (and cancel my flight plan) at 4,000 so I would ease into the pattern at Oneonta. Overall, it was an uneventful trip. The kind I like.

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One hot summer's day, the rumble of thunder through the study served to emphasize the reason why I wasn't up flying. Saturday afternoon. All the honey-do projects were done and Ma Nature had to kick up her heels. A gust of rain rattling the sliding glass doors on the den made me realize that the even those who regularly fly in Class A airspace would be attempting to avoid this storm.

When I enter Class A airspace, I do it from a passenger seat of a airliner. For now, I'll let the pilots of the jet aircraft worry about a headwind which is approaching the redline on the Piper. The meteorology upstairs is of major concern to me because what happens upstairs often affects the air I fly in. So too, the weather downstairs almost always affects the flow aloft. Figure 2-6 is a 500 mb chart, made the same way and plotted the same as the 700 mb chart in Figure 2-5. This is a favorite of mine to use with one-day appearances in science classes. The kids almost always pick up immediately on the fact that the winds don't blow directly from high to low but move parallel to the lines.

The 500 mb map, as with the other upper level pressure charts, is a contour map of height of the middle of the atmosphere by weight. It shows the flow of the winds and the weather patterns between 17,000 and 19,000 feet. While flying at this cabin altitude requires oxygen, and is beyond the service ceilings of unturbocharged small singles, the level is one of the most important in meteorology. This is the place where jet streams are most easily noticed on the charts. And, because of its middle position in the atmosphere, the features on this level generally reflect what's going on through a considerable depth of the atmosphere.

The jet streams do not go entirely around the globe as they would appear to do so from most TV weather shows. They appear along the edges of the troughs where the slopes of the surface are the steepest. They disappear when the steepness diminishes. Yet, there is a line which follows the maximum slope of the sides of the troughs and ridges which does go round the globe. The 500 mb level is usually thought of as the bottom of the Jet stream, which seems to peak out at 300 mbs or even higher.

At the 500 mb level the winds can be upwards of 100 knots, none of the winds at this level reached that this day. Typical winds for the winter will peak out in the jet at 130 knots, with most of them being in the 30 to 50 knot range elsewhere. For most of us whose green line tops out at 140 knots, running upwind is generally not a profitable venture. But, since the level is beyond the small single's service ceiling, it is usually not a problem.

The wind directions are along the contours, not across the contours. It's as though wind doesn't blow from high pressure to low. Of course, the winds blow from high pressure to low pressure. Without a pressure difference at the same elevation (or, equivalently, a height difference at the same pressure) we wouldn't have any winds at all.

Like anything else, there are complications. The complication is simply that we live on a rotating Earth; the reason the winds blow along contour lines is the fact that the Earth turns on its axis once a day.

Professor Coriolis is credited with the reasoning and the mathematics in the middle 1800s, but to simplify the results of his derivations, the Coriolis Force is:

Figure 2-7 shows a simplified weather map with the straight lines being isobars which give the pressure gradient between simple high pressure and low pressure areas.

Figure 2-7 Simplified Weather Map to Show Coriolis Effect

One side of the diagram has a pressure higher than normal and the other side is at a lower pressure than normal. It could be at any level if you substitute contours for isobars. If this were a 500 mb map, the more dense air in the lower half of the atmosphere would be in the area marked "Low Pressure" only in that case it should be marked "Low Height."

At time zero, a thermal shoves air aloft at A and injects a bubble of air into the level of the map. Initially it is at rest with respect to the compass directions. Assume, for the moment, that the other winds don't just carry it along, but that it moves because it is pushed by the sum of just the pressure gradient and Coriolis forces. Sensing the pressure difference, it begins to move directly to the Low Pressure side.

As soon as it moves in the horizontal, the Coriolis force starts to push it to its right. So the Coriolis force needs to be added to the pressure gradient force vectorially. The result of the vector additions are the solid arrows. A little while later, the parcel will have sped up and arrive at point C. It is now moving faster than it was at B.

The Coriolis force, which is dependent on the speed of the wind, will be pulling to the right more and the force diagram will be that shown at point D. When the parcel gets going as fast as it is going to go, the Coriolis force will just balance the pressure gradient force as at point E. The wind is moving right along there and if you were to slow it down, perhaps by adding friction, the Coriolis force will decrease and the parcel will edge across the isobars towards the Low pressure side.

At this speed, the total force (the solid arrow has disappeared) is zero, so as Isaac Newton once said "... an object in motion continues in motion until forced to change its motion." Of course all of the air around the air at point E has undergone a similar change so the wind is moving right along. The bubble of air will move along parallel to the contour lines as long as it isn't either slowed or speeded up by something.

Friction might slow it down. When frictional forces enter the picture, you simply add the frictional force vector into the sum and the total force pulls it ever so slightly towards the Low. This is most noticeable near the ground, but rarely ever shows up on 700 mb or higher maps.

Looking back at figure 2-6, notice the winds at 500 mb follow the contour lines, and that the closer the lines are together, the faster the winds. Where the lines are really close together, the winds have speeds of 50 knots or greater, the traditional lower wind speed limit for the jet streams. The jet streams are only winds which are caused by the same thing every other wind is - difference in pressures over horizontal distances. There is no magic additional energy source for the jet streams.

I said before that only a few contours on the upper air charts are needed for the analyst to pick up a lot of information from the charts. The sophistication shows by looking at the closeness of the contour lines. Notice that the places where the lines are far apart have weak winds. Where the lines are close together, the winds are strong. This effect did not go unnoticed and is a confirmation that the Coriolis force and the pressure gradient force are well balanced at these levels. Friction is small. Only in the high mountains is friction reasonably strong at 500 mb. Of course, the 850 mb map is closer to the ground in the east and shows frictional effects more frequently.

The middle of the atmosphere is of interest for other reasons. Except in the highest mountain areas, it is far from surface friction. Only the Himalayas cause real friction problems to analysts at this level because the whole region has a surface pressure about 500 mbs. If you plan to climb one of these peaks you probably will use a pressure altimeter which is corrected to sea level the same way the altimeter in the cockpit is.

Figure 2-8 is a polar stereographic map. The contour lines are the same as in figure 2-6, but this chart gives the flow over the Northern Hemisphere. Look for the waves again. They're still there. But now you can see that they are part of a wave pattern extending all around the globe.

The height of the 500 mb surface is lowest in the Arctic, where the lack of sunlight in winter causes it to be cold. Coppermine, Northwest Territories has an average January temperature of -54 F. True, sometimes it gets up to a balmy -20, but other January days can be mighty cold. The air molecules are much less violent to their brethren air molecules than in the tropics. Because they aren't as vigorous there, they tend to be closer together and the air more dense than in warmer climes. This brings the middle of the atmosphere (the 500 mb height) closer to the surface.

The 500 mb height chart shows the waviness of the atmosphere quite well. Since the middle of the atmosphere by weight is always somewhere, the waves are continuous around the globe; that is, there are no places where the waves stop. There are no big beaches or jetties which stop these waves from flowing. They just keep rolling slowly around the hemisphere.

Other charts such as the 200 mb are regularly drawn but they generally don't have much additional different information for pilots who don't have turbochargers mounted on their engines. The contours on these charts generally mirror the ones below but may be shifted slightly. Expect the winds to be faster. If you happen to be flying jet driven aircraft and have oxygen handy, the upper level charts are good to look at as they provide wind information for fuel conservation.

The NWS has produced tropopause charts in the past; however, there are difficulties. For the most part, the tropopause is relatively easy to define; however, in a significant number of cases each day, the tropopause is not easily defined. Indeed at times there are two or three trops at some sites. We think this happens when a short wave turns the lower edge of the stratosphere over, much like an upside down ocean breaker. Knowing where there are multiple trops does provide some information about potential turbulence.

On to Chapter 3 or back to Table of Contents. ©