speeds
up the winds aloft and causes the fastest winds in the Jet to be above and just
west of the cold front. Chapter 6 Seeing the Big Picture
Perhaps the best way of describing the flow of the atmosphere is to start at the big picture and then narrow down to where you are most interested. Winter shows the most contrast in the Northern Hemisphere so lets start there. (Shift six months for readers in the Southern Hemisphere.) Winter brings thoughts of cold for most of us as the sun takes a longer and longer sleep behind the Earth and it does not appear to rise as high in the sky during the day. Of course, people who are north of the Arctic Circle don't see the sun at all. It gets really cold up there as the only heat transfer is infrared heat energy radiating out into space. The average January temperature at Coppermine North West Territories is around -38o F. Sometimes it gets up to balmy -20, but on other days it really gets cold. As a contrast, my great aunt in Florida starts to get her woolens out as the temperature dips into the 70's. The tilt of the axis and the curvature of the Earth works wonders in providing a difference in heating, and it is the contrast in solar heating which drives the weather.
Molecules of air are continuously bumping into each other. If the air is cold, the molecules huddle together but are still bouncing. Hot air involves lots of bounce, driving molecules farther apart and making the air less dense. Cooling the air means removing some of the bounce and the molecules settle closer together. North of Coppermine, the winter loss of heat slows the molecules down and the reduced bouncing allows gravity to compact them closer to the ground. It isn't surprising, then, that the middle of the atmosphere by weight, the 500-millibar (mb) surface, moves closer to the ground in the Arctic while in the tropics the 500 mb surface remains high. The slope of this surface isn't much but it is significant.
Figure 6-1 is a hemispheric map of the 500 mb surface during the cool season. The North Pole is where the lines cross in the upper center. Below it is the Arctic ice pack. The lowest height is over Northern Siberia at 4,980 meters above mean sea level while the highest height is 5,900 meters in a couple of places. The 920 meter difference (~3,000 feet) isn't much in an airplane but in the atmosphere, it is enough engine to drive the winds and make the weather. So far, we haven't found the throttle of the engine but we do know the gas tank is the balance between incoming sunlight and outgoing radiation which warms the surface in some places and cools others.
The pattern of waves on figure 6-1 goes all around the globe, looking for all the world like a giant amoeba. The lobes or troughs extend southward and ridges extend northward. In this case there are troughs or trofs over the eastern and western U.S. with a ridge in between in the central U.S. Ireland has a trof centered on it and there is one which is north of Hawaii. And, of course the big one in Siberia . Under each trof is a pool of cool or cold air.
These are big waves, almost 3,000 miles from the center of the trof to center of the next trof. Yet, the biggest wave is the with one trof and one ridge. It encircles the globe. These patterns move relatively slowly as they cross the country. It may take from two days to two weeks for the ridge of a wave to move from the West Coast to the Eastern Seaboard. Meteorologists number these waves from wave number 1 which encircles the globe through wave number 2 which has two lobes around the globe to shorter and shorter waves. The amplitude of the wave is the amount of energy in the wave. In the illustration of figure 6-1, wave number 4 appears to have the most amplitude, the most energy. Long waves are the first six of these waves.
Estimating the speed of these trofs isn't easy, but fortunately they travel slowly and relatively regularly from day to day. We leave calculation of their speeds to the numerical models which can do a very good job of it. The first models did remarkably well for their simplicity but as the modelers incorporated more physics, the calculated speeds improved. This means that the speed of these trofs is governed by the total physics of the atmosphere.
Because we live and view the weather from a rotating Earth, the winds blow parallel to the isobars as mentioned earlier. The winds on the 500 mb surface blow in a wavy fashion from west to east, a west wind. The speed is dependent on the closeness of the isobars and the latitude. In the tropics, a little pressure gradient goes a long way to produce winds but in the far reaches of the North; it takes a lot of pressure gradient to produce even a mediocre wind.
A friend of mine talked of a balloon flight to describe the flow of the air and the energy around the globe. But, since it takes from a week to six months to make the circuit, to say nothing about it being a tad chilly towards the end of a cycle, a helium filled dirigible might be more in order. I've always been fascinated by them anyhow. Let's bring one out from deep storage in our imagination, and stock it with everything we might need for a leisurely trip. That little seaplane docked underneath can be used for picking up fresh caviar if the supply runs low. It can also be used to probe clouds with ATC's permission.
Starting in the tropics near the Windward Islands where the sun is nearly overhead all of the year, we go aloft and rig the craft for neutral density to follow the winds. Drifting gently westward in the Tropical Easterlies the haze out the gondola comes from sunlight heating the Gulf of Mexico's waters. The moisture is, in a very real sense, stored solar energy, for the warmer the water, the more water will evaporate storing some 600 calories for every gram of evaporated water. Moving gently along with the trade winds we pass lines of clouds and some apparent cells where the air moves downward in the middle and upward along the edges. They are very similar to the patterns you see in a thin layer of hot oil in a frying pan. Other lines of clouds appear off in the distance but made up of gentle lines of thermals which start at small islands. Often the clouds start along the upwind shore, but by the time rain forms, the cloud has moved off the island and the rain falls in the ocean. Only the big islands get the benefit of the rain the island causes. A little pressure gradient goes a long way here. The errors in barometers are often too large for tropical weather analysis that pressure gradients are often not used for everyday work. If you are basking on a tropical island and your barometer readings start to decrease rapidly, think hurricane and start paying attention to the radio and television. It might not be; but, then again...
The Intertropical Convergence Zone which follows the Sun has moved into the Southern Hemisphere, providing thunderheads over South America, so we'll avoid these. As the craft approaches the Yucatan Peninsula, it is time to send out the plane for supplies and a passenger who wants to come. A week or two later it is time to gain altitude as the Mexican coast comes into view. Once aloft and above the tropical inversion the winds switch to be Westerlies which move us out over the Gulf again, but this time in clear air, not hazy. The haze below extends as far as one can see. Floating back over the Gulf, the air parcels around lose enough heat through outgoing radiation so their temperature drops about one degree per day. This causes them to descend slowly, keeping the tropical inversion in place.
Keeping track of the weather to the north, there is a complex low pressure area over the Rockies as in figure 6-2. At 3,500 feet approaching the Florida Peninsula the airflow moves towards the Northeast, a Southwest wind laden with moisture. Dew points are in the 70s through a thick humid layer below and above. The forecasts for the weather map (Fig. 6-2) take us into the warm sector of the storm. Winds accelerate and careful coordination with ATC is necessary. Soon the ground speed is 50 knots and the clouds start forming in rows, some starting over hills near the coast and others may be stretched out from some of the cells over the ocean. There is clear weather between the bands which generally stretch out parallel to the cold front to the west but if you keep between the bands, there is some gentle downward motion as the air rises in the clouds and descends in the clear air between the cloud streets. As we move rapidly along the Piedmont, the forecasts indicate the storm has moved into western Pennsylvania. Keeping in close contact with ATC, the dirigible moves with the wind up the coast towards the warm front.
As we approach the warm front, there is a slight turning toward the low but
we aren't close enough to get wrapped around it. Moving upward over the warm
front the clouds thicken. One nice thing about meteorology, if you have upward
moving moist air, you get clouds. The air buoys us up and away over the front.
After a while, the clouds dissipate and we are over
Since the air emits infrared radiation, it cools a degree Celsius a day, so we will be a while circling the globe while slowly descending into the Siberian Plains or Northern Canada. As we descend, the wind speed slows and after a few weeks, the craft is gently moving above the tundra. The air outside continues cooling, now aided by the snow pack on the ground and the ice on the lakes and bogs. Turning on the lights to see what's going on, ice needles provide a dazzling glitter as they form and float gently towards the ground. While arctic winds can be strong, for the most part the ones we meet are weak and blow us gently southward. A strong pressure gradient is needed to produce even a moderate wind. As the air we are drifting with works its way southward, occasionally deviating around mountains but finally becoming part of the air over the Canadian Plains, the large scale wave pattern becomes unstable and an "air mass" or high pressure area moves out of the Plains headed for the U.S.
Moving near the surface over the Dakotas and southeastward we are borne
along by the northwest winds at around 850 mb, some 1,300 meters (4,000 feet)
above mean sea level. Other cold pools exist in each potential trof around the
globe. The wave patterns dig in and the whole pool moves south behind a cold
front in the Northern Plains. It surges towards the southeast and we are
carried along for the ride over the Mississippi river
over the Southern Appalachians towards
The slow movement of the waves around the Earth makes weather forecasting possible. The speeds of the long waves are not trivial to calculate. The mathematics of their motion has not been worked out except for the simplest case, where the wave speed is dependent on its wavelength. For more complicated waves such as those in the atmosphere, the whole range of physics equations is needed, solved in the numerical simulations on the big computers. The timing is what the models are all about. However, as a rule of thumb, the longer the wave, the slower it generally moves but it must be modified by the time the major part of energy is in the wave. The longer it has been around, the slower it moves. There are seasonal effects here too. During the summer, sunlight bathes the land north of the Arctic Circle all the time. The difference in heat energy between there and the equatorial regions diminishes so the energy or amplitude in the long waves decreases as well. In winter the difference in heat energy is much larger and the amplitude or energy in waves 1 through 6 increases. Figure 6-3 illustrates this for the two extreme seasons in the Northern Hemisphere. Going from Summer to Winter involves the energy going into shorter and shorter waves from wave one to wave two, then three, four, five and then six. Each transition is different; some are mathematically stable while others unstable. And, when making the transitions, "blocks" which stop the waves may occur.
Shorter waves, with a wave length of four or five mid-western states in size
move faster and move along the wave complex. Low pressure areas can be thought
of as short waves (wave numbers 7 and higher) in the system. The speed of the
lows and their fronts may be very rapid, or very slow. Again, we use the models
to try to gauge their speed, but there are some other clues. The best clue for
us pilots comes from the jet stream. The line on the TV map which represents
the jet stream is drawn by looking at an upper level chart such as in Figure
6-1 and drawing the line following the contours where the contours are the
closest. These days the charts are drawn automatically by the computer graphics
generator in the back room. The jet stream chart outlines the edge of these
ridges and trofs on the upper level charts and you won't go far wrong by
assuming most storms travel just to the South of the jet streams. The reasoning
is relatively simple. In mid-latitudes, the southernmost point of the 500 mb
trof is over the coldest air below with the warm air under the ridges. The cold
air is coupled with the upper level trofs and both move eastward. Friction with
the Earth sharpens the contrast in temperature along the leading edge of the
cold air causing the pressure gradient aloft to intensify. This
speeds
up the winds aloft and causes the fastest winds in the Jet to be above and just
west of the cold front.
Figure 6-4 shows a jet maximum and its position a day later. Imagine on day 1 we were able to place a big mylar balloon in the shape of a big cross to the front and left of the jet max. We do need to make it rigid so only the large scale winds will affect it. Because the wind speeds to the left of the jet max get slower the further you go from the center, by day 2 the cross has turned in a cyclonic curvature, the same spin as the low pressure area below. To be sure, if the cross were on the other side, the curvature would be the other way, but that is anti-cyclonic curvature, and it will have the same spin as the high below. Surface low pressure areas are coupled to the upper air by the spins or vorticity of the upper air. The place where the vorticity or spin is maximum is called, not unreasonably, a vort. max.
I once was riding on a 727 when the captain came on the intercom and mentioned that his weather people told him we would be flying over a "vort. max.," the place where this spin would centered. The pilot's tone indicated that he wasn't at all sure what it meant and that jargon of any type was to be avoided. I'm pretty sure that many of the people in the plane didn't have any idea what was going on. But, some of us were headed to a meteorological conference, so probably twenty percent of the passengers did. I thanked him for the information as I was departing the plane at the terminal.
The mathematics for showing that in areas of cyclonic curvature the vertical
velocity is upward is pretty heavy, but the results are simple. Where moist air
rises clouds occur, and in the cyclones clouds abound. Where the air is turning
the other way (anti-cyclonic curvature) the air descends and clouds, if there
were any to start with, decay. Clear air is nice to fly in, but when I'm
standing on the ground, clouds do make for an interesting sky. Clouds can make for interesting flying as well.
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