Troughs are given short shrift in aviation weather courses, but they are major weathermakers. Not so much the troughs occurring at the surface (which are shown by dashed lines, colored brown on colorized surface analysis charts), but those aloft. Troughs aloft can be found on constant pressure charts, and are best identified on constant pressure charts above 700 millibars, which roughly corresponds to 10,000 feet msl. This means checking the 700-millibar constant pressure chart, as well as the 500-millibar (18,000 feet) and 300-millibar (30,000 feet) charts, to learn the telltale shapes of bad-weather troughs.
Constant pressure charts have lines that look quite similar to the isobars (lines showing equal barometric pressure levels) you see on surface analysis charts, but that's where the similarity ends. The lines on constant pressure charts connect points having the same height (i.e., altitude) of a given pressure level. For example, the lines flowing around a low aloft on a 500-millibar constant pressure chart indicate the varying heights of the 500-millibar pressure surface. These height contours are labeled in meters. On the 500-millibar constant pressure chart, you may see a low (represented by an "L") surrounded at its perimeter by a line labeled "567," then followed by a blank line (every other line is labeled, to minimize clutter, so unlabeled lines must be interpolated), then followed by a line labeled "561" right next to the L — the center of low pressure. Add a zero to these numbers, and you learn the height in meters of the 500-millibar pressure surface.
So the 567 refers to a height of 5,670 meters, which works out to about 18,711 feet. Not exactly 18,000 feet, but close enough for analytical purposes. Next comes the blank line, which we can interpolate as 5,640 meters (or 18,612 feet). Then the 561 line, which translates into 5,610 meters, or 18,513 feet. Then, at the center is the L — the center of lowest pressure, which may or may not be labeled with a height value, depending on the scale of the chart.
What does it mean?
This particular segment of our sample constant pressure chart shows us that the altitude of the pressure surface equating to 500 millibars is sloping downward, toward the center of the low aloft. The surface starts out at 18,711 feet, then drops to 18,612 feet, then 18,513 feet before reaching the lowest height at the low's center.
Pressure surfaces aloft can be wavy, and descend in altitude at varying rates — sometimes slowly, sometimes very steeply — toward lows aloft. If the height contours are spaced farther apart, then the height falls are gradual. If they are packed closely together, the height fall will be abrupt.
As with the isobars on surface analysis charts, closely spaced height contours on constant pressure charts indicate stronger winds. And winds aloft parallel height contours, the same way surface winds follow the patterns made by isobars. Furthermore, on most constant pressure charts you'll see wind barbs showing the strength and direction of the winds aloft.
Wait a minute, you might wonder. If sharp pressure differences, as shown by narrow isobar spacing on surface charts, indicate strong winds at the surface, how can sharp drops in the height of a pressure surface aloft also cause high winds?
The answer is that there's a relationship between wind and temperature. First, assume that atmospheric pressure is a constant, which, of course, is the premise behind constant pressure charts. Second, assume that a warmer air mass is right next to a colder one. Will the height of the pressure surface change with temperature?
You bet it will. Air molecules are farther apart in warmer temperatures, and closer together where the air is colder. This explains why the height contours in warmer air (to the south of the trough aloft) are higher than those closer to lower temperatures aloft (to the north of the trough), and why constant pressure surfaces slope downward, toward colder air masses. This is also the theory behind altimeter errors when flying in colder-than-standard temperatures (see " Proficient Pilot: Look Out Below," October 2004 Pilot).
And this also explains why closely spaced height contours indicate strong temperature gradients, which in turn generate the contrasting pressure gradients that produce winds aloft. There's a meteorological equation that explains all this — the thermal-wind equation — but take my word for it. Strong temperature differences mean stronger winds.
Fast facts
Like surface troughs, troughs aloft are extensions of low pressure.
- They appear as U-shape, southward-dipping bulges in the atmosphere.
- They mark the delineation between colder and warmer temperatures, at the surface and aloft.
- They produce weather-making jet streams in the southeast portions of the trough. The fast-moving jet-stream winds are the direct result of the strong temperature gradients in this section of the trough — where cold air moving south abuts warm air moving north. Rising motions in the fastest-moving cores of jet-stream winds cause air to diverge aloft, and converge at lower levels. This intensifies and sustains convection, and makes the most severe thunderstorms.
- Cutoff lows, or closed lows aloft, are marked by a circular height contour of lowest pressure in the center of a trough. Cutoff lows mean stagnant, adverse weather patterns at lower levels of the atmosphere that can last for days.
- Those "cold pools of air aloft" that TV weathermen often talk about? They're troughs aloft.
Case study
This past January gave us a great example of a trough aloft in action. For the first half of the month, an upper-level trough — complete with a cutoff low — was parked off the California coast. The counterclockwise flow around this trough sent loads of moisture in the form of record-setting rainfalls, as well as snowfalls in the higher terrain. Then came the mud slides.
Jet-stream winds aloft affected altitudes as low as 6,000 feet, and icing conditions, turbulence, and low ceilings and visibilities occurred in nearly every western airmet of the first two weeks of January.
Weather maps for January 4 (pages 129 and 130) show a number of interesting features of this winter West Coast system. On the 500-millibar constant pressure chart, notice the cutoff low's circular height contour parked over Southern California. It had been there for the previous three days, and would remain off the West Coast for the next four. Look at the surface analysis chart, and see how this low pressure extends all the way to the surface, where it sends a cold front/occluded front hybrid eastward to Arizona and Nevada. This vertical "stacking" of low pressure is a sign that the system is slowing down. But it would be several days before this low threw in the towel.
Now look at the temperature maps and see how the temperature bands are aligned so well with the orientation of the trough aloft. This east-northeast/west-southwest alignment stretches across the entire United States, demonstrating the continentwide effects of the trough. It also shows how troughs demarcate warmer zones from colder ones.
The precipitation map confirms the way this huge trough sent moisture and instrument meteorological conditions from coast to coast. Backing all of this up is a stationary front, which runs from Texas to the Ohio Valley and beyond — all of it a reflection of the trough pattern.
Finally, this trough served as a steering current or pipeline for future surface lows and fronts. Wave after wave of instrument weather cycled from west to east, following the height contour orientations across the central United States. One of them caused a nasty, two-day-long freezing-rain episode that affect-ed Kansas and Missouri on January 2 and 3. How do I know? I was trapped in Wichita, watching deicing trucks hose down airliners until it was safe to depart. There was plenty of time to reflect on trough dynamics, and get started on this article.
E-mail the author at [email protected].
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