Ever hear your local TV weatherpeople preface a warning of bad weather by mentioning “spin” in the atmosphere as a culprit? What they really mean is that parcels of air at high altitude are moving in such a way as to create a net upward movement of air.
In other words, forces aloft are causing lift at the surface. And you know what that means when unstable conditions are present: Rising air cools, saturates, gives off latent heat, and keeps on rising. The end result can be heavy precipitation and thunderstorms. So the lesson here is simple—“spin” can be bad news.
When most of us think of thunderstorms, we tend to think of them as “bottom up” creations. We all have a pretty good grip on the idea of surface heating causing the lifting forces that kick off thunderstorms. That’s a useful concept, but remember that thunderstorms fed only by surface heating are apt to be less severe. The tallest, meanest, most far-ranging, longest-lasting convective weather is characterized by the additional lifting produced by that “spin” generated well into the flight levels.
Troughs and height gradients
So where do you find this spin? The first step is to consult constant pressure charts. These are charts that plot height contours—lines of equal heights of pressure surfaces. These contours give three-dimensional views of various pressure surfaces, but they look just like the isobars we see on surface analysis charts. And as with isobars, closely spaced height contours also indicate steeper gradients and therefore stronger winds.
For spin-location purposes, it’s often most useful to use the 500-millibar (this roughly corresponds to 18,000 feet msl), 300-millibar (about 30,000 feet msl), and 250-millibar (about 34,000 feet msl) constant pressure charts. These charts show not just height contours, but also station models that show temperature and dew points aloft. The 300- and 250-millibar charts have the added benefit of showing isotachs—lines that define areas of maximum wind speeds. These are useful in pinpointing the locations of the strongest jet stream winds.
Constant pressure charts also show troughs aloft (U-shape, southward-extending areas of lower pressure surfaces), ridges aloft, and centers of low and high pressure. They’re easy to identify, too—isobars and height contours look similar, and the pressure patterns they depict bear strong resemblances.
It’s the steep-gradient troughs aloft that call for the most scrutiny when looking for possible areas of severe weather. That’s because counterclockwise rotating air parcels moving around troughs aloft speed up, then slow down, as they make their way from west to east.
Skating around troughs
Think of those rotating air parcels as ice skaters, spinning their way around a trough aloft. To the west of the trough axis, where height contours are far apart, the air parcels/skaters are spinning slowly. But when the contours come together, the parcel-skaters spin up dramatically—the way skaters do when they draw their arms inward. That’s because the narrowed gradient imposes higher winds and stronger shear forces on the parcel-skaters.
But once the parcel-skaters round the bend in the trough axis, the height contours widen, and the parcel skaters spin down. Metaphorically, their “arms” extend outward as shear forces decrease.
Convergence and divergence
What does all this rotational speed and arm-spreading have to do with planting the seeds of strong thunderstorms? Good question.
West of the trough axis, where rotational speed is highest, air parcels contract—just like the skater’s arms. This contraction causes convergence aloft. This convergence helps suppress any thermal or convective lifting in the atmosphere below. Looking at a vertical slice of the atmosphere, if converging forces aloft are greater than those nearer the surface, convection will be suppressed.
But it’s a much different story east of the trough axis, where the height contours fan out and the spinning slows down. This lets the parcels in the spin-down zone expand. This creates divergence aloft, which in turn causes the kind of expansion that accelerates the growth and boosts the intensity of any nearby surface lows or storm complexes.
Think of the vertical column of air beneath the area of divergence aloft. If there’s more divergence aloft than convergence at the surface, then a surface low will be created; if one already exists, it will deepen. It’s almost as though the divergence aloft sucks the surface low’s air upwards, causing it to deepen even more—not to mention creating the kind of turbocharged lift that leads to severe weather.
And that, ladies and gentlemen, is what weatherpeople mean by “spin” causing adverse weather. Just remember: Divergence aloft bad, convergence aloft good.
Look for signs of divergence east of any troughs aloft, where height contours fan out. Compare this fanning-out pattern with any lows or frontal complexes on the surface analysis and low-level prognosis (“prog”) charts.
If the air in the lower levels of the atmosphere is unstable, or if any surface lows are parked—or heading for—areas of divergence aloft, be prepared for the weather to deteriorate.
Divergence aloft works to build snowstorms in the winter as well as thunderstorms in the warmer months.
Of course, there’s a lot more to convective weather than identifying spin zones. Next month we’ll take a brief look at another great tool for analyzing potential thunderstorm conditions—the Skew-T Log P charts.
E-mail the author at [email protected].
Web site of the month
The Unisys upper air Web site lets you call up constant pressure charts for all the mandatory pressure levels. These levels, together with their rough equivalents in msl altitudes, include the 850-mb (5,000 feet), 700-mb (10,000 feet), 500-mb (18,000 feet), 300-mb (30,000 feet), and 250-mb (34,000 feet) constant pressure charts. What’s more, the site lets you look ahead as far as 72 hours for winds aloft and pressure analysis purposes. Along with identifying troughs, shaded isotach contouring on charts above 500 mb are especially helpful in seeing areas of likely divergence aloft. In addition to upper air information, the Unisys site posts a huge amount of other weather data.— TAH
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