Midlevel guidance

Forecast clues are evident in reported data. Information gleaned from 500-mb weather charts gives valuable insight to coming fronts.

Negatively tilted trough off the east coast of the US contains strong positive vorticity, with the Earth vorticity (Coriolis) enhanced by the negative tilting. Divergence aloft and warm, humid maritime air south of Bermuda will likely produce strong convection ahead of the trough axis.

Remember that as air moves poleward its positive vorticity increases. With air moving from west to east in the midlatitudes, this means that the outflow from a vort max at the base of a positively tilted trough will not move as rapidly poleward as the air flowing from a negatively tilted trough.

As a result, there is likely to be far less divergence taking place above the positively tilted trough than over one with a negative tilt.

Furthermore, a negatively tilted trough is more likely to draw cold mid and upper-level air to the right side of the trough, placing it over warmer, humid surface layer air.

This increases the temperature differences between the surface and upper air, and increases the overall instability of the atmosphere. Some of the worst weather conditions are produced by a negatively tilted trough containing a strong vort max.

Thick or thin

A last important component of 500-mb analysis is something known as thickness. On average, the 500-mb pressure plane sits at about 18,000 ft MSL (roughly 5400 m). In the summer, the heated surface air will expand, raising this height, and in the winter, as the cold air compresses, it will be lower.

So the amount of altitude—or thickness—between that 500-mb level and sea level (or, more precisely, the 1000-mb level) is a function of the average temperature of the air in between.

Interestingly, the average thickness of about 18,000 ft is also a dividing line for helping meteorologists determine whether any precipitation that is fore­cast might fall as rain, snow or freez­ing rain/mixed precipitation.

Although very approximate, thicknesses greater than about 18,000 ft (5400 m) indicate low-level air warm enough to support rain. Thicknesses below 5400 m are associated with air sufficiently cold for precipitation to fall as snow.

Thicknesses of 1000–500 mb that are within about 100 m either side of the 5400-m line place the lower troposphere temperatures in a range where pilots may encounter a mix of rain and snow, or freezing rain. Something to bear in mind, however, is that those thickness levels correspond to surface precipitation at around 1000 mb, or roughly sea level.

The higher you are in the lower troposphere, such as on ap­proach or at a mountain airport, the thicker the 1000–500 mb can be and still support snow or freezing rain. For example, at 3000 ft (about 1000 m), you may encounter freezing rain even though the overlying 1000–500 mb thickness is 5600 m.

However, in general, a transition zone around 5400 m thickness, which can be prominently displayed on many 500-mb and even some surface charts will provide pilots with a rough idea of where icing may be a danger, or at least where rain may start changing over into snow.

Coupled with surface weather maps, atmospheric soundings and upper-air charts, a pilot can begin to gain a fairly comprehensive 3D picture of the atmosphere through which they will be flying.

These are all the same information to which a meteorologist will be referring when they provide you with your preflight weather briefing, so a familiarity with the prominent information will enhance your ability to visualize what the briefer may be describing, as well as improving your ability to ask sound questions that may help you steer clear of any atmospheric trouble.

As with many meteorological products, most of today's 500-mb charts are produced by weather forecast models from point source observations. While conditions aloft are a bit easier to interpolate than are surface conditions, there is still a chance that a contour line on the map doesn't line up with the actual conditions in the sky.

This is why it is important not to rely solely on any one source of information, and not to forgo a proper weather briefing in favor of perusing the latest weather charts. It is also why filing Pireps is important, especially if the conditions you experience are different from the ones that were forecast.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg Unversity. Shein holds a commercial license with instrument rating.


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