Midlevel guidance

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

By Karsten Shein
Comm-Inst, Climate Scientist

Flightcrew watches snow fall on the tarmac ahead of a flight. Information contained on 500-mb charts can be extremely useful for understanding how such weather conditions might evolve over the next several hours.

As pilots, we tend to be wary of things like lows and fronts. After all, these bring us all sorts of adverse weather conditions, such as thunderstorms, low ceilings, icing and strong, gusty winds.

But we also know that those "L"s and frontal lines we find on a surface weather chart are not all created equal. Some are viciously powerful—dropping tornadoes and forming raging blizzards—while others wimper through a region with nary a drop of rain or ominous cloud.

Unfortunately, it is usually quite difficult to discern the behavioral differences between 2 fronts or areas of low pressure just from looking at a surface weather chart.

While a series of surface charts can provide a pilot with a good deal of information about how fast a particular front might be moving, whether it is strengthening or weakening, or where conditions are worst, they lack the information needed to really show how quickly a system might develop or how strong it might become.

For that sort of information, we need to look skyward, to the middle levels of the troposphere and the 500-mb chart. The 500-mb chart is actually a series of charts that include things like wind and temperature, pressure height, vorticity and thickness. The first few variables are the same as what you would expect to see on a map of any other level.

The most basic of the 500-mb charts will show wind and pressure height. This information is normally sufficient to help a pilot to determine the general flow of the atmosphere at roughly 18,000 ft MSL.

This can be compared against the patterns at higher (300-mb) or lower (750-mb) levels to determine a route of flight that will take optimal advantage of the winds. More detail­ed 500-mb charts will also show station models corresponding to weather balloon launch locations.

The station models will display temperature (°C), dew point (°C) and pressure height (decameters). Most temperatures at 500 mb will be negative. The flow of wind at 500 mb will also help a pilot determine if there is cold or warm air being blown into a region at the middle levels.

This has implications for the stability of the atmosphere in general. Cold air being advected into the middle altitudes of a region will normally destabilize the atmosphere, especially if the surface air is substantially warmer.

Colder air aloft will allow any air rising from the surface to remain warmer than its surroundings as it rises, permitting it to continue rising to much higher altitudes. Conversely, warm air advection in the middle levels may help to suppress potential convection if it means that rising surface air will become cooler than its surroundings and cease its ascent.

Level of nondivergence

Vorticity-diagram 500-mb chart showing areas of positive and negative vorticity factors (shear, curvature and Earth), as well as the axes of troughs (red dashed lines), shortwaves (yellow dashed lines) and vorticity maxima (black 'x's). Vort maxes indicate the highest positive vorticity, and thus the areas of greatest upper air divergence.

The troposphere—the lowest layer of the atmosphere, where most of us fly and where most of what we consider "weather" occurs—extends from the surface to about FL300. Above the troposphere is the tropopause, where air temperature levels out before beginning to increase with altitude into the stratosphere.

That stratospheric inversion caps the rise of surface heated air that is responsible for generating density and pressure differences which, in turn produce our weather.

Although air is in constant movement through all parts of the troposphere, the layer itself can be divided, meteorologically, in half. The lower half is the layer that provides the heat and moisture needed to support convection, while the upper half is the layer that promotes or suppresses the vertical motion of the air from below. The halfway point—around 500 mb or roughly FL180—is known by meteorologists as the level of nondivergence.

The level of nondivergence (LND) is so called because it rests beneath the upper levels of the troposphere, where a great deal of convergence or divergence of air flow takes place. This convergence and divergence is what helps to enhance or suppress the pressure systems moving along the surface.

For example, an area of diverging air in the upper troposphere will lower the air density aloft, encouraging the uplift of lower-level air and enhancing a surface low beneath it.

Conversely, upper troposphere convergence will increase density there, resulting in increased surface pressure. The strength of convergence or divergence aloft can best be captured by evaluating conditions at the LND.

One of the most important measures of the potential of the upper levels to support convergence or divergence is vorticity.

Vorticity is a measure of the spin of the air. This spin is due to 3 factors. The first is the effect of the rotation of the Earth and the resulting Coriolis effect. The Coriolis effect is the appearance of a curvature in the path of something moving above a spinning surface.

On Earth, that effect is zero at the equator and greatest at the poles—so air moving poleward will be increasing its vorticity due to the increasing Coriolis effect, and air moving equatorward will be decreasing in vorticity. Vorticity due to Earth's rotation is always positive (except at the equator, where it is zero).


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