500-millibar charts

The middle troposphere influences weather conditions near the surface as well as in the flight levels.

By Karsten Shein
Comm-Multi-Inst
Climate Scientist

The pilot responded to ATC, “8387 Zulu, climbing to FL330, proceeding direct to PIE.” He then added power, passing through 18,000 ft into the flight levels. The copilot adjusted the pressure altimeter to 29.92 inches of mercury. “I always wondered why 18,000 ft is the transition altitude,” said the younger pilot.

The senior pilot replied, “It’s just an altitude meant to give enough terrain clearance when switching over to standard pressure.

At least in the US, the highest spot in the lower 48 states is Mt Whitney, at 14,500 ft. It’s different in other places.

Last month I was flying to AMS (Amsterdam, Netherlands), and they had us maintain pressure altitude all the way down to 3000 ft.”

While the transition altitude of 18,000 ft above mean sea level (AMSL) does provide sufficient terrain clearance over any spot in the contiguous US, it also serves as an important altitude in the dynamics of the atmosphere.

Understanding conditions at this level can help pilots anticipate upcoming weather conditions both in the flight levels and closer to the surface.

Meteorologically speaking, 18,000 ft AMSL marks an important halfway point in the atmosphere.

With gravity exerting its force on the various molecules in the air, around 50% of the mass of those molecules is concentrated below approximately 18,000 ft – or, to be exact, 17,970 ft at average sea level pressure.

This mass, and therefore the atmospheric pressure it exerts, decreases exponentially with increasing altitude, with the greatest decreases occurring closest to the surface.

Above 18,000 ft, pressure decreases more slowly with altitude, until at around 51,000 ft, the atmosphere is just 10% as dense as it is at sea level.

Spin

Because the average surface pressure is just over 1000 millibars (or hectopascals), 50% of that value is 500-mb and 10% is 100-mb. Naturally, because aircraft rely on air density to generate thrust and lift, the mass of molecules in the air at a given altitude is all-important.

More critically for forecasters, the 500-mb constant pressure surface is a level at which the vertical movement of air in the troposphere can be evaluated and the movement of weather systems can be determined.

Knowing the vertical movement of air helps forecasters predict how quickly or intensely a low or high pressure will develop or decay. Vertical motion is measured as a function of vorticity, which is basically the “spin” in the air.

When that spin increases, it is known as positive vorticity advection (PVA), and it forces surface air to rise, creating or strengthening surface low-pressure systems and supporting the formation of thunderstorms.

Conversely, when the 500-mb flow exhibits negative vorticity advection (NVA), or a decrease in spin, it means air is sinking from the upper levels to the surface, filling in surface lows and generating high pressure.

Normally, positive vorticity occurs within troughs in the 500-mb flow, which normally corresponds to the position of surface lows and troughing in the jet stream above, with surface lows occurring out ahead of the 500-mb trough, beneath the area of greatest increase in vorticity.

The greatest decreases in vorticity, on the other hand, occur downwind of the axis of ridges in the 500-mb flow. These areas correspond to the locations of surface high pressures.

At jet stream levels, air flowing out of the base of a trough will diverge, lowering pressure and enhancing the upward flow of air from below. This can be an area of potentially strong turbulence created by both vertical and horizontal shear.

Similarly, turbulence frequently occurs downwind of ridges in the jet as the air flow converges going toward a leading trough. Convergence increases pressure and forces air to descend, clearing skies below.

Steering

While changes in vorticity at 500 millibars indicate the potential growth or decay of convective systems and surface pressure, the winds (or, more correctly, the pressure gradient force) provide guidance about the speed and path of mid-latitude weather systems. In general, large-scale surface weather systems tend to travel along a path that follows the wind patterns at 500 millibars.

This is largely due to the dependence on the vertical motion of the troposphere – the motion driven by changes in the 500-mb vorticity. As waves move through the 500-mb constant pressure surface, the locations of strong PVA and NVA move with them.

However, if it were always the case that surface lows remain beneath the exit region of the upper air trough, they would persist much longer than they do.

In reality, surface weather systems move at about half the speed of the winds at 500 millibars.

Because the flow patterns at 500 millibars normally take a day or two to change appreciably, these relationships between surface weather systems and the speed and direction of the 500-mb flow allow forecasters to predict where the storm will move over the next 24 to 48 hours with reasonable accuracy.

One other useful bit of information on 500-mb charts is the height of the 500-mb level. Because it is a constant pressure, the altitude at which it occurs varies with air density.

Places where air density is high are areas of high pressure at the surface and corresponding ridges in the wave flow at 500 millibars, while low air density is where one finds surface lows beneath the 500-mb troughs.

This has relevance when flying the flight levels, which is basically flying along a constant pressure surface, rather than at a constant height.

So, while you may be assigned FL300, in areas of high pressure, the actual altitude may be 32,000 ft, but in low-pressure trough regions, the altitude of FL300 may be 28,000 ft.

This behavior is behind the saying “When flying from high to low, look out below.” Fortunately, because all aircraft cruising the flight levels are varying their actual altitude equally, they can maintain clearance.

The height of the 500-mb pressure level, especially when referenced against the 1000-mb (near-surface) pressure level, can give pilots and airport personnel a better understanding of the potential of freezing rain or snow falling from a passing system.

Cold air is more compressed than warm air, so the altitude difference between the 500- and 1000-mb height can give an indication of the air temperature through which precipitation must fall.

It turns out that, when the 1000- to 500-mb height is below 5400 meters or around 17,700 ft, the lower atmosphere is cold enough to support snow, while above that thickness it is warm enough for rain.

Freezing rain is most likely to occur right around 5400-m thickness – indicated as 540 decameters on most charts, or, more commonly, referred to as the 540 line. Of course, other factors, such as a warm surface layer in the first few thousand feet above the ground, can affect the accuracy of this rule of thumb.

Karsten Shein is cofounder of 2DegreesC.org. He was director of the Midwestern Regional Climate Center at the University of Illinois, and a NOAA and NASA climatologist. Shein holds a comm-inst pilot license.