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Hazardous Weather: Upper Level Trough

Source:, Featuring Scott Dennstaedt

upper level trough

trough axis


"For most pilots, the 500 millibar constant pressure chart is unfamiliar. The 500 millibar chart shows the weather features that are roughly 18,000 feet above sea level. Everywhere on this chart the pressure is 500 millibars. What is changing is the mean sea level "height" of the 500 millibar surface.

The height contours are shown in black and are labeled in decameters. For example, the 582 line shown through western North Dakota indicates that the height of the 500 millibar surface is 582 decameters or 5,820 meters above sea level. So why does the height of the 500 millibar surface vary? There are many factors with air density driven by temperature and moisture being one of the primary reasons. For now, let me make a couple of observations using this "generic" 500 millibar chart.

The first thing you notice is this "U-shaped" feature in the 500 millibar flow. That feature is called an upper-level trough. The first thing to do is to split the trough into two "symmetric halves" with a line oriented north and south as shown by the red vertical line. This is called the trough axis. On the east side of the axis, cyclonic circulation is favored and is where you'll likely encounter the most serious adverse weather or degrading conditions.

On the west side of the trough axis anti-cyclonic circulation is favored. Adverse weather can occur on the western side of the trough, but is much less likely. For example, turbulence, including severe or extreme turbulence, can be found on both sides of the axis of some very sharp upper-level troughs.

The trough can also be split into quadrants by drawing a line perpendicular to the trough axis producing four distinct sectors. The lower right sector will contain the most likely region for adverse weather."

Note: The 500 MB Constant Pressure Chart and other helpful weather tools can be found at the National Weather Service website: