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EZTip No. 28 - The 500 mb chart and clear air turbulence

If you fly at or above 15,000 feet on a regular basis, you need to be checking the 500 mb chart in the EZWxBrief EZImagery during your preflight planning. It is one of the best tools to determine the potential for widespread mid-level severe clear air turbulence especially for the extended range forecast. The 500 mb level is ~18,000 feet MSL, but provides a good indication of the turbulence potential from 15,000 feet to 21,000 feet MSL.


We know that turbulence is caused when air is allowed to mix. This can occur when the atmosphere is unstable (e.g., convection) and when there is large-scale ascent in the atmosphere due to an upper level feature or orographic lifting. If the atmosphere is poised to mix, then any marked changes of wind direction and/or speed also becomes a candidate to generate clear air turbulence. The key is to recognize the various patterns in the 500 mb chart that may signify a rough ride so you can plan to avoid those areas from a timing or route perspective.


The image below is a 500 mb forecast from the GFS model. The GFS provides a forecast out to 16 days, but EZWxBrief only provides forecasts with a lead time out to 132 hours or 5.5 days given this is the length of time where the GFS has some reasonable skill to depict an upper-level flow that can be useful for those extended-range decisions.

Notice in this 500 mb forecast above, there is a very elongated positive tilted trough extending from Quebec Canada through the Midwest and southern Plains. This V-shaped pattern is a strong signal that the ride may be rough. In the magnified area below, notice how the wind direction (shown by the blue arrows) changes direction from the north to west over a relative short distance. The other parameter shown on this chart by shades of green, yellow, orange and red is called absolute vorticity. Vorticity is generated as spin is imparted on the air as it makes the turn around the upper-level trough. When the wind is perpendicular to the vorticity axis, it is called positive vorticity advection and will enhance large-scale ascent in the atmosphere leading to atmospheric mixing. Given the mixing potential, wind shift and wind speed change, this region is very likely to exhibit severe clear air turbulence. In fact, a SIGMET (not shown) was issued for severe mid-level turbulence from 10,000 feet to 25,000 feet as discussed in The Daily EZ Weather Brief for this day.

Of course, right before you are to depart, it's always best to check pilot weather reports (PIREPs) to determine what altitudes may be problematic for moderate or greater turbulence. Moreover, if the reports of severe turbulence become widespread, it's often the case that the Aviation Weather Center (AWC) will issue a SIGMET for severe turbulence as was this case. For more isolated areas of moderate or greater turbulence, also consider looking for any Center Weather Advisories (CWAs). PIREPs, SIGMETs and CWAs are available as layers on the EZMap in the EZWxBrief progressive web app. And don't forget to drill down in these areas and along your route with a Skew-T log (p) sounding analysis or forecast.


If you want to learn more about the Skew-T diagram, you can order your copy of The Skew-T log (p) and Me: A Primer for Pilots that is available in softcover and eBook format.


Most pilots are weatherwise, but some are otherwise™


Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief™

CFI & former NWS meteorologist

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