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EZTip No. 22 - The 700 mb constant pressure chart

Updated: Jan 29, 2023

Do you ever look at the 700 mb constant pressure chart forecast as you are planning a flight? If not, then it's likely a good idea to read on further. The 700 mb chart is perhaps one of the most useful for general aviation pilots. It represents the weather at approximately 10,000 feet MSL. This is a very common flight planning altitude for many pilots flying light aircraft.

There are several locations in the EZWxBrief static weather imagery where you can find a 700 mb chart. It is located under the GFS, NAM and RAP/HRRR models. Shown below is the 700 mb constant pressure chart from the GFS model (recommended). Why use a constant pressure chart? First, take a look at this post that describes the reasons these charts exist in the first place.

There four fields presented on this chart. It is quite busy, but they include the 700 mb geopotential height field (black lines), relative humidity (green solid filled contours and green 50% unfilled contours), wind barbs (indicating wind speed and direction) and omega (red and blue contours). For brevity, let's take a look at the latter.

Omega is presented on these charts as red and blue solid contours. The units are in microbars/second (more on that later). Air readily moves horizontally throughout the atmosphere. That's what we call wind. However, air also retains a vertical component as well. Certainly thunderstorms are great models of rather disorganized air moving in the vertical or even downward in a convective downdraft. However, at the large scale, most air moves up or down in a more organized fashion, one that is called large scale ascent or descent, respectively. It is this large scale ascent that causes much of the weather we experience that isn't related to convection.

Omega simply shows where in the atmosphere the air is moving up or down through the 700 mb level. Negative omega (red contours) depicts where air is moving away from the surface of the earth. Again, this isn't an updraft like you might see in a healthy thunderstorm, but large scale you might see ahead of a strong cold front or associated with a large scale weather system or even mountain wave activity. Think about wading out in the ocean far off the beach and a wave (that has yet to break) moves through your body. You'll feel an upward push as the wave first connects with your body...that's similar to the large scale ascent in the atmosphere, but on a much, much smaller scale.

When you see areas of concentric red contours like you see below from the Great Lakes down through the central Gulf Coast, that's a great indication there is plenty large scale ascent to the weather and likely one you should avoid in a light aircraft. This doesn't say if the weather will have a convective component; but even if convection is unlikely, this is clearly a location where you'd will see some serious adverse weather.

On the other side of the coin are the blue contours of omega or what is referred to as positive omega. These depict areas of motion toward the surface of the earth. Again, this is not like a downdraft or microburst in association with a thunderstorm, but indicative of large scale atmospheric descent that may include mountain wave activity. You can see that the negative omega in red above that is likely associated with a strong cold front, but just to the west of that line, you can see a contour or two of positive omega that is indicative of stable, cold air pouring in behind the front. Negative omega is usually a sign of subsiding air which tends to dry out.

What about those units of microbars/second and why is negative designated as upward? The "microbars" is associated with pressure. We know that pressure decreases as altitude increases. So in order to get the signs right on the various equations, upward motion is deemed to be negative. The seconds in the units implies the rate of vertical ascent or descent. So highly negative or highly positive omega correspond to greater large scale ascent or descent in the atmosphere, respectively.

One last point. Given that much of the elevation west of the Continental Divide is above 5,000 feet, omega is likely to pick up on thermal activity. So it is not unusual during the warm season to see vast regions of negative omega during the afternoon in the higher terrain in the western U.S. - that simply is telling you about that annoying thermal turbulence and not necessarily about large scale ascent.

However, regions of high positive omega that are downwind of the various ridgelines in the western U.S. should get your attention. For example, below you can see concentric areas of blue contours (positive omega) that are downwind of the Rocky Mountains in southern Wyoming and norther Colorado. This is likely a scenario where downsloping winds could be developing during this time and may mean some serious mountain wave activity and possible turbulence.

Interested in learning more about the Skew-T? You can order your copy of The Skew-T log (p) and Me book today in soft cover or 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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