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EZTip No. 22 - The 700 mb constant pressure chart

Updated: Jan 29

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.