What Are Constant Pressure Charts?

Meteorologists use them instead of constant altitude charts for several reasons.

Scott Dennstaedt, Ph.D

Wednesday, April 23, 2025 at 08:00 AM ET

Verified

Edited By:

Travis Tingle

TAFs are one way to identify the likelihood of nonconvective low-level wind shear for your departure or destination airport. [Adobe Stock]

Key Takeaways:

Constant pressure charts depict the varying *height* of a specific constant pressure surface (e.g., 500 mb), contrasting with surface charts that show pressure changes at a constant height (mean sea level).
On these charts, "H" and "L" represent high and low *heights* of the pressure surface, respectively, not high or low-pressure centers, and the lines indicate constant heights.
These charts are favored because aircraft at higher altitudes often fly on constant pressure surfaces, weather balloon data is reported by pressure, and using pressure as a vertical coordinate simplifies meteorological computations.
Pilots commonly use constant pressure charts to ascertain winds aloft, noting that the associated mean sea level altitude is an approximation of the true, varying height of that pressure surface.

See a mistake? Contact us.

Question: What are constant pressure charts?

Answer: Constant pressure charts like the 500 mb chart shown below may seem obvious to interpret. It has many features similar to what you might see on the surface analysis chart. There are H’s and L’s that represent highs and lows, and there are lines that resemble isobars.

However, those H’s and L’s are not marking the position of high and low pressure centers, and the lines are not lines of constant pressure or isobars. So why is this different from the surface analysis chart?