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 map the varying *height* of a specific constant pressure level (e.g., 500 mb), differing from surface charts that show pressure adjusted to a constant height (mean sea level).
On these charts, "H" and "L" denote high and low *heights* of the pressure surface, not high or low pressure centers.
Their widespread use stems from historical aviation practices (aircraft flying constant pressure surfaces), the way radiosonde data is reported, and the simplification of thermodynamic equations for weather prediction.
Pilots utilize these charts primarily for winds aloft, understanding that any displayed altitude (e.g., 18,000 ft msl) is an approximation for the true constant pressure surface.

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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?