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 *heights* of a specific pressure level (e.g., 500 mb) where the pressure itself is held constant, unlike surface charts which show varying pressure at a constant height.
These charts are used because aircraft often fly on constant pressure surfaces, meteorological data from weather balloons are reported as a function of pressure, and using pressure simplifies complex weather prediction equations.
On constant pressure charts, features like high (H) and low (L) indicate high and low *heights* of the pressure surface, respectively, and pilots use them for winds aloft, noting that labeled altitudes are approximations of the true varying altitude of that 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?