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 level (e.g., 500 mb), in contrast to surface analysis charts which show pressure changes at a constant height (mean sea level).
They are primarily used because aircraft often fly on constant pressure surfaces (especially at higher altitudes) and weather balloon (radiosonde) data are reported as a function of pressure.
Meteorologists also utilize these charts because holding pressure constant simplifies the thermodynamic equations crucial for numerical weather prediction models.

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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?