Definition
A weather chart that shows the height, in feet above mean sea level, at which a selected atmospheric pressure is found, along with temperature, wind, and moisture data at that level. Common pressure surfaces charted include 850 mb, 700 mb, 500 mb, 300 mb, 250 mb, and 200 mb, each corresponding to a typical altitude band used in upper-air analysis and flight planning.
Plain English
Instead of showing what the weather looks like at a fixed altitude, this chart picks a fixed air pressure and shows how high you have to go to find it. The lines on the chart show where that pressure sits higher or lower in the atmosphere, which tells forecasters about winds and weather systems aloft.
Context Anchor
Seen in upper-air weather analysis, forecast discussions, and preflight planning for winds and temperatures above the surface.
Derivation
"Constant" means unchanging, and "pressure" refers to atmospheric pressure. The chart holds the pressure value constant and lets the altitude vary, which is the reverse of a typical altitude-based weather map. Naming it after what stays fixed makes the concept easier to recognize on sight.
Why Pilots Care
Reveals jet streams, wind patterns, and turbulence areas that affect fuel planning, routing, and passenger comfort at typical jet cruising levels.
Analogy
Imagine measuring how deep the snow is by walking around with a ruler set to a fixed depth and marking every spot where you hit that depth. The pattern of those marks shows you where snow is piled high and where it's thin. A constant-pressure chart works the same way, but with air pressure instead of snow depth.
Grounding Statement
Picture a pressure layer in the atmosphere as a flexible sheet that rises and dips over the Earth; the chart maps weather along that sheet.
Intuition Check
Do not read constant-pressure as constant altitude. The pressure is the fixed value; the height of that pressure level changes from place to place.
Example Sentence 1
Before the long flight at FL340, she reviewed the 250 mb constant-pressure chart to locate the jet stream and pick an altitude with a strong tailwind.
Example Sentence 2
Reviewing the constant-pressure chart helped the crew select an altitude with favorable tailwinds.