Definition
On a constant pressure chart, contour lines are lines that connect points of equal height (altitude) at which a specified pressure is found. They show how the pressure surface rises and falls across the atmosphere, much like elevation contours show how terrain rises and falls on a topographic map.
Plain English
Lines on a weather chart that join places where a chosen pressure level sits at the same altitude. Where the lines bunch together, the pressure surface is sloping steeply, which means stronger winds aloft.
Context Anchor
Seen on surface weather maps when reading pressure patterns around highs, lows, and areas of changing wind.
Derivation
From the French contour, meaning 'outline' or 'the line that bounds a shape.' On a weather chart, each line outlines the shape of a pressure surface in the atmosphere — its hills, valleys, and slopes.
Why Pilots Care
They reveal pressure patterns that drive wind direction and strength, directly affecting route planning and turbulence expectations.
Analogy
On a hiking map, a contour line connects places at the same height. On a weather map, a contour line connects places with the same weather reading.
Intuition Check
Do not think of contour lines as just decorative curves or outlines. Each line has meaning: it connects places where the same value exists.
Example Sentence 1
The briefer pointed to the tightly packed contour lines on the 500 millibar chart and warned of strong winds aloft along the route.
Example Sentence 2
Widely spaced contour lines indicated light winds across the planned route.