Definition
Lines drawn on a surface weather map connecting points of equal atmospheric pressure, with each line representing a single pressure value (typically in millibars or hectopascals) reduced to sea level for consistency.
Plain English
Lines on a weather map that join places where the air pressure is the same. Where the lines are close together, pressure changes quickly across a short distance; where they are far apart, pressure changes slowly.
Context Anchor
Seen on surface weather maps when comparing high-pressure and low-pressure areas and the wind patterns around them.
Derivation
From the Greek 'isos' meaning 'equal' and 'baros' meaning 'weight' or 'pressure.' So an isobar is literally an 'equal-pressure' line — a useful reminder of exactly what the line represents.
Why Pilots Care
Isobars reveal pressure gradients that determine wind speed and direction, directly affecting flight planning, fuel use, and turbulence risk.
Analogy
Think of contour lines on a topographic map: instead of joining points of equal elevation, isobars join points of equal pressure. Steep terrain shows up as closely spaced contours; strong winds show up as closely spaced isobars.
Grounding Statement
Tightly spaced isobars indicate a steep pressure change and stronger winds.
Intuition Check
Isobars are not physical lines in the sky and they are not fronts. They are map lines that show equal air pressure.
Example Sentence 1
The tightly packed isobars across the route warned the pilot to expect strong surface winds.
Example Sentence 2
Widely spaced isobars on the map suggested light winds along the route.