Plain English Meaning

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.

Full 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.

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.