Definition
A horizontal wind that flows in a straight line, parallel to isobars (lines of equal pressure), at a constant speed. It results from a balance between the pressure gradient force (which pushes air from high to low pressure) and the Coriolis force (which deflects moving air due to the Earth's rotation). Geostrophic wind occurs in the upper atmosphere where surface friction is absent.
Plain English
A high-altitude wind that blows in a straight line because two forces — one pushing it sideways, and one bending it from the Earth's spin — cancel each other out perfectly.
Context Anchor
Seen in aviation weather discussions about winds aloft, pressure patterns, and why upper-level winds often flow nearly parallel to isobars on weather charts.
Derivation
From the Greek 'geo' (earth) and 'strophe' (turning). The name reflects that the wind exists because of the Earth's rotation — the turning of the Earth creates the Coriolis force that shapes this wind.
Why Pilots Care
It explains the direction and speed of winds that form jet streams and affect high-altitude navigation and fuel planning.
Analogy
Think of two steady influences on a moving object: one pushes it one way, and another turns it sideways. When those two effects balance, the object keeps moving along a steady path instead of crossing directly toward the push.
Grounding Statement
Picture air high above the ground, far from any hills or surface drag, settling into a steady straight-line flow because the force pushing it toward low pressure is exactly matched by the Earth's rotational deflection pulling it sideways.
Intuition Check
Geostrophic wind is not usually the exact wind at the surface. It is an idealized wind used to understand winds above the surface, where friction has much less effect.
Example Sentence 1
At cruise altitude, the winds aloft closely approximate geostrophic wind, flowing parallel to the isobars on the upper-level chart.
Example Sentence 2
Flight planning software uses geostrophic wind values to estimate true winds aloft.