Marine Glossary

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pressure gradient

The pressure gradient is the driving force of the wind. It is how much the pressure changes in some given distance in the horizontal direction. When the isobars run east-west for example, and the pressure is 1020 mb at 47° latitude and 1016 mb at 46° latitude, then the gradient is 4 mb per 1° or 4mb per 60 nautical miles.

Wind speed is proportional to the pressure gradient. A gradient of 4 mb in 60 miles will create twice the wind that a change of 2 mb over 60 miles would, or 4 mb over 120 miles would. When the pressure drop is large over a short distance, the pressure gradient is said to be "high" or "strong" or "steep," when the pressure drop is low the gradient is said to be "low" or "weak."

Wind speed can be computed from a pressure gradient using the wind computer (G224). Notice in G426 that the pressure gradient alone is not enough to predict actual surface winds. The curvature of the isobars also influences the wind speed.

The direction of the pressure gradient is perpendicular to the isobars, from high to low. Thus with parallel isobars running east-west, with the high to the south, we would refer to this as a northerly gradient... meaning it would cause a wind blowing to the north.

See also gradient wind.