Isobars are lines on weather maps that connect points of equal atmospheric pressure, similar to contour lines for elevation. Analyzing their pattern and spacing determines the strength and direction of wind flow across a region. The relationship between isobars and wind is a dynamic interplay of atmospheric forces.
The Driving Force: Pressure Gradient
The Pressure Gradient Force (PGF) is the initial cause of all wind movement, driving air from areas of higher pressure to areas of lower pressure. The PGF always acts perpendicular to the isobars, pointing directly toward the lower pressure center.
Wind speed is determined by the steepness of the pressure gradient, visualized by the spacing of the isobars on a map. Closely packed isobars indicate a steep gradient and result in strong winds. Conversely, widely spaced isobars signify a gentle pressure gradient, where the air moves more slowly. While the PGF dictates the initial motion, the wind does not ultimately flow directly across the isobars.
The Deflection: Coriolis Effect
As air begins to move due to the PGF, Earth’s rotation introduces the Coriolis Effect, an apparent deflection that modifies the wind’s direction. The deflection is always to the right of the wind’s path in the Northern Hemisphere and to the left in the Southern Hemisphere.
The strength of the Coriolis force increases with both wind speed and latitude, being zero at the equator and maximum at the poles. When the Coriolis force eventually balances the PGF, the resulting motion is the theoretical Geostrophic Wind. This wind flows parallel to the straight isobars. This parallel flow is the model used to determine wind direction in the upper atmosphere, typically above 1 to 2 kilometers, where friction is negligible.
Surface vs. Upper Air Flow
Wind flow patterns high in the atmosphere differ from what is experienced at the Earth’s surface. Above the friction layer, the Geostrophic balance of PGF and Coriolis force dominates, causing wind to flow parallel to the isobars. The friction layer, also known as the planetary boundary layer, typically extends up to 1,000 to 2,000 meters.
Within this boundary layer, drag caused by terrain features slows the wind down. Since the Coriolis force is proportional to wind speed, friction weakens the Coriolis force. The PGF, which remains unchanged, then becomes slightly dominant, pulling air inward across the isobars toward the lower pressure area. Surface wind crosses the isobars at a small angle, typically between 10 and 30 degrees, toward the low-pressure center.
Applying the Rules: Reading Wind Direction on a Map
To determine the surface wind direction from an isobar map, the combined effects of the three forces simplify into predictable rotation patterns around pressure centers. In the Northern Hemisphere, winds spiral inward and counter-clockwise around a Low-Pressure system (cyclone). Conversely, winds spiral outward and clockwise around a High-Pressure system (anticyclone).
The inward flow around a low means the wind crosses the isobars toward the center, causing air to converge and rise, often leading to cloud cover and precipitation. The outward flow around a high means the wind crosses the isobars away from the center, causing air to diverge and sink, which typically leads to fair weather. The flow is opposite in the Southern Hemisphere: clockwise around a low and counter-clockwise around a high.