Dear Tom,
What do isobars represent on weather maps?
—Mal Wallis, Lyons
Dear Mal,
A bar is a metric unit of pressure and isobars are lines on a weather map that connect points of equal air pressure. Because variations in air pressure drive atmospheric winds, isobars give meteorologists an easy way to assess wind direction and speed.
Closely spaced isobars indicate large pressure changes over a small area and suggest strong winds. Widely spaced isobars portray a weak pressure gradient typical of light wind situations.
Isobars also determine wind direction. Winds in the Northern Hemisphere blow clockwise around highs and counterclockwise around lows. Frictional effects at ground level cause winds to blow across isobars at about a 30 degree angle toward lower air pressure.