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Isobars on a synoptic (weather) chart are lines along which the atmospheric pressure is the same. They are of the same nature as height contours on a geographical map. Usually they are drawn at intervals of 2 or 4 millibars. By definition, isobars can never cross each other.

The completed isobars usually reveal a few standard patterns. A set of curved isobars surrounding an area of low pressure reveals a depression, with the wind in the Northern Hemisphere blowing anticlockwise around its centre. A set of curved isobars surrounding a high pressure reveals an anticyclone, with the winds in the Northern Hemisphere blowing clockwise around its centre. Open V-shaped isobars with low pressure inside mark a trough of low pressure. In contrast N-shaped isobars with high pressure inside mark a ridge of high pressure. A col. is the indefinite isobar configuration between two highs and two lows arranged alternately, and has no particular type of weather associated with it other than light winds.

When isobaric patterns are plotted with information gathered at weather stations in the form of weather symbols, the weather forecaster can use his skill to predict the weather over the next few hours or days.

Simple isobars