Changes in atmospheric pressure are one of the most commonly used ways to forecast changes in the weather because weather patterns are carried around in regions of high and low pressure. Weather maps use lines of equal pressure called isobars to indicate areas of equal pressure. (Learn more about weather map symbols).A slowly rising atmospheric pressure, over a week or two, typically indicates settled weather that will last a long time. A sudden drop in atmospheric pressure over a few hours often forecasts an approaching storm, which will not last long, with heavy rain and strong winds.