Most weather maps show sea level pressure, which is the atmospheric pressure at sea level. Weather maps show atmospheric pressure using lines called isobars, which are similar to contours on a topographic map. Just as topographic contours show hills, basins, ridges and valleys, isobars show areas of high and lows pressure called highs and lows, ridges and troughs. These are formed by the circulation of the atmosphere around the Earth, and by movement of air upwards and downwards