Isobars

http://www.weatheronline.co.uk/reports/wxfacts/Isobars-on-surface-maps.htm

Isobars are lines that are drawn on weather maps that connect equal lines of pressure.In the example above you can see a red line that connects pressures at 1020 and indicates it at high. There is another line that connects 1010 and is represented using a blue line to indicate low pressure. This maps it easy for someone to look at and easily read the high and low pressures that are connected using lines.