Isobars

http://education.gsfc.nasa.gov/ess/units/unit2/u2l8a.html

Isobars are lines of equal value in pressure and can be represented on a graph, map, or plot. This is very similar to contour lines where the closer the lines are together, the smaller the gap between them, meaning a steeper rise in pressure and also like contour lines, they are not just random numbers, normally they are presented in intervals, and this specific isobar map, they are separated by intervals of 4. This is a isobar map of the united states portraying the pressure across the whole country and it even shows where a low and a high pressure system is located. One can see, that a low pressure system has, obviously, a lower pressure than the normal and the high pressure system has a higher pressure than the normal, or rest of the pressures.