Isobars

Isobars are contour lines of equal pressure on a weather map. Each line passes through a pressure of given value. Isobars, as seen in this map, pass through atmospheric pressures of 1000 + 4 or - 4.

http://www.weatherchannel.com.au/Magic94scripts/mgrqispi94.dll?appname=WC&prgname=WC&Function=Maps&Map=Syn