I understand isobars are lines of equal pressure but I've never heard anyone mention what the difference in pressure is from one line to the next, is there a standard that everyone uses?

MIKE MOSS SAYS:       Randy,      There isn't a single, enformced standard for what we would call the "contour interval" for pressure analyses. However, there are three such spacings that are by far the most commonly used. Those three show an isobar for every 1, 2, or 4 millibars. Which is used depends on the scale of the map involved and the preferences of the meteorologists, along with his/her intent as it relates to seeing or displaying details in the pressure pattern. At the scale of the maps I use for showing WeatherScope isobars on TV, for example, I tend to set the isobar spacing at 2 millibars. I find that on those map projections, 4 millibars leaves out too much detail, especially in the warmer months when pressure patterns are weaker, and 1 millibar just results in way too many lines on the map.