Certain weather maps allow us to view High and Low pressure systems that control the weather. An isobar is a line connecting locations of equal barometric pressure. Isobar maps show where pressures are relatively high and low, and show us where pressure changes are gradual or dramatic over a distance.
The white numbers are station reports of sea-level pressure in millibars (mb). Isobars are normally drawn at 4 mb intervals, with 1000 mb being the base value. The small yellow numbers are contour labels, which identify the value of an isobar (for example 1004 mb, 1012 mb, etc.).
The two maps below show the isobars drawn for the entire United States. The top map has lines only and the bottom map is color coded.