Regions of high and low atmospheric pressure develop at the earth's surface for a variety of reasons, including heating or cooling, and the rising or sinking of air due to surface topography or movements of air at higher levels in the troposphere. When pressure changes occur gradually over a large area, the isobars describing the horizontal change in pressure, or the pressure gradient, will take on a banded appearance, as seen below.
Example of parallel isobars
Much of the convection that takes place on earth is more localised. When low or high pressure develops more locally, the resulting high- and low-pressure regions are cell-like systems, and isobars describing the patterns of air pressure will include one or more closed, circular isobars, as shown below.
Closed isobars around a low pressure cell.
Closed Isobars
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