Closely space isobars often indicate a steep change in atmospheric pressure across pressure gradients. Because isobars are used to indicate connected points of equal or constant atmospheric pressure, the more closely these lines are placed means that strong winds are likely to be observed.
Isobars aid meteorologists in forecasting the behavior of weather systems on large scales by synthesizing the data gathered by individual weather stations into a bigger picture. When isobars are spaced close together, this means that there is a large pressure gradient. Conversely, when isobars are spaced far apart, this translates to a small pressure gradient. In the latter event, this means that calm conditions are more likely to prevail.
In low pressure systems in the Northern Hemisphere, where the air circulates counter-clockwise around the center of the system, isobars are likely to be spaced close together. Low pressure systems favor convective uplift, or the lifting of an air mass. This lifting results in the formation of clouds, windy conditions and precipitation. For example, the eye of a hurricane typically has closely spaced isobars on its periphery, where the winds are the strongest.
In high pressure systems in the Northern Hemisphere, where the air circulates clockwise around the center of the system, isobars are likely to be spaced far apart. High-pressure systems favor the sinking of air masses, which tends to spread apart regions of uniform atmospheric pressure. This reduces the likelihood of windy conditions and is typically associated with clear conditions.