An isobar is a line connecting points of equal atmospheric pressure on a weather map. The word comes from the Greek words isos — equal — and baros — weight. By plotting isobars at intervals based on pressure readings, areas of high and low pressure can be depicted on a map, just like hills and valleys on a contour map of a landscape. From studying the isobars on a map, meteorologists can predict whether the weather will be clear or cloudy, the wind strengths and directions and — taking into account latitude and time of year — the temperatures over a wide area.
Since it is not possible to measure atmospheric pressure at every point within the area covered by a weather map, isobars are based on air pressure readings taken at weather stations. Air pressure falls with altitude, so the readings are adjusted to sea level values to allow for variations in elevation. In the USA, pressure readings are normally taken every hour, and the isobars are normally at 4 millibar (mb) intervals, using a pressure of 1000 mb as the base. From a set of air pressure readings taken at the same time at various locations within an area, isobars can be plotted by estimating where the pressure would have the appropriate value.
For example, if a weather station reports a pressure of 1002 mb and another station a few miles to the north reports 1006 mb, it can be estimated that the 1004 isobar would pass between the two. On an isobar map, the isobars will be labeled with the pressure values they represent, for example 996 mb, 1000mb, 1004 mb and so on. The map will also show the individual readings at the various stations.
From an isobar map, meteorologists can determine the likely weather over the next few days. Low-pressure areas, known as cyclones, feature inflowing air that ascends at the center and are generally associated with cloud and precipitation. High-pressure areas, known as anticyclones, are associated with descending, outflowing air and usually bring dry, clear weather.
Wind flows from areas of higher pressure to areas of lower pressure. The isobars on a weather map show pressure gradients. If the isobars are far apart, this indicates as gentle pressure gradient and light winds. Where the isobars are close together, this indicates a steep gradient. The steeper the pressure gradient, the higher the wind speeds.
Pressure gradients tend to be steeper surrounding areas of low pressure than around areas of high pressure. If an isobar map is pictured as a landscape, high-pressure areas would look like gently sloping hills and low-pressure areas like steep-sided depressions. Low-pressure areas are, in fact, called “depressions” in some areas.
If friction is ignored, wind speed is determined by the pressure gradient force (PGF). This can be calculated as the result of the high-pressure value minus the low-pressure value, divided by distance, and is normally expressed as millibars per kilometer (mb/km). For example, if an isobar map shows a drop in pressure from 1008 mb to 996 mb over a distance of about 12 miles (20 km), the pressure gradient is 12 mb/20 km, which equals 0.12 mb/km. That is quite a steep pressure gradient, so strong winds would be predicted for this area.
Wind direction is affected not only by the orientation of the pressure gradient, but also by the Coriolis force that results from the Earth’s rotation. In the northern hemisphere, this causes the winds around a low-pressure area to rotate counterclockwise and those around a high-pressure area to rotate clockwise. The reverse is true in the southern hemisphere. The amount of deflection due to the Coriolis force is greater toward the poles and is also proportional to the wind speed.
Disregarding friction, the PGF and the Coriolis force can balance out, resulting in winds that flow parallel to the isobars. These are known as geostrophic winds and can occur high above the ground, where friction is not important. At the surface, however, friction slows the wind, lessening the Coriolis effect, and the winds tend to cross the isobars, spiraling inwards towards cyclones and outwards away from anticyclones, clockwise or counterclockwise according to the hemisphere.