Isobars are lines connecting points of equal atmospheric pressure on a weather map. By plotting isobars, meteorologists can predict weather, wind strength and direction, and temperatures. Pressure readings are taken at weather stations and adjusted to sea level values. Low-pressure areas bring clouds and precipitation, while high-pressure areas bring clear weather. Wind flows from areas of higher pressure to areas of lower pressure, and wind speed is determined by the pressure gradient force. Wind direction is influenced by the Coriolis force, resulting in counterclockwise winds around low pressure areas in the Northern Hemisphere.
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, high and low pressure areas can be represented on a map, much like hills and valleys on a landscape contour map. By studying the isobars on a map, meteorologists can predict whether the weather will be clear or cloudy, the strength and direction of the wind and, taking into account the latitude and time of year, the temperatures over a large area.
Because it is not possible to measure air pressure at every point within the area covered by a weather map, contours are based on air pressure readings taken at weather stations. Barometric pressure decreases with altitude, so readings are adjusted to sea level values to allow for elevation changes. In the United States, pressure readings are normally taken every hour and contours are normally in 4 millibar (mb) intervals, using a pressure of 1000 mb as a basis. From a series of air pressure readings taken simultaneously at various points within an area, contours can be plotted by estimating where the pressure would have the appropriate value.
For example, if one weather station reports a pressure of 1002 mb and another station a few miles north reports 1006 mb, the 1004 isobar can be estimated to lie between the two. On a contour map, contour lines will be labeled with the pressure values they represent, such as 996mb, 1000mb, 1004mb, and so on. The map will also show the individual readings at the various stations.
From an isobar map, forecasters can determine the likely weather for the next few days. Low-pressure areas, known as cyclones, have airflow rising in the center and are usually associated with clouds and precipitation. High-pressure areas, known as anticyclones, are associated with descending and outgoing air and usually bring dry, clear weather.
Wind flows from areas of higher pressure to areas of lower pressure. Isobars on a weather map show pressure gradients. If the isobars are far apart, this indicates a slight pressure gradient and light winds. Where the isobars are close together, this indicates a steep slope. The steeper the pressure gradient, the higher the wind speed.
Pressure gradients tend to be steeper in surrounding low-pressure areas than in surrounding high-pressure areas. If an isobar map is depicted as a landscape, high pressure areas would appear as gently sloping hills and low pressure areas as steep depressions. Low-pressure areas are, in fact, called “depressions” in some areas.
If friction is ignored, the 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 the distance, and is normally expressed in millibars per kilometer (mb/km). For example, if an isobar map shows a pressure drop from 1008 mb to 996 mb over a distance of approximately 12 miles (20 km), the pressure gradient is 12 mb/20 km, which equals 0.12 mb/km. This is quite a steep pressure gradient, so strong winds would be expected for this area.
Wind direction is influenced not only by the orientation of the pressure gradient, but also by the Coriolis force that results from the rotation of the Earth. 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. In the Southern Hemisphere the opposite is true. The amount of deflection due to the Coriolis force is greatest towards the poles and is also proportional to wind speed.
By ignoring friction, 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 of importance. At the surface, however, friction slows the wind, decreasing the Coriolis effect, and the winds tend to cross the isobars, spiraling inward towards the cyclones and outward away from the anticyclones, clockwise or counterclockwise at depending on the hemisphere.
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