Isobars

Isobars are lines that connect points of equal atmospheric pressure on weather maps. Isobars are similar to elevation contours on topographic maps, and can never cross each other. Meteorologists use isobars on weather maps to depict atmospheric pressure changes over an area and to make predictions concerning wind flow.

The term “isobar” originates from the Greek, isos (equal) and baros (weight).

Isobars are drawn using data from mean sea-level pressure reports. Because most of the weather stations are not located at sea level, the pressure measured at every location is converted into sea level pressure before the isobars are drawn. The normal atmospheric pressure at sea level is defined as 1 atm of pressure or 29.92 inHg (760 mmHg or 760 torr). This normalization process is necessary because atmospheric pressure lapses (decreases) with increasing altitude, and the pressure difference on the maps is intended to depict weather conditions, not elevation differences.

Wind is a direct consequence of air pressure differences. The greater the pressure contrast over an area, the shorter the distance between isobars on a weather map depicting the area. Wind blows from areas of high to low pressure. The greater the contrast in pressure difference between two areas, the faster the wind will blow, so closer isobars on a weather map predict higher velocity winds.

Although the wind initially is controlled by the pressure differences, it is also modified by the influence of the Coriolis effect and friction close to Earth’s surface. This is why isobars can only give a general idea about the wind direction and wind strength.

A rule observed first in 1857 by Dutch meteorologist Christoph Buys-Ballott (1817–1890) described the link between isobars and wind: In the Northern Hemisphere, if you stand with your back to the wind, the low pressure area is located on the left. In the Southern Hemisphere, standing with your back to the wind means that the low-pressure area is on the right. This is called Buys-Ballott’s law.

Isobars can form patterns useful for weather analysis or forecasting. A cyclone or depression is an area of curved isobars surrounding a low-pressure region with winds blowing counterclockwise in its center in the Northern Hemisphere. An anticyclone is an area of curved isobars surrounding a high-pressure area, and the wind blows clockwise in the center of an anticyclone in the Northern Hemisphere. Open isobars forming a V-shape define a through of low pressure while high-pressured, N-shaped, open isobars define a ridge of high pressure. These features are usually predictable, and associated with certain kinds of weather, making it easier to forecast weather for a particular area.

See also Aerodynamics; Atmosphere, composition and structure; Atmospheric circulation; Barometer; Weather forecasting; Weather mapping.