Isobars

Isobars are lines that connect points of equal pressure on weather maps. The word originates from Greek, where isos means equal and baros means weight. Isobars are designed to describe the horizontal pressure distribution of an area, and are created from mean sea-level pressure reports. Because most of the weather stations are not located at sea level, but at a certain elevation, the pressure measured at every location has to be converted into sea level pressure before the isobars are drawn. This normalization is necessary because atmospheric pressure decreases with increasing altitude, and the pressure difference on the maps has to be due to the weather conditions, not due to the elevation differences of the locations.

Isobars are similar to height lines on a topographic map, and they are defined such that they can never cross each other. An important consequence of air pressure differences is wind, because wind blows from areas of high pressure to areas of low pressure. The greater the pressure contrast and the shorter the distance, the faster the wind will blow, so closer isobars mean faster wind. 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 certain patterns, making it useful for weather analysis or forecast. 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 a certain kind of weather, making it easier to forecast weather for a certain area.

See also Atmospheric pressure