Contour line factors in a weather system. An isobar (from or "baros", meaning 'weight') is a line of equal or constant pressure on a graph, plot, or map; an isopleth or contour line of pressure. More accurately, isobars are lines drawn on a map joining places of equal average atmospheric pressure reduced to sea level for a specified period of time. In meteorology, the barometric pressures shown are reduced to sea level, not the surface pressures at the map locations. The distribution of isobars is closely related to the magnitude and direction of the wind field, and can be used to predict
Surface weather observation concerns during takeoffs and landings. The ICAO defines the International Standard Atmosphere (also known as ICAO Standard Atmosphere), which is the model of the standard variation of pressure, temperature, density, and viscosity with elevation/altitude in the Earth's atmosphere. This is useful in calibrating instruments and designing aircraft, and is used to reduce a station's pressure to sea level pressure (SLP) where it can then be used on weather maps. In the United States, the FAA mandates the taking of weather observations at larger airports for safety reasons. To help facilitate the purchase of an automated airport weather station, such as