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
Static pressure to the atmospheric pressure at the altitude at which the aircraft is flying, across the operating range of weight and airspeed. Many authors describe the atmospheric pressure at the altitude at which the aircraft is flying as the "freestream static pressure". At least one author takes a different approach in order to avoid a need for the expression "freestream static pressure". Gracey has written "The static pressure is the atmospheric pressure at the flight level of the aircraft". Gracey then refers to the air pressure at any point close to the aircraft as the "local static pressure". The concept of