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
Moody chart (Pa) (or head loss, "h" (m)) and flow rate through pipes. Head loss can be calculated using the Darcy–Weisbach equation in which the Darcy friction factor "f" appears : This formula must not be confused with the Fanning equation, using the Fanning friction factor "f", equal to one fourth the Darcy-Weisbach friction factor "f": Pressure drop can then be evaluated as: or directly from where "ρ" is the density of the fluid, "V" is the average velocity in the pipe, "f" is the friction factor from the Moody chart, "L" is the length of the pipe and "D" is the