The decrease of pressure between two points along a line perpendicular to the isobars divided by the distance between the points is called the pressure gradient. In other words, pressure gradient is the rate of change of pressure per unit horizontal distance.
It usually means a change in a direction perpendicular to the isobars. In fact, that is the direction in which the change is most rapid. The rate and direction of the change in air pressure are also known as ‘barometric slope’.
It is expressed in millibars per hundred kilometers, or per degree of latitude. A decrease of 34 mb in a horizontal distance of 24 km is considered to be a unit of pressure gradient.
Thus, a decrease of 3.4 mb in 48 km or a fall of 10 mb in 160 km signifies a steep pressure gradient. Isobars drawn closely on a weather map represent a steep pressure gradient, while isobars drawn farther apart indicate a weak pressure gradient.