PRESSURE GRADIENTS

PRESSURE GRADIENTS The change in baromet­ric pressure across a horizontal surface constitutes a pressure gradient. Where a pressure gradient exists, air molecules tend to drift in the same direction as that gradient. This tendency of mass movement of air is referred to as the pressure gradient force; on a weather chart, this is indicated by the spacing of isobars: the gradient being 'steep' if they are close together and 'gentle' if they are far apart. A steep gradient indicates strong winds and a gentle one slight winds. Wind is thus the horizontal motion of air in response to the pressure gradient force.

CORIOLIS FORCE Winds would follow the direction of
the pressure gradient.. if the earth did not rotate on its axis. . But the earth's rotation produces another force called the'
Coriolis force which tends to turn the flow of air. It was named after the French mathematician, Gaspard de Corio­lis, who first described it in 1835. It is a 'fictitious' force, needed to relate the movement of air masses over the earth's surface to its rotating coordinate system (the grid).

The direction of action of the Coriolis force is stated in Ferrel's law: "any object or fluid moving horizontally in the northern hemisphere tends to deflect to the right of its path of motion regardless of the compass direction of the path." A deflection towards the left is experienced in the southern hemisphere. Coriolis force is absent at the equator and increases towards the poles. The force is responsible for the formation and direction of movement of the anticyclones and whirlpools.