Coriolis Force: Definition and Characteristics | Climatology | Geography

In this article we will discuss about the definition and characteristics of coriolis force.

Definition of Coriolis Force:

The direction of surface winds is usually con­trolled by the pressure gradient and rotation of the earth. Because of rotation of the earth along its axis the winds are deflected. The force which deflects the direction of winds is called deflection force. This force is also called coriolis force on the basis of famous scientist G.G. Coriolis.

Because of coriolis force all the winds are deflected to the right in the northern hemi­sphere while they are deflected to the left in the southern hemisphere with respect to the rotating earth. This is why winds blow counter-clockwise around the centre of low pressure (to make cyclonic circulation) in the northern hemisphere while they blow clockwise in the southern hemisphere.

The direction of surface winds is usually control­led by the pressure gradient and rotation of the earth. Because of rotation of the earth along its axis the winds are deflected. The force which deflects the direction of winds is called deflection force. This force is also called coriolis force on the basis of the name of famous scientist G.G. Coriolis (1792-1843) who observed and explained the process of deflection in wind direction for the first time.

Because of coriolis force all the winds are de­flected to the right in the northern hemisphere while they are deflected to the left in the southern hemisphere with respect to the rotating earth. This is why winds blow counter clockwise around the center of low pres­sure (to make a cyclonic circulation) in the northern hemisphere while they blow in clockwise direction in the southern hemisphere.

It may be mentioned that Coriolis force is not in force is real sense rather it is an effect of the rotational movement of the earth and hence it is also called as Coriolis Effect.

Characteristics of Coriolis Force:

The characteristic features of Coriolis Effect may be summarized as fol­lows:

(1) Coriolis force is not in itself a force rather is an effect of rotational movement of the earth.

(2) Coriolis force becomes effective on any object which is in motion (i.e., wind, flying birds, aircrafts, ballistic missiles, long-range artillery fire etc.).

(3) Coriolis force affects wind direction and not the wind speed as it deflects the wind (and other moving objects) direction from expected path.

(4) The magnitude of Coriolis force is determined by wind speed. The higher the wind speed, the greater is the deflection of wind direction due to resultant greater deflective (Coriolis) force.

(5) It becomes maximum at the poles due to minimum rotational speed of the earth while it becomes zero at the equator.

(6) It always acts at right angles to the horizontally moving air and other moving objects. The net effects is that the horizontal winds are de­flected to the right in the northern hemisphere and to the left in the southern hemisphere.

(7) The magnitude of deflection (Coriolis effects) is directionally proportional to:

(i) the sine of the latitude (sin 0° =0, 90° = 1)

(ii) the mass of the moving body, and

(iii) horizontal velocity of the wind.

It may be remembered that the direction of pressure gradient is always from high pressure to low pressure. The earth rotates from west to east. Every latitude is a complete circle. Equatorial latitudinal circle is the largest one and the latitudinal circles decrease pole-ward wherein polar circle is the smallest one. The whole earth completes one rotation along its axis roughly in 24 hours.

Thus, the rotational speed of the earth is highest at the equator and decreases pole-ward. When the wind moves either northward or southward following straight path in equatorial region it does not reach its destination because by that time the destina­tion place moves ahead and the wind lags behind because of high rotational speed of the earth (fig. 35.6).

Contrary to this the wind moving either northward or southward in high latitudes reaches ahead of its desti­nation because of decreasing rotational speed of the earth. Based on the deflective force (coriolis force) of the earth and deflection of winds Ferrel has propounded his law which is popularly known as Ferret’s Law.

This law states that ‘if one stands with one’s back towards the direction from where winds are coming they (winds) are deflected to the right in the northern hemisphere and to the left in the southern hemisphere.’

Buys Ballot, a Dutch meteorologist, formulated his law of wind direction in 1837 on the basis of relationship between wind direction and pressure. According to his law ‘In the northern hemisphere if you stand with your back to the wind, there will be low pressure to your left and high pres­sure to your right. In the southern hemisphere the coriolis deflection being to the left the situation is reversed.’

Frictional force not only retards the speed of winds but also deflects them. The frictional force is effective upto a height of a few thousand metres only. The hill-slope or ground slope facing the winds is called windward slope or onward side while the opposite slope is called leeward slope.