Coriolis Force and Pressure Gradient Force

Coriolis Force and Pressure Gradient Force (PGF) along with Frictional Force are three important forces which determine the direction, speed and curvature of winds both horizontally and vertically.

What is Coriolis Force?

Fig. 1: Coriolis Force
  • Coriolis force is the force which is exerted by the rotating Earth on the objects moving on its surface.
  • For example, any moving object on Earth such as winds, ocean currents, gun bullets etc. deflect towards the right in the northern hemisphere and towards the left on the southern hemisphere.
  • In relation to winds, they blow from tropical high pressure belts to the equatorial low pressure belt. However, due to Coriolis force, they deflect towards the right and left in northern and southern hemisphere, respectively (Fig. 1).
  • Further, Coriolis force increases towards the poles. As a result, the moving object experience greater curvature in its trajectory.

Pressure Gradient Force

  • The rate of change in atmospheric pressure from one place to other is called pressure gradient.
  • The difference in the atmospheric pressure between those two places causes the winds to blow from the high pressure zone to low pressure zone.
  • The impact of difference in atmospheric pressure between two places on the movement of air is called pressure gradient force.
  • Pressure gradient force is perpendicular to the ISOBARS.
  • If the isobars are closer to each other, the pressure gradient force is high. Therefore, the wind speed is also higher.

Frictional Force

  • Frictional force is the force exerted by surface of the earth on the winds.
  • It is inversely related to the wind speed.
  • The wind closer to the ground faces greater friction which inhibits the speed of wind.
  • As the altitude increases, the wind gains momentum and greater speed due to lack of friction.

Relationship between Coriolis Force, Pressure Gradient Force and Frictional Force

Fig. 2: Impact of Friction on Wind Speed
  • The coriolis force is positively related to speed of wind. Higher the speed, higher the deflection of winds. Therefore, the frictional force is inversely related to coriolis force because it reduces wind speed. Hence, the deflection of wind is low at the ground due to higher friction but the deflection increases as we move to higher altitude.
  • The pressure gradient force and frictional force is also inversely related. Therefore, the speed of wind increases as the altitude rises. The pattern of winds looks as shown in Fig. 2.
  • The pressure gradient force has a positive relationship with coriolis force. Greater PGF leads to greater wind speed. Therefore, the coriolis force is also higher.

    Fig. 3: Ekman Spiral Source: Benoit Cushman-Roisin, Jean-Marie Beckers, in International Geophysics, 2011

In short, the relationship between these three forces leads to higher curvature in the trajectory of fluid as the altitude increases. As a result, Geostrophic Wind (Fig. 3) are formed in atmosphere and Ekman Spiral is formed in the oceans.