Concepts of Atmospheric Forces

Wind refers to the horizontal air motion which is theoretically parallel to the pressure gradient force and perpendicular to the isobars between two places having differences in the pressure. Thus, Coriolis force, Pressure Gradient Force, Ferrel s Law play a major role in determining the velocity and direction of a wind.

Pressure Gradient Force

Pressure Gradient Force is created due to the difference in the atmospheric pressure, and it is directed from the high pressure to the low pressure.

Characteristics of the Pressure Gradient Force (PGF)

  • PGF is the primary cause behind the wind movement since the direction and magnitude of pressure gradient ultimately determines the wind direction and velocity.
  • The primary cause of the pressure gradient force is unequal heating of earth s land-sea surface or it is the temperature difference which creates a pressure gradient and thereby creates winds and there exists a close relationship between temperature and pressure as greater the temperature difference, steeper the pressure gradient and the resultant wind.
  • When the pressure gradient is steep, the velocity of wind is higher while due to weak pressure gradient force the wind blows at a low speed. Thus, it can be concluded that more the pressure difference, more the wind velocity.
  • The direction of the pressure gradient force is perpendicular to the isobars, as the direction of wind motion is always from the higher pressure to the lower pressure.
  • The spacing between various isobars shows the magnitude of pressure change which occurs over a particular distance, thus the velocity of wind can also be estimated through the analysis of spacing of isobars. The isobars which are closely spaced signify a steep pressure gradient and high-velocity winds while the widely spaced isobars indicate a weak pressure gradient and wind with low speed.
  • The diurnal (daily) variation of the temperature and pressure, and the related effect thereof like PGF, are limited to a much shallow layer of the atmosphere while latitudinal variation in the amount of insolation received is the cause of atmospheric circulation at much larger scale.

Coriolis Force

Coriolis force is an apparent force created due to the rotation of the earth about its axis. Coriolis force was demonstrated by the Gaspard de Coriolis in 1844. Strictly speaking, Coriolis force is not a force but an effect which results due to rotational movement of the earth and the movement of air relative to the earth, and has its impact on every moving object, whether a bullet fired from the gun or an ocean current.

Characteristics of the Coriolis Force

  • The direction of this apparent effect is stated through Ferrel s Law, as due to the Coriolis effect all winds/objects move towards their right in the northern hemisphere and towards their left in the southern hemisphere with respect to the rotating earth. Thus, in the northern hemisphere winds blow counter-clockwise along the centres of low pressure while clockwise in the southern hemisphere.
  • The Coriolis effect has a zero value at the equator while it increases progressively towards the poles. Coriolis force acts at an angle of 900 to the horizontal direction of the wind, and at the same time, it is directly proportional to horizontal wind speed. Since vertical velocities are so small, due to this the Coriolis effect on them is negligible.
  • With the increase in the pressure gradient force, the velocity of wind increases, and this leads to the more deflection in the direction of the wind.

Ferrel s Law

Because of the rotation of earth all winds/objects are deflected to their right in the northern hemisphere and to their left in the southern hemisphere with respect to the rotation of the earth, this phenomenon is known as Ferrel's law, as it was first proved by an American meteorologist named William Ferrel.

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