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Which of the following statements correctly describes the theoretical development of the geostrophic wind within the Northern Hemisphere?
  • A
    An air parcel initially at rest will move from high pressure to low pressure because of the coriolis force, but as the air moves it is deflected to the right and is eventually balanced by the pressure gradient force.
  • B
    As a surface pressure difference begins, air will start to move towards the lower pressure. As the wind gains speed, the pressure gradient force deflection increases to the right until it equals the coriolis force and the wind now blows perpendicular to the isobars.
  • C
    The pressure gradient force moves the air towards the low pressure, but surface friction starts to pull the air anticlockwise, eventually causing the wind to blow parallel to the isobars with the low pressure positioned downstream to the left.
  • D
    As the air starts to move and is accelerated by the pressure gradient force, it is increasingly deflected to the right by the coriolis force until both forces are balanced and opposite to each other.

Refer to figure.
Learning Objective 050.02.02.01.04: Explain the development of the geostrophic wind.


The GEOSTROPHIC WIND blows parallel to straight and equidistant isobars.
1. Wind begins as a flow of air perpendicular to the isobars, initially under the influence of the Pressure Gradient Force (PGF), from high to low pressure.
2. As the movement begins, the Coriolis Force (CF) begins to influence the moving air causing it to be deflected to the right of its path in the Northern Hemisphere, and to the left in the Southern Hemisphere. If Coriolis force was not present, the air would flow directly from the high to the low pressure, at 90º angle to the isobars, under the influence of the PGF.
3. This deflection continues until PGF and CF are opposite and balance each other and the airflow becomes parallel to the isobars – PGF and CF act perpendicular to the airflow direction and opposite to each other (balanced forces).

GEOSTROPHIC WIND:

  • Occurs above the friction layer.
  • Occurs at a latitude greater than 15 degrees.
  • Occurs when the isobars are straight, parallel and equidistant (constant gradient).
  • Occurs when the pressure situation is not changing rapidly (constant gradient/equidistant).

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