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Wind Systems

Wind Systems

Definition: Wind refers to the horizontal movement of air from areas of high pressure to low pressure due to the Earth’s attempt to balance out pressure differences. The Coriolis force, caused by the Earth’s rotation, deflects winds from moving in a straight path.A wind vane is used to determine the direction of the wind, while an anemometer is employed to measure its speed.

Pressure and Wind

Geostrophic Wind

  • Definition: Geostrophic wind refers to the wind pattern in the upper atmosphere, approximately 2-3 km above the Earth’s surface, where frictional effects from the surface are negligible.
  • ForcesInvolved: It is primarily influenced by the balance between the pressure gradient force and the Coriolis force.
  • Characteristics:
    • Occurs when isobars (lines of equal pressure) are straight.
    • The pressure gradient force (PGF) is balanced exactly by the Coriolis force, resulting in wind that flows parallel to the isobars.
    • Direction:
      • In the Northern Hemisphere, geostrophic winds blow parallel to isobars with lower pressure to the left.
      • In the Southern Hemisphere, they blow parallel with lower pressure to the right.

Cyclonic Circulation

  • Around Lows (Cyclonic Circulation):
    • Definition: Refers to wind circulation around a low-pressure area.
    • Characteristics:
      • Air converges towards the center of low pressure at the surface, rises, and then diverges aloft.
      • The direction of circulation:
        • Northern Hemisphere: Counterclockwise (cyclonic).
        • Southern Hemisphere: Clockwise (anticyclonic).
  • Linked to upward-moving air, which can result in cloud formation and precipitation.
  • Around Highs (Anticyclonic Circulation):
    • Definition: Refers to wind circulation around a high-pressure area.
    • Characteristics:
      • Air subsides from above towards the surface of high pressure, then diverges at the surface.
      • The direction of circulation:
        • Northern Hemisphere: Clockwise (anticyclonic).
        • Southern Hemisphere: Counterclockwise (cyclonic).
      • Generally associated with fair weather due to descending air suppressing cloud formation.
  • Relation to Upper-Level Circulation:
    • Wind circulation patterns at the Earth’s surface are often related to those at higher levels.
    • Convergence, convection currents, orographic uplift (caused by mountains), and frontal uplift contribute to rising air, essential for cloud formation and precipitation.

Forces Affecting the Velocity and Direction of Wind

Wind Definition: Wind is air in motion, blowing from high-pressure to low-pressure areas.

1. Pressure Gradient Force (PGF):

  • The force resulting from differences in atmospheric pressure.
  • The pressure gradient refers to the rate at which pressure changes over a given distance.
  • Strong pressure gradient = close isobars, weak pressure gradient = widely spaced isobars.
  • Drives wind from high-pressure to low-pressure areas.

2. Frictional Force:

  • Acts on the wind near the Earth’s surface.
  • Reduces wind speed, greatest at the surface, influencing up to 1-3 km elevation.
  • Minimal over sea surfaces due to less friction.

3. Coriolis Force:

  • Result of Earth’s rotation, described by French physicist Gaspard-Gustave de Coriolis in 1844.
  • Causes wind to shift to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
  • Increases with wind velocity and is directly proportional to latitude.
  • Maximum at the poles, absent at the equator.
  • Acts at a right angle to the pressure gradient force, resulting in wind blowing around low-pressure zones rather than straight into them.

4. Gravitational Force: Acts downward, contributing to the atmospheric pressure and aiding in maintaining the vertical structure of the atmosphere.

Interaction of Forces

  • Pressure Gradient Force (PGF): Drives wind from high to low pressure.
  • Coriolis Force: Deflects wind perpendicular to the PGF, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
  • Frictional Force: Slows down wind near the surface, reducing the Coriolis effect.
  • Combined Effect: Near the Earth’s surface, the interaction of these forces causes wind to blow at an angle to the isobars. Higher up, where friction is minimal, winds tend to flow parallel to isobars due to the balance between the PGF and Coriolis force.
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