Lesson 5 -Pressure gradient^J Coriolis force; geostrophic global air circulation

Lesson Overview

  • Pressure Gradient

  • Coriolis Force

  • Geostrophic Global Air Circulation

Pressure Gradient Force (PGF)

  • The Pressure Gradient is the horizontal difference in air pressure at Earth's surface.

  • A difference in pressure generates a force called Pressure Gradient Force, causing air to move from high-pressure areas to low-pressure areas.

  • The steeper the pressure gradient, the stronger the pressure gradient force, leading to stronger winds.

  • Isobars:

    • Lines on a map that connect areas of equal pressure.

    • Used in weather maps to assess the strength of the pressure gradient force.

    • Pressure measured in millibars (mb), typically drawn at 4 mb intervals.

  • Closer isobars indicate a stronger pressure gradient and therefore, stronger winds.

  • Wind direction is influenced by the location of low-pressure vs. high-pressure areas.

Illustrating Pressure Gradient Force

  • When isobars are close together, the PGF is steeper, resulting in stronger winds.

  • When isobars are further apart, the PGF is weaker, resulting in gentler winds.

Coriolis Force (CF)

  • The Coriolis Force refers to the deflection of wind due to Earth's rotation.

  • Strongest at the poles and weakest at the equator (especially absent at 5° N and S).

  • The strength of the Coriolis force relies on wind speed; faster winds mean stronger CF.

  • Wind deflection directions:

    • Southern Hemisphere: Winds deflected to the left.

    • Northern Hemisphere: Winds deflected to the right.

  • This directional behavior is known as Ferrel's Law.

Illustrating Coriolis Force

  • Maximum deflection occurs at the poles, with no deflection at the equator.

  • CF characteristics differ between the two hemispheres:

    • Northern Hemisphere: deflection to the right.

    • Southern Hemisphere: deflection to the left.

Geostrophic Flow

  • Geostrophic Air Flow: Occurs when air moves parallel to isobars, at right angles to the pressure gradient, rather than across it.

  • Geostrophic flow arises when PGF is balanced by the Coriolis force.

  • This flow predominantly occurs in the upper atmosphere and over oceans, where friction is minimal.

  • Near the land surface, friction slows the wind due to resistance (e.g., buildings, mountains).

Geostrophic Wind

  • Geostrophic wind blows parallel to isobars, is perpendicular to PGF.

  • Occurs specifically when PGF equals CF, which happens in upper atmospheres and oceans.

Diagram Summary

  • Pressure Gradient Force (PGF) indicates air moving from high pressure to low pressure across isobars.

  • Coriolis Force (CF) causes wind deflection based on hemisphere:

    • Left in Southern Hemisphere

    • Right in Northern Hemisphere

  • When PGF and CF are balanced, geostrophic flow occurs, moving parallel to isobars.

Pressure Cells Rotation

  • Pressure cells spin in different directions according to hemisphere:

    • Clockwise in High-Pressure systems of the Southern Hemisphere.

    • Counter-clockwise in Low-Pressure systems of the Southern Hemisphere.

  • Northern Hemisphere:

    • Clockwise in Low-Pressure systems.

    • Counter-clockwise in High-Pressure systems.

Exam Questions

  1. Identify the force caused by Earth's rotation (Coriolis force).

  2. Explain the initial northward movement of air.

  3. Determine wind direction at point A.

  4. Identify the hemisphere of developing geostrophic wind and justify (deflection to the right).

  5. Substantiate the statement regarding wind at point B being geostrophic (parallel to isobars, balance of PGF and CF).

  6. Explain how geostrophic wind develops in eight lines of text (movement from high to low pressure, deflection by CF until balance).

Memo

  1. Coriolis Force (CF)

  2. Pressure higher in the south.

  3. Southwest (SW).

  4. Northern hemisphere; deflection to the right.

  5. Parallel to isobars and PGF balances CF.

  6. Initial movement due to PGF, deflection caused by CF until balanced.