Notes on Atmospheric Forces and Wind: 4/7 Discussion

Wind and Atmospheric Force Balances

• Pressure Gradient Force (PGF)

  • Acts from high to low pressure.

• Real Wind

• Friction

• Geostrophic Wind

• Coriolis Force

  • Deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Why Do We Have Wind?

• Differential Heating: Imbalance from solar radiation leading to temperature differences.

• Pressure Relationship: P = ho R T.

  • Density ($
    ho$), gas constant ($R$), temperature ($T$).

• Balance in the Atmosphere: Constant attempt to maintain equilibrium.

Forces in the Atmosphere

• Key Forces:

  • Pressure Gradient Force (PGF): Result of pressure differences.

  • Coriolis Force: Caused by the Earth's rotation, affects wind direction.

  • Frictional Force: Resistance from the Earth's surface.

  • Centrifugal Force: Effects of rotating systems.

Understanding Gradients

• Definition: A gradient indicates a change between two points (e.g., temperature, pressure, height).

• Example: Movement from higher to lower height is walking down a gradient.

Pressure Gradient Force

• Formula:
  PGF = \frac{\Delta Pressure}{\Delta Distance}.

• Characteristics:

  • Always points from high to low pressure.

  • Stronger where isobars (or heights) are closer together.

Pressure Gradient Force at Upper Levels

• Involves height gradients, not just pressure gradients.

• Direction is down the height gradient.

• Formula:
  PGF = \frac{\Delta Height}{\Delta Distance}.

Coriolis Force

• Characteristics:

  • Apparent force due to Earth's rotation.

  • Turns right in the Northern Hemisphere and left in the Southern Hemisphere.

  • Maximum deflection occurs at poles.

  • No deflection at the equator.

Geostrophic Balance

• Definition: Equilibrium between Pressure Gradient Force and Coriolis Force.

• Friction: Minimal in upper atmosphere, leading to balance.

• Wind Direction: Flows parallel to isobars with low pressure to the left.

Surface Winds and Friction

• Surface Effects: Friction alters wind direction.

• Frictional Force: Acts against the wind direction due to surface drag.

• Balancing Forces: The magnitude of Coriolis and Friction must balance PGF.

Mass Continuity in the Atmosphere

• Convergence: If air is forced into a low-pressure area, it must rise, causing potential cloud formation.

• Divergence: Air moves outward from high pressure, leading to sinking and warming, typically no clouds.

• Cycle of Air: Sinking and rising motions lead to weather patterns and conditions.