Notes on Force Balances in Atmospheric Winds: Lecture 4/9

Manifestation of Force Balances in Atmospheric Movements

• Focus on two scenarios:
• Winds well above the surface (friction negligible)
• Winds near the surface (friction significant)

Winds Well Above the Surface

• Only two forces to consider:
• Pressure gradient force (PgF)
• Coriolis force (CF)
• Concept of Isobars:
• Use isobars to represent pressure distribution:
  • P + delta P (higher pressure)
  • P - delta P (lower pressure)
• Pressure Gradient Force:
• Draws fluids from high to low pressure.
• Always directed perpendicular to isobars toward low pressure.
• Coriolis Force:
• Acts on moving objects, deflects to the right in the Northern Hemisphere.
• Balances with pressure gradient force when in equilibrium.

Geostrophic Balance

• When pressure gradient force and Coriolis force balance, results in geostrophic wind.
• Characteristics of Geostrophic Wind:
• Always parallel to isobars.
• Stronger wind correlates with tightly spaced isobars (high PgF).
• Weaker wind correlates with widely spaced isobars (low PgF).

Example of Wind Analysis at 500 millibars:

• Observational data collected from weather balloons.
• Analysis shows winds align closely with isobars - confirming geostrophic balance.
• Wind intensity increases with stronger pressure gradients; vice versa.

Winds Near the Surface

• Must consider friction in addition to PgF and CF.
• Forces at Work:
• Pressure gradient force points from high to low pressure.
• Newton's second law applies for balance: total forces = 0 for steady wind (not accelerating).
• Friction always acts opposite to wind motion, affecting speed and direction.

Force Diagram Insights:

• Sum of CF and friction forces must equal the PgF to maintain steady wind.
• CF acts to the right (in NH) at right angles to wind direction.
• Friction reduces wind speed, affecting CF magnitude, thus altering wind's direction across isobars.
• Result: wind flows at an angle from high to low pressure, not parallel to isobars.

Implications of Friction:

• More friction (e.g., in forest areas vs. ocean) leads to a greater angle across isobars.
• If friction is negligible, wind flows parallel to isobars (geostrophic balance).

Cyclones and Anticyclones:

• Winds around cyclones (low pressure):
• Counterclockwise circulation in the Northern Hemisphere.
• Winds around anticyclones (high pressure):
• Clockwise circulation in the Northern Hemisphere.
• Understanding circulation patterns is crucial for weather prediction.

Conclusion and Links to Weather Systems:

• The study of geostrophic and frictional balances is fundamental in meteorology, providing insights into wind behaviors that influence weather patterns.