Notes on Forces Affecting Wind and Weather Patterns

Overview of Forces in Meteorology

• Four primary forces affecting wind and weather patterns:

  • Pressure Gradient Force

  • Coriolis Force

  • Centrifugal Force

  • Friction

Definitions and Mathematical Representations

• Force:

  • An influence producing or preventing motion; mathematically described as a vector (has both magnitude and direction).

  • Example: Weight is a force due to gravity acting on mass (weight = mass * gravitational acceleration).

• Pressure Gradient Force (PGF):

  • It arises from differences in atmospheric pressure across space.

  • Defined mathematically as F_{PG} = \frac{\Delta P}{\Delta x} (change in pressure divided by distance).

  • Moves air from high-pressure areas to low-pressure areas.

Wind Patterns and Forces

Wind Schematics

• Two configurations of wind based on forces acting on parcels of air:

  1. Geostrophic Wind:

  • Straight-line flow where PGF and Coriolis forces balance each other:

    • Wind flows parallel to isobars (lines of constant pressure).

    • In Northern Hemisphere, winds deflect right of PGF; in Southern Hemisphere, deflect left.

    • Valid for low friction conditions, mostly above 200-300 meters above ground level.

  1. Gradient Wind:

  • Involves three forces: PGF, Coriolis, and centripetal acceleration due to curvature of flow.

  • Flow is typically around curves and doesn't parallel the isobars exactly.

Coriolis Effect

• Caused by Earth's rotation, affecting wind direction:

  • Deflection to the right in the Northern Hemisphere and left in the Southern Hemisphere.

  • Mathematical representation: C = 2 \cdot \omega \cdot v \cdot \sin(\phi) where:

  • \omega = Earth’s rotation rate,

  • v = velocity of the moving air.

  • \phi = latitude (Coriolis force is maximum at poles and zero at the equator).

Friction's Role in Wind Dynamics

• Friction affects wind speed and direction, especially closer to the Earth's surface:

  • Frictional force acts opposite the wind direction, reducing speed.

  • Due to friction, near the surface, winds tend to flow inward towards low-pressure areas rather than parallel to isobars, unlike geostrophic flow.

Wind Around Pressure Systems

• High-Pressure Systems (Anticyclones):

  • Air diverges from high-pressure areas, typically resulting in clear, dry conditions.

  • Clockwise flow in the Northern Hemisphere, counterclockwise in the Southern Hemisphere.

• Low-Pressure Systems (Cyclones):

  • Air converges towards low-pressure centers, leading to cloud formation and potential storms.

  • Counterclockwise flow in the Northern Hemisphere, clockwise in the Southern Hemisphere.

Conclusion

• Understanding these forces and their interactions allows us to predict wind behavior and weather patterns effectively.

• The interplay between pressure gradients, rotational dynamics, and friction is crucial for interpreting and forecasting meteorological phenomena.

• Remember:

  • Geostrophic Wind: balanced forces, straight-line, high speeds.

  • Gradient Wind: curved flow, incorporates the effect of additional forces (centrifugal acceleration).

  • Friction: acts primarily at the surface, altering wind speeds and directions significantly in lower atmospheric layers.