Mid-latitude Surface Winds Explained

Understanding the forces acting on winds helps determine their general direction and speed, especially near the Earth's surface in mid-latitudes.

Key Forces Affecting Winds

• Pressure Gradient Force ($PGF$): Acts from high pressure towards low pressure, initiating wind.
• Coriolis Force ($C$): Deflects moving air due to Earth's rotation (right in N. Hemisphere, left in S. Hemisphere). Its strength is proportional to wind speed ($C \propto \text{speed}$).
• Friction ($F$): Opposes wind motion near the surface, significantly reducing its speed.

Geostrophic Wind Conditions

Higher in the atmosphere, friction is negligible. The wind (geostrophic wind) flows roughly parallel to isobars where the Pressure Gradient Force ($PGF$) balances the Coriolis Force ($C$). The equation is approximately $PGF \approx C$.

Surface Wind Modifications

Near the ground, surface friction ($F$) is a major factor. It:

• Slows down the wind speed.
• Reduces the effect of the Coriolis force ($C$), as $C$ depends on speed.
• Causes the wind to blow across isobars at an angle towards lower pressure, rather than parallel to them.

Subgeostrophic Wind Definition

Because friction reduces wind speed, the Coriolis force becomes weaker than the Pressure Gradient Force ($C < PGF$). This state, where the wind is slower than the geostrophic speed and blows slightly across isobars, is termed subgeostrophic.

In mid-latitudes, surface friction consistently modifies wind patterns. Therefore, the surface winds are generally subgeostrophic.