What causes the 45-degree wind shift in surface winds?

The phenomenon of a 45-degree wind shift in surface winds is primarily attributed to the way Earth's surface interacts with the winds and the Coriolis effect. As winds blow across the Earth's surface, they are influenced by both friction with the ground and the Coriolis force.

At the surface, the wind slows down due to friction with various terrains, such as mountains, forests, or urban areas. This slowing down alters the balance between the pressure gradient force driving the wind and the Coriolis force that acts perpendicular to the motion.

In the absence of friction, winds are largely unaffected by Earth’s surface and follow a straight path that aligns with the isobars. However, friction causes the winds to deviate slightly, resulting in a shift of about 45 degrees from the isobars. This means that surface winds will flow at an angle—typically toward lower pressure areas but at an angle to the isobars—due to the dual influence of these forces, hence capturing the essence of how surface conditions impact wind direction.

This understanding explains why the interaction between Earth's friction and the Coriolis effect is critical in determining the surface wind shift rather than merely being a function of Earth's rotation or temperature gradients alone.