What causes surface winds to flow across isobars at an angle?

Surface winds flow across isobars at an angle primarily due to surface friction. As wind moves along the atmosphere, it encounters resistance from the Earth's surface, including obstacles such as terrain, vegetation, and buildings. This friction slows down the wind and alters its direction, causing it to deviate from the geostrophic wind flow, which is parallel to the isobars.

This frictional effect causes two primary changes: a reduction in wind speed and a resultant change in the balance between the pressure gradient force and the Coriolis effect. While the Coriolis force is responsible for causing the wind to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, it does not directly result in the angle at which wind crosses isobars at the surface. Surface friction effectively dampens the Coriolis force's influence, allowing the wind to cross isobars rather than flow parallel to them.

In summary, surface friction is the key factor that causes winds to cross isobars at an angle, leading to the characteristic spiral of winds in high and low-pressure systems as they interact with the Earth's surface.