Why do surface winds tend to cross the isobars?

Geostrophic wind blows parallel to the isobars because the Coriolis force and pressure gradient force are in balance. However it should be realized that the actual wind is not always geostrophic. Especially near the surface. The surface of the Earth exerts a frictional drag on the air blowing just above it.

Geostrophic winds blows parallel to the isobars. That is because the Coriolis force and pressure gradient force (PGF) are in balance. But near the surface the friction can act to change the direction of the wind and to slow it down. Coriolis force decreases at the surface and PGF stays the same. The difference in terrain conditions affects how much friction is exerted. Hills and forests force the wind to change direction more than flat areas. Answer: Friction reduces the speed so Coriolis is weakened.