What causes wind to flow parallel to the isobars above the friction level?

Wind flows parallel to isobars above the friction level primarily due to the Coriolis force. This force results from the rotation of the Earth, which causes moving air to deflect to the right in the northern hemisphere and to the left in the southern hemisphere. When air begins to move from high pressure to low pressure due to the pressure gradient force, the Coriolis force acts on it, pushing the air sideways and resulting in a wind pattern that runs parallel to the isobars.

At higher altitudes, where the effects of surface friction are minimal, the balance between the pressure gradient force and the Coriolis force leads to this parallel flow. Thus, the interaction between these forces is crucial in determining wind patterns in the atmosphere, especially in the context of larger-scale weather systems, where the influence of the Earth's rotation is significant.