Raindrops hitting the side windows of a car in motion often leave diagonal streaks even if there is no wind. Why? Is the explanation the same or different for diagonal streaks on the windshield?

because of the raindrop velocity relative of the car has a vertical and horizontal component

Explanation:

The car moves in a horizontal direction relative to the ground. The raindrops fall in the vertical direction relative to the ground. Their velocity relative to the moving car has both vertical and horizontal components and this is the reason for the diagonal streaks on the side window. The diagonal streaks on the windshield arise from a different reason. The drops are pushed off to one side of the windshield because of air resistance.