Answer
The motion of the raindrops relative to the car's motion has a vertical component (the raindrops are falling vertically) and a horizontal component (from the car's horizontal motion). Therefore, when raindrops hit the side windows, the relative motion causes the raindrops to leave diagonal streaks.
When raindrops leave diagonal streaks on the windshield, there must be relative horizontal motion between the car and the raindrops, a horizontal motion which is perpendicular to the windshield's direction of motion. The most likely explanation is that a crosswind is blowing the raindrops across the face of the windshield as the raindrops fall vertically, thus resulting in diagonal streaks on the windshield.
Work Step by Step
The motion of the raindrops relative to the car's motion has a vertical component (the raindrops are falling vertically) and a horizontal component (from the car's horizontal motion). Therefore, when raindrops hit the side windows, the relative motion causes the raindrops to leave diagonal streaks.
When raindrops leave diagonal streaks on the windshield, there must be relative horizontal motion between the car and the raindrops, a horizontal motion which is perpendicular to the windshield's direction of motion. The most likely explanation is that a crosswind is blowing the raindrops across the face of the windshield as the raindrops fall vertically, thus resulting in diagonal streaks on the windshield.
