Rain drops hitting the side window of a car in motion often leave diagonal streaks if there is no wind. Why?

result of downward and sideways motion.

The diagonal streaks left by raindrops on a side window of a moving car with no wind are caused by a combination of factors: gravity, the motion of the car, and the shape of the raindrop.

When the car is in motion, the raindrops that hit the side window are also moving horizontally relative to the car. As these raindrops hit the window, gravity pulls them downward due to their weight. However, since the car is moving forward, the raindrops are also moving horizontally. This horizontal motion creates an angled path for the raindrop as it slides down the window under the influence of gravity.

The shape of the raindrop also plays a role. Raindrops are not perfectly spherical; they tend to be slightly elongated or oblong. As the raindrop slides down the window, its oblong shape causes it to tilt and rotate, which further contributes to the diagonal streak.

In summary, the combination of the car's motion, gravity, and the oblong shape of the raindrop results in the formation of diagonal streaks on the side window of a moving car with no wind.