# Rain drops hitting the side windows of a car in motion often leave diagonal streaks. Why?

This happens due to the relative nature of velocity. The relative velocity of rain with respect to car makes certain angle with vertical.

Let $\theta$ be the angle made by the relative velocity of rain with the vertical. Then,

$\tan\theta=\frac{v_{\text{car}}}{v_{\text{rain}}}$

Hence, rain drops hit the side windows of the car by making an angle $\theta$ with the vertical and leave diagonal streaks.

[Read: Motion in a Straight Line]