Rain is falling vertically with a speed of 4 ms^-1  After some time, wind starts blowing with a speed of 3 ms^-1 in the north to south direction. In order to protect himself from rain, a man standing on the ground should hold his umbrella at an angle $\theta$ given by

Velocity of rain (v_r) = 4 ms^-1 vertically downwards. Velocity of wind (v_w) = 3 ms^-1 from north to south direction. A rain drop is acted upon by two velocities v_r and v_w as shown in Fig. 2.55. From the triangle law, the resultant velocity of the rain drop is v = OW. In order to protect himself from rain, he must hold his umbrella at an angle $\theta$ with the vertical (towards north) given by 

                                       $\tan \theta =\frac{RW}{OR}=\frac{v_w}{v_r}=\frac{3}{4}$