When a boy moves down an inclined plane of inclination  ${37}^0$ with horizontal with a constant speed of $5m{s}^{-1}$, he finds that his umbrella is directed vertically upward. When he moves up on the same inclined plane with the same speed, finds that his umbrella is directed at angle  $\theta$  with horizontal, where  $\tan \theta =7/8$ .The speed of rain drops is