WHY DOES THE RAIN SPLASH ON YOUR FACE WHEN YOU RUN?

     Suppose a boat crosses a flowing river from one bank to another straightly.  The boat will  appear to go and actually go in a inclined path.  This is due to net effect of river's velocity and boat's velocity.  To find boat's relative velocity with respect to the flowing river, we should use parallelogram law of vector addition.  That is, both the velocities should be added taking their directions into account.  
     Next, a man is running horizontally on the ground and the rain is falling vertically downwards (without wind).  To find the relative velocity of the rain with respect to the running man, we have to adopt the same method mentioned above.  Then we get that the rain will fall slantingly towards the man.  Here we add man's horizontal velocity and rain's vertical velocity vectorially.  Slanting rainfall can be observed in a moving car's window or in a moving train's window.  
     All velocities are measured relative to something.  No velocity is absolute.