I wonder if anyone can help me? I'm trying to find out if any research has been done on how fast a car would have to be travelling in order for vertically falling rain drops not to hit your back windscreen??!! I guess all sorts of things will come into play, like turbulence etc. but this really isn't my area and any help would be gratefully received!!

An excellent investigation topic, something i've thought about many a time when driving along the motorway. I assume it's a boundary layer development problem.

Regards James

We have some experience of this issue, and there are a number of factors. At most sensible speeds rain droplets will not hit the rear windscreen, but will be caught up in the separation region, or by the vortex regions behind the rear screen. This usually results in the water being dispersed into a mist or spray which will recirculate and find its way back to the screen. However, the more important issue is the path that water on roof etc takes along the vehicle, but that's a different story.

Hope this is of some help.