Ask A Scientist© Environmental Science Archive

Author:      t lai
While in a car when it is raining, why does the windshield get wetter when you 
are driving than when you stop (like at a stoplight)?  We noticed this because 
at a stoplight, we have to turn off the windshield wipers or it would make a 
squeaky noise.  Is not the rain coming down everywhere at the same time so 
that you would get the same amount of rain on the windshield?
Oh another thing:  Is it true that you get wetter when you run in the rain 
then if you were to walk in the rain?

Response #:  1 of 3
Author:      rickru
You can imagine how, when moving along a horizontal surface, you will pass 
more road signs than if you are standing still.  The faster you travel, the 
more road signs you will pass (until a friendly policeman stops you, that is).  
With the raindrops, your body or a car at rest will encounter a given number 
of drops.  Move your body or the car horizontally through a rainstorm and you 
will encounter more drops, thereby making yourself wetter.  There is naturally 
a variety of precipitation speeds in a storm, that is, in a storm the rains 
falls at different rates.  You therefore might experience less "wetting" than 
you might expect when traveling, just because the storm is made that way.  
But, given a theoretical "set-speed" storm, the faster you move through the 
storm the more raindrops you will encounter.

An interesting footnote is, that, above a given speed, you might actually 
become LESS wet, as the wind and vacuum created by your super-fast travel 
might cause less drops to actually be able to contact your car/plane/jet 
surface.  I do not have the figures available for how fast this would have to 
be, you might ask the question in the physics section.



Response #:  2 of 3
Author:      mortis
You might actually get wetter running than you would walking since your body 
exudes more moisture when you are more active.  I can get perfectly drenched 
when running on a hot day even if it is not raining.

Response #:  3 of 3
Author:      allsopp
Assuming no wind and the rain coming straight down, the only explanation for 
the windshield getting wetter while driving would be that spray from the tires 
of other cars is hitting the windshield in addition to the rain when the car 
is moving.  Even if it stops raining, spray from the wet road will cause you 
to need the wipers sometimes.  In answer to the second question, again 
assuming no wind and rain coming straight down, your body tends to lean 
forward when running, but is more upright when walking.  Thus more of
your body's surface area is exposed to the rain when running.  But if you are 
running, you get inside quicker and are exposed to the rain for a shorter time 
period.