Smell of Rain

name         Johnny
status       student
grade        6-8
location     CO

Question -   What makes rain smell the way it does?
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Johnny,

Really pure water does not usually smell of anything to most people. 
However, rain water (depending on how you get to it) can contain 
ions, or trace organic compounds. For example, rain water that has 
collected from drainage pipes or barrels always seem to smell 
metallic to me. This might be because I am actually smelling certain 
compounds that I associate with metals. Also, the first time it 
rains after a dry spell, the rain kicks up dust from the ground, and 
gives us that "first day of rain smell". In some languages, there is 
actually a word for this smell. So I guess, the answer to your 
question is that it is not the water that smells but usually is 
brought along with the water/rain.

Greg (Roberto Gregorius)
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Johnny,

To get right to the point, what the humidity in the air increases, 
it changes the vapor pressure of the atmosphere (locally 
anyway).  Nitrates from the ground are then released and that is 
what you are smelling.  An interesting thing is that the nitrates 
that are released will actually rise up and hit the underside of 
leaves on trees.  When this occurs, many leaves will undergo a 
chemical reaction that will make the leaf actually twist so that it 
is upside down.  Why do the leaves do this?  It is a evolutionary 
process by which the trees get to wash the bugs off of the under 
side of their leaves!  So, next time you can smell the rain, go and 
look at the trees, especially if there is a slight breeze, and you 
will see the leaves turn upside down!

Matt Voss
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