after long dry periods of weather. when we get thi first few drops of rain, why does the air smell?

Answer 1

There are many things that release aromatics when moistened. Ever smell wet asphalt?

Answer 2

some people are good at predicting weather by the smell in the air.. normally though, we are able to smell the moisture in the air (water content). Same as if you walk into a bathroom or a sauna/steam room.
Most sailors are also able to smell land becuase of the industrial pollution carried by the wind.

Answer 3

Sorry not a good question to ask on this UK site after all the rain we have had as not many people will remember that wonderful smell when it rains after a long dry period.

Answer 4

There are a few answers to your question depending on the environment that you are referring to specifically, but I think the best place to clear this up is How Stuff Works