Why does it smell so nice after it rains?

The answer to this was quite slow coming and no one really knew for sure, perhaps we still don't know for certain, but there was certainly some work done on this in the 1960s and the paper got published in 1966 where scientists actually, they think got the answer.

The theories where that this could either be something coming out of the soil, something reacting with water in the soil to produce the smell, or perhaps something organic, something living and it turns out, it's probably the latter.

A group of scientists analyzed the air and they found that when you took soil, you find a very common soil bacterium called actinomyces. These are filamentous bacteria and they grows lots of little filaments that ramify through the soil, picking up nutrients.

But it also has another form which it uses to protect itself when the soil is very, very dry. So, when there's severe arid, dry conditions, it recedes into a spore and this is a dormant form of the bacterium from which it can reactivate when water comes back and the soil is fresh and there's lots of good environment for it to exploit again.

So what scientists think happens when you get a rain shower and it produces that beautiful earthly smell in the air, is that the rain comes down, it hits dry soil where all these bacteria have formed these little spores, the spores then get ejected up into the air, and they drift around in a cloud.

Because they're so tiny, they stay drifting around in the cloud for quite sometime. You then breathe them in and they smell the way they smell. That's their smell.

But it's also a form of, sort of, dispersal for the bacterium because it then descends on another patch of ground, out of the air and can germinate and grow.

So, I suppose that's one point. Another thing to bear in mind is of course, there's the other possibility that was also raised by scientists historically and that is that there are various chemical reactions that can occur when water hits soil or dry soil or a rock. And so, it might be that some of these smells, because of particular rocks getting wetted, then chemical reactions are being elaborated and then they produce various chemicals that go up in to the air.

But we think it's mainly the actinomycetes, that's the main cause.