It’s the rain and the dirt that mix together aromatically to set the mood for the day. The most refreshing of fragrances, the most ambrosial smell that brings us closer to emotions and sentiments as soft as the wet mud is one of the most cherished mysticism of nature. Now, the physics and logic, which in laymen terms would be some divine alchemy, is right there to back the romanticism up with science. How do we smell it? There is a common substance in the forests areas known as geosmin, produced by a bacteria called actinomycetes that swell the soil. Now, when the bacteria creates spores, the compound is secreted which are bounced into the air with force of the falling rain. The spores containing geosmin get splashed and spread in the air where they remain as we walk into them and to be received by the enticing fragrance.

Some smell the arrival of rain:

This is the smell that you can feel before the drops hit the earth. The smell is electrically induced, generally accompanied by thunderstorms, that breaks the nitrogen and oxygen molecules in the atmosphere into several parts reacting with other particles, often creating Ozone which reaches your nostrils as the sign of oncoming rains.

That explains why people often say they can smell the arrival of rains.