Question

why does a different smell comes when it just starts to rain

Answer 1

In the soil we can find many types of soil bacteria. Actinomycetes are once such bacteria that produce spores in the damp and warm soil. They remain dormant in the soil when it is too hot in order to overcome the unfavourable conditions and protect them from desiccation. When it rains the water that hits the ground releases millions of these spores into the air which enter our nose contributing to the fresh and earthy smell.