The smell after rain does not come from the rain itself. It comes from the soil. Tiny living things in the soil, called bacteria, make a chemical named geosmin. It has an earthy smell.

Dry ground also collects oils from plants. When raindrops hit the soil, small bubbles of air go up and pop. The pops throw the smell into the air. This happens most in light or normal rain, not in heavy rain.

The pleasant smell after a summer shower is not the smell of rainwater, which has almost no odor. In 1964, two Australian scientists collected a yellowish oil from dry rocks and soil and named the smell "petrichor," from Greek words for "stone" and the fluid in the veins of the gods. A year later, American researchers identified geosmin, an earthy-smelling chemical made by soil bacteria. Humans can notice it even at very low levels.

But how does the smell reach our noses? In 2015, researchers at MIT carried out about 600 experiments, filming water drops with a high-speed camera. When a drop hit the soil, tiny bubbles rose and burst, sending small particles into the air.

The distinctive scent that follows rain after a dry spell has a surprisingly complex origin. Rainwater itself is nearly odorless; the smell comes from compounds already present on the ground. In 1964, researchers at Australia's CSIRO extracted a yellowish oil from dry rocks and soil and coined the term "petrichor" to describe its smell. Shortly afterwards, geosmin, an earthy-smelling substance produced by soil bacteria such as Streptomyces, was identified, and the human nose turned out to be extremely sensitive to it.

The question of how these compounds become airborne was addressed in 2015, when MIT engineers ran roughly 600 experiments on 28 different surfaces using high-speed cameras. They observed that air trapped in porous soil formed bubbles that rose through each drop and burst, releasing fine aerosols, mainly during light or moderate rain rather than downpours. A 2020 study suggested why bacteria produce geosmin at all: it attracts tiny soil animals called springtails, which eat the bacteria and spread their spores. However, the MIT work described the physics of aerosol release without directly measuring odor molecules, and geosmin may well serve other ecological functions.