The distinctive smell that often accompanies rain, especially after a dry spell, is called **petrichor**. It’s a familiar, earthy scent that many people find refreshing and nostalgic. This smell is caused by a combination of natural processes involving plants, soil, and bacteria. Here’s how it works:

### 1. **Oils from Plants**
During dry periods, certain plants produce oils that accumulate on soil and rocks. When it rains, the falling water droplets hit the ground, displacing and releasing these oils into the air. These oils contribute to the overall scent but aren't the main cause of petrichor.

### 2. **Geosmin from Soil Bacteria**
The primary source of petrichor is a compound called **geosmin**, which is produced by soil-dwelling bacteria, particularly *Streptomyces*. These bacteria break down organic matter in the soil, and geosmin is one of the byproducts. When it rains, raindrops disturb the soil and release geosmin into the air. Humans are incredibly sensitive to geosmin, even in small amounts, which is why we can detect it so easily after rain.

### 3. **The Mechanism: Raindrop Impact**
When raindrops hit porous surfaces like soil or pavement, they trap tiny air bubbles beneath them. These bubbles rise to the surface and burst, creating aerosols—tiny particles that carry the plant oils, geosmin, and other compounds into the air. This process disperses the molecules that create the petrichor smell, which we can then inhale.

### 4. **Ozone Contribution (Before the Rain)**
Sometimes, people can smell a distinct, sharp scent in the air just before it rains, especially during thunderstorms. This is caused by **ozone**. Lightning can split oxygen molecules (O₂) into individual atoms, which can recombine to form ozone (O₃). Winds from the storm carry this ozone to ground level, where people may smell its crisp, clean scent.

### Why Do We Like the Smell of Rain?
The pleasant association many people have with the smell of rain might be due to an evolutionary trait. Since rain is critical for life, especially for plants, crops, and drinking water, humans may have developed a sensitivity to the smell of petrichor as a way to sense and appreciate rainfall, which would signal good conditions for survival. 

Overall, the smell of rain, or petrichor, is a unique natural phenomenon resulting from a fascinating interaction between the earth, plants, and microorganisms.