雨の匂いの理由｜petrichorという英語も一緒に覚える

Why Does Rain Smell So Good?

Have you ever noticed a fresh, earthy smell right after it starts to rain?

Scientists have a special word for that smell: petrichor.

The word comes from Greek: petra means "stone," and ichor means "the blood of the gods."

Two Australian scientists created this word in 1964 to describe a smell that had no proper name.

Petrichor is not made by the rain itself, but by things already sitting on the ground.

During dry weather, plants slowly release tiny oils, and soil bacteria called actinobacteria produce a compound called geosmin.

These substances sit quietly on rocks and soil, waiting.

When a raindrop hits dry ground, it traps a tiny bubble of air.

That bubble shoots upward and bursts, launching oils and geosmin into the air as a fine mist.

Wind then carries that mist to your nose, so you smell rain before you even feel a drop.

Humans are extremely sensitive to geosmin.

We can detect geosmin at about five parts per trillion, an incredibly tiny amount.

That is why even a short, light rain after a dry spell can smell so strong.

Interestingly, heavy rain often smells weaker than a gentle shower.

Slow, soft raindrops trap and release more bubbles than fast, hard ones do.

Petrichor also fades once the ground is wet, because most of the oils and geosmin have already been released.

Next time you catch that smell, remember it was quietly forming on the ground long before the first drop fell.