ウェザーウォッチ: science behind the ‘heaven scent’ aroma of rain

There is something satisfying about the smell of rain. It is an aroma that is particularly noticeable at the end of a dry spell in summer. Some would claim it is a figment of the imagination but it is a real smell called petrichor, although the name is rather pretentious, made up by two Australian scientists who discovered its origins in 1964. The word comes from the Greek words for stone, petra, and ichor” the golden fluid said to be in the veins of the immortals.

The source of the smell is a combination of oils and chemicals. In a dry spell plants secrete the oils to signal the halt of root growth and seed germination, while the chemicals come from the reactions of bacteria.

Rain falling on dusty soils is infused with this mixture and the smell is released when tiny air bubbles rise and burst out of a raindrop, throwing out aerosols.

Using high-speed cameras, Massachusetts Institute of Technology researchers have been able to film air bubbles rising through raindrops like they do in sparkling wine.

So much for facts: why does it seem such a pleasant smell? Perhaps it is the primitive instinct in us that knows that without water there is no life. The smell is welcome because it heralds the end of a drought.

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