Smell of rain

On Oct. 27, most locations along the Central Coast received more than an inch of rain.

Contributed by John Lindsey

It’s been a dry and warm November in the San Joaquin Valley this year. In fact, overnight temperatures have been averaging five degrees warmer than usual during the overnight hours and three degrees warmer during the day this month in Hanford.

The first rain of the November occurred on Nov. 17 when a little over a tenth of an inch of precious precipitation was recorded at the National Weather Service (NWS) Forecast Office in Hanford; otherwise, it has been dry this month.

The lack of rain combined with well above average temperatures has turned the landscape dry across the San Joaquin Valley.

This condition may lead to a California early rain season phenomenon. A weak cold front is forecast to move through the Hanford area on Sunday. If the weather models verify and you’re out and about as the first raindrops hit the ground, you may notice an earthy or fresh, pungent scent.

This “smell of rain” aroma is most noticeable after a long dry spell, especially in arid regions. This fragrance is gradually washed away by continuous rains.

The official weather term for this unique smell is “petrichor.” On Oct. 27, when most locations along the Central Coast received more than an inch of rain, loyal Tribune reader John Gower emailed me to say he noticed the petrichor fragrance during the early stages of the storm.

Petrichor comes from the Greek words “petra,” meaning “stone,” and “ichor,” which refers to the fluid that flows like blood in the veins of the gods in mythology. In other words, the blood of the stone.

Two Australian Commonwealth Scientific and Industrial Research Organization researchers, Isabel Joy Bear and Richard G. Thomas, coined the term in 1964. They hypothesize that some plants exuded oil during dry periods as a mechanism to reduce new plant growth and retard seed germination when vegetation is under moisture distress.

Over a long dry spell, minute amounts of this yellow oil from these plants gradually accumulate on almost everything, including driveways, soils and especially rocks with nooks and crannies.

When the first raindrops slam into the earth, this oil is thrown into the atmosphere as an aerosol along with another compound, geosmin, which is produced by algae and soil-dwelling bacteria. These oils and compounds combine in unison to make this reassuring and pleasant odor of first rainfall.

It’s interesting to note that the petrichor aroma often seems to lift everybody’s spirits. Perhaps, deep down, we instantly know we are all dependent on the rains to grow and raise our food.

John Lindsey is Pacific Gas and Electric Co.’s Diablo Canyon Power Plant marine meteorologist and a media relations representative.

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