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Smelling flowers and rain; Seeing the Leonid meteor shower

How do we smell a flower? Susan K. and her father, Ron, Albuquerque, New Mexico

We inhale gardenia molecules (blue) and some drift to the smellers (yellow) lining the upper nasal cavity.  Photo courtesy of Bartleby.com, Gray's Anatomy

A: You smell the sweet scent of a gardenia because molecules escape the flower's surface, and float into the air where your nose can detect them. A smell travels from gardenia to nose because molecules in the air bop around, banging into other molecules like zigzagging, tiny pool balls. Gardenia molecules eventually reach your nose. A gardenia smells stronger when it's warmer because warm molecules move faster. Faster molecules have more energy to escape the surface and so more do.

Smelling is a strange sense: not like seeing or hearing. You can see a far-away object because you receive its light waves. Likewise you hear something from afar by receiving sound waves. Smelling--perceived in the most primitive part of our brain--is an ancient sense, chemical in nature.

Let's return to the gardenia. You breathe in the gardenia molecules that meandered to your nose. A paltry five percent (by volume) of your nasal cavity receives the gardenia molecules and smells them. The smellers are postage-stamp sized, thin patches of yellowish gray moist tissues. One on each side of the nose, they are located behind and barely beneath the bridge of your nose. They line the upper part of the nasal cavity directly below the brain. These are the smell targets. Only about two to ten percent of the inhaled gardenia molecules reach the target. There they dissolve in the wet mucus that coats the target lining so they can reach the smell receptors.

Tiny hair-like threads--the smell receptors--cover the lining, dangle into mucus, and reach out like antennae for matching molecules. The hairs, made of protein and separated only by the thin goo layer from the outside world, are extensions of your brain. And they do get damaged in that exposed location. These unusual nerve cells last only about four to six weeks before new ones replace them. Most nerve cells never regenerate.

Some searching hairs--which respond chemically--detect, connect, and react to the gardenia molecules. If a receptor nerve cell reacts, it sends an ON signal. Otherwise its message is OFF. This begins a cascade of reactions that, in a fraction of a second, creates a perception of gardenia odor in the brain.

The olfactory circuitry can send signals corresponding to a theoretical limit of sixteen million different smells. In 1991 neuroscientists at Harvard and Columbia Universities discovered nearly 1,000 distinct odor receptors in humans. Moreover, our nerve cells can recognize and respond to some 10,000 chemical odors--an amazing repertoire.

Why does it smell so nice those first few minutes just when it starts to rain? Susan K. and her father, Ron, Albuquerque, New Mexico

Approaching thunderstorm with lead gust front. Rain-cooled air scuttles in front of the storm.  Photo courtesy of NOAA.

A: What a delightful question! Several things cause the good smell of rain--but the main one is humidity. Much water exists in the air just before it rains. That moisture makes plants release aromas so there are more scents in the air both immediately before and during rains. Also your nose functions better when cleaned by moist air so you notice more.

This is our present knowledge. Now let's speculate on some causes. I asked scientists around the world.

Ken Doxtader, professor of soils and crop science at Colorado State University, says that soil contains bacteria that make aromatic gas. When you dig in soil, you disturb the gas and free its aroma. Likewise when it rains. The rain floods the space between soil particles, displacing the bacteria's gas, and you smell an earthy fragrance.

Cloud water droplets form around tiny airborne clay particles. Steve Goodwin with NASA's Lightning and Atmospheric Research thinks downdrafts associated with rainstorms can bring those clay particles close to your nose and you may notice an earth scent,.

The breezes and winds that swirl rain also stir up smells around you--plants, soil, gases--so you notice them, says Robin Hicks of Melbourne, Australia's Bureau of Meteorology.

J. Hallet at the Desert Research Institute mentions how even surface bugs release odors when they get wet.

Why does rain stop smelling so good as it rains longer? Susan K. and her father, Ron, Albuquerque, New Mexico

When an odor molecule reaches a nerve hair on the smell lining in the nose, it fills a spot there. If the smell is strong (i.e., many molecules), soon all the hairs that respond to that particular odor are filled. Those receptor hairs have sent their smell messages to the brain and cannot send more until they detatch their present odor chemicals. So the receptors are quiet and the brain no longer perceives that smell. That is why you quit noticing the good smell of rain.

Which day (18 or 19 November) will I be able to see a good meteor shower from Mumbai, India? "About meteor shower", Mumbai, India

You can see the shower in Mumbai, India at its peak about 6 am, your time on Nov. 19, 2007.  You're lucky, because the moon sets a little after midnight, so the skies should be dark — at least out in the country.  Of course, Mumbai (formerly Bombay and the biggest city in India) is another story.  Good luck!

Albuquerque — half a world away — gets to see the shower 3 am the day before, on the 18th.

Further Reading:

Leonids, NASA estimator

U.S. Naval Observatory: World Time Zone Map

U.S. Naval Observatory: U.S. Time zones

U.S. Naval Observatory: Universal Time

(Answered Nov. 15, 2002; updated April 11, 2009)

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