Tag: science

  • Petrichor: Why Rain on Dry Ground Smells So Good

    Petrichor: Why Rain on Dry Ground Smells So Good

    After a run of hot, dry days in a Georgia summer, the first fat raindrops on the driveway release a smell almost everyone recognizes. It is earthy, clean and a little sweet, and it seems to rise from the ground itself. For centuries people simply called it the smell of rain. Since the 1960s, it has had a proper name: petrichor.

    A word invented in 1964

    Two Australian scientists, Isabel Joy Bear and Richard Thomas, coined the term in a paper published in the journal Nature in 1964. They built it from two Greek words: petra, meaning stone, and ichor, the ethereal fluid said to flow in the veins of the gods in Greek mythology. The name suits a scent that seems to be drawn out of rock and soil.

    Their research found that during dry periods, oils given off by plants collect on rocks and in soil. When rain finally arrives, those oils are released into the air along with other compounds, creating the familiar smell.

    The earthy note: geosmin

    A large part of the scent comes from a compound called geosmin, whose name means earth smell. It is produced by microbes that live in soil, including bacteria in the group known as Streptomyces. Human noses are remarkably sensitive to geosmin and can detect astonishingly small amounts of it, which helps explain why the smell of rain on dry ground can feel so strong and so sudden.

    There is also a sharper, cleaner smell that sometimes arrives just before a thunderstorm. That one is often ozone, a form of oxygen created when lightning splits molecules in the air, which storm winds can carry ahead of the rain.

    How the scent reaches us

    In 2015, researchers at the Massachusetts Institute of Technology used high-speed cameras to watch raindrops land on porous surfaces such as soil. They found that a drop can trap tiny bubbles of air that rise and burst, flinging a fine mist of particles upward, a little like the fizz from a glass of soda. Those particles carry scent with them. Light and moderate rain released more of them than heavy downpours, which may be why a gentle first shower often smells strongest.

    Bottling the smell of earth

    In the town of Kannauj in northern India, long known for its traditional perfumes, artisans make a fragrance called mitti attar, or earth attar. Clay is baked and then distilled, and its scent is captured in sandalwood oil, creating a perfume that smells like the first monsoon rain on parched ground. It is a lovely example of people cherishing this smell enough to preserve it.

    The next time a summer shower begins, step to an open door or a covered porch and take a slow breath. If there is thunder, enjoy the moment from inside, since lightning can strike well away from the storm itself. Notice the earthiness, the sweetness, the sudden cool. It is one of the oldest smells there is, and one of the few that seems to make everyone stop and breathe a little deeper.

  • Why Fresh-Cut Grass Smells So Good: The Science of a Green Summer Scent

    Why Fresh-Cut Grass Smells So Good: The Science of a Green Summer Scent

    Few smells are as instantly recognizable as a freshly mowed lawn. On a Saturday morning in May, the scent drifts across whole neighborhoods in Middle Georgia, sharp and sweet and green. Many people name it among their favorite smells of the year. What we are enjoying, it turns out, is the grass reacting to being cut.

    What is happening inside the grass

    When a mower slices through grass blades, it breaks open plant cells. Within seconds, the damaged tissue releases a burst of small, fast-evaporating molecules that scientists call green leaf volatiles. They include compounds such as cis-3-hexenal and cis-3-hexenol, the latter sometimes nicknamed leaf alcohol. These are what give cut grass, crushed leaves and torn stems their sharp, fresh scent.

    Researchers who study plants have found that these compounds play roles in how plants respond to damage. They can act as signals, for example by attracting insects that prey on leaf-eating pests, and neighboring plants may respond to them, too. The smell we find so pleasant is, in a sense, part of the plant’s communication system.

    Why the smell feels like summer

    Part of the pleasure is chemistry, and part of it is memory. Our sense of smell is closely linked to the parts of the brain involved in memory and emotion, so a scent we meet again and again in a particular season tends to carry that season with it. For many people, cut grass means school letting out, ballfields, long evenings and the sound of a mower down the street.

    The same green note turns up in many other places once you start noticing it: a tomato leaf brushed in the garden, a snapped green bean, a handful of crushed basil, the stem end of a cut flower.

    Green notes in the perfume bottle

    Perfumers call this family of scents green, and they have been building it into fragrances for a long time. Cis-3-hexenol is a common ingredient for adding a fresh, leafy lift to a blend. Natural materials with a green character include galbanum, a resin with a sharp, leafy smell, and violet leaf, whose scent is more like cucumber and wet greenery than like the flower. If you love the smell of a mowed lawn, these are the notes to look for on a fragrance or candle description.

    Noticing it on purpose

    The next time the scent drifts over the fence, stop for a few seconds. Close your eyes and take a slow breath. Notice whether the smell seems sweet, sharp or watery, and what it brings to mind. It is a small, free mindfulness practice that the season hands you every week.

    If you are the one mowing, try doing the first and last passes with full attention to the sound, the smell and the stripes appearing behind you. Wear sturdy closed shoes and hearing protection, work in the cooler morning hours and take water breaks. If grass pollen bothers you, you may prefer to enjoy this scent through an open window, and your doctor can help with seasonal allergy questions.