Why We Can Sense If It's About To Rain

You don't always need to look at a weather report to know when it's going to rain. Obviously if the sun disappears and the sky darkens, that's a pretty clear sign that the heavens are about to open. But some people actually feel the rain approaching. According to Jaspal Singh, MD, and co-director of the Weill Cornell Medicine Center for Comprehensive Spine Care, our bodies can predict changes in weather thanks to barometric pressure, "AKA the weight of the atmosphere" (via the New York Times).

On sunny days, normal and high bouts of barometric pressure keep the body from expanding, "but when the weather is gloomy (think dampness, rain, snow), the pressure drops, thereby making your body expand." This is why those with joint conditions like arthritis can often tell rain is about to come as the body expanding can "cause jostling among your joints, leading to discomfort, aches, and pains."

But it's not just the barometric pressure that can be an indicator for rainy weather. Even before a storm arrives, some may notice a distinct, earthy aroma wafting around the air. But what is it, and why can you only smell it before – and just after – it rains?

Before the rain starts, you'll notice a "sweet, pungent" smell according to Scientific American, which happens to be the "sharp, fresh aroma of ozone" which is formed by oxygen. This is then followed by the chemical petrichor, which is the scent you'll be most familiar with.

First described in 1964 by Australian mineralogists Isabel Joy Bear and R. G. Thomas, petrichor occurs when airborne molecules like actinobacteria (via Earth Sky) "from decomposing plant or animal matter become attached to mineral or clay surfaces." Just before rain arrives, a bout of humidity in the air moistens the ground which releases the actinobacteria, as well as the byproduct geosmin which contributes heavily to the familiar scent. As geosmin is a type of alcohol, its smell "is especially noticeable to people at extremely low levels," according to Earth Sky.

When raindrops fall and disrupt the soil, the petrichor and geosmin can splatter into tinier particles as an aerosol. Enough of these particles can result in these chemicals traveling downwind, resulting in people smelling the rain before it even arrives. As Professor Mark Buttner, head of molecular microbiology at the John Innes Centre told BBC News, "Lots of animals are sensitive [to these chemicals] but humans are extremely sensitive," resulting in this almost superhuman ability to predict the weather.