Four benefits of going out in the rain

From its distinctive smell to the negative ions it releases, rain has some fascinating benefits for our bodies – especially when it comes to our mood.

It was the sixth straight day of the heat index topping 100F (38C) in New Milford, Connecticut, when the sky turned a sickening shade of puce. I was leading an outdoor theatre workshop, and noticed campers gawking at an ominous, miles-long shelf cloud advancing overhead. Then a thunderclap shook the ground, and lightning stretched across the sky like a jagged web. 

We heard the rain before we felt it, then suddenly we were drenched in a torrent of heavy raindrops. No one could hear each other, so I motioned to a nearby covered platform for laundry bags, and my students and I made a run for it. Soaked and laughing, we fell into the laundry pile and watched the storm rage on. After 30 minutes or so, the sky cleared, and the air felt astonishingly clean and cold. The strong, distinctive smell of rain was overwhelming. 

As we walked back to rehearsal, the grass and trees looked greener and healthier somehow. Everyone seemed lighter, smiled more easily, and I felt as if a brain fog had finally lifted. Was it due to the heat wave breaking or the adrenaline of the dash to shelter, or did the rain have some hand in our collective better mood?

It turns out that after decades of studying the elements of rain's potential mood-boosting abilities, scientists have found some decent evidence to support it. And it's not the only benefit of rain: research shows rain also removes harmful substances from the air, while its unmistakable smell may even enhance our memories. 

The serotonin boost

The predominant reason may be that rain releases negative air ions – oxygen molecules with an extra electron, which form as raindrops collide or hit a surface and split apart. In high levels, these are known to boost serotonin and alpha waves in the brain, leading to a happier, more relaxed state. 

When raindrops hit the ground, they can splatter and release negative ions into the air – a process called the Lenard effect. So if you're hoping to get a sizeable dose of serotonin-boosting negative ions, try taking a walk in a rainstorm. Just make sure to get inside if you see lightning – for safety's sake.

Some scientists believe these positive effects may be due to negative air ions increasing oxygen levels in the blood, resulting in a mood boost akin to what you experience from intense exercise.

However, there still isn't conclusive evidence to explain exactly what mechanism is going on to produce these effects. Pam Dalton, a cognitive scientist at the Monell Chemical Senses Center in Pennsylvania in the US, notes it is still not well understood why negative ions have effects such as changes in mood as well as fatigue, cardiovascular status and blood pressure. "While intriguing, there just isn't much consensus on the physiological benefits, and there is even less understood about possible mechanisms by which negative ions would elicit these effects," she says. 

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Studies into the effects of negative ions on mood began in the 1950s, though results were inconclusive until more advanced, high-voltage ionisers that more efficiently produced negative ions became available in the 1990s. In one notable 1995 study, researchers found that participants with seasonal affective disorder (Sad) who were given daily sessions with high-voltage ionisers were far more likely to see a large reduction in their symptoms than those given a low-output treatment.

According to Columbia University professor Michael Tehan, who led the study, heavy rain produces similar levels of negative air ions to the high-voltage ionisers used by his team. He notes, though, that so far no study has directly demonstrated this, nor directly correlated time in the rain with mood changes. 

The cleaner air 

Rain's negative ions also appear to clear the air of particulate matter such as pollution and allergens, making it easier to breathe. This could have an impact on mood and health as well: since poor air quality is associated with increased anxiety and risk of developing more severe mental health disorders, it stands to reason that cleaner air would promote the opposite. 

"There is reasonably good evidence that the negative ions can clear dust, bacteria, allergens, and other particles from air, which for many can have a positive effect on respiratory health," says Dalton.

Just how adept negative ions are at cleaning was less clear until about a decade ago. In a 2015 study, researchers replicated this ability in miniature by pumping different kinds of particulate matter into a raindrop-generating glass chamber. After the raindrops had evaporated, researchers collected the remaining particles, noting their position to determine whether the raindrops had, in fact, attracted them. They found that smaller raindrops were particularly adept at attracting these airborne particles. 

When raindrops fall to the ground, they're essentially "sweeping up" tiny airborne particles in their path, says Dan Cziczo, co-author of the study and a professor of earth, atmospheric and planetary sciences at Purdue University in Indiana in the US. The electrical charge (the ions) within a raindrop acts like a magnet for these particles, resulting in a sweeping-up process known as coagulation.

Cziczo likens it to what happens when a construction crew sprays a dusty site with water; the suspended dust is pushed back down to the ground, leaving the air cleaner.

The intensity of the rain also matters. "The heavier the rain, the more of a cleaning effect you're going to get on the atmosphere," says Cziczo. This includes reducing the amount of positive air ions, which have been associated with irritability and heightened anxiety. 

So next time there's a heavy rainfall, consider opening your windows right after it stops. You'll likely notice the air seems clearer, and if it came on the heels of a cold front (as heavy rains often do), the accompanying wind may blow some of that clean air into your home, improving the air quality.

The memory-enhancing smell

Rain's distinct smell can also have a psychological impact. Known as petrichor, this odour rises out of the ground after a storm and is often described as pungent and earthy, yet somehow clean.

"Petrichor arises when rain releases aerosols from the ground," says Phil Stevenson, a professor of plant chemistry at the University of Greenwich who leads work on plant and fungal traits at Kew Gardens in the UK. "During dry weather, organic molecules from plants, animals and soil accumulate on surfaces. When raindrops strike, these molecules – including volatile plant oils – are shattered into airborne particles."

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The "clean" smell is thought to be due to ozone, which rainstorm downdrafts can pull down to earth. Another part of the smell comes from geosmin, a chemical compound that actinomycete bacteria produce as they form spores in soil.

"Rain liberates the spores and geosmin, creating the familiar 'first‑rain‑after‑drought' smell, most noticeable in warm seasons," says Stevenson. This might explain why humans are so sensitive to it – more sensitive than sharks are to blood. Scientists hypothesise that we evolved to understand that petrichor signifies the renewed abundance of fresh water, which likely helped our ancestors feel safe and calm.

These smells have been shown to yield distinct changes in alpha and beta wave activity in the brain that are linked to a calmer, more relaxed state.

And thanks to its unique fragrance and how dramatically it changes the environment, rain can also be a powerful nostalgia trigger. My camp thunderstorm experience happened over 20 years ago, yet every time it rains, my mind paints the picture of that day with remarkable clarity.

"A sensory experience like the smell of approaching rain or the aftermath can become the backdrop or context that becomes attached to our memories of many different places or emotions," says Dalton, who has extensively studied the psychological significance of smell. 

Any smell, she says, can activate the amygdala, the part of the brain responsible for processing emotions and emotionally significant memories. That tie to our emotional epicentre is why memories associated with smells tend to stick in the brain and remain vivid. So whether you perceive a smell like rain as good or bad doesn't matter; the context within which you experience that smell is what makes it evocative.

So take a whiff the next time it rains from an open window or during a post-rain stroll and see what details of long-past moments surface.

The relaxing sound

It's not just smelling and breathing in rain that can make us feel good, though; it's hearing it too, which is why you often find rain tracks on sound machines. A steady rain can reduce cortisol levels, inducing a sense of calm, as well as drowning out disruptive noises.

"Water sounds have been associated with activation of the parasympathetic nervous system, which is the branch of the nervous system involved in relaxation and recovery," says Amy Sarow, a clinical audiologist who works in outpatient care in Southfield, Michigan in the US. "When this system is activated, we may see physiological effects such as lower heart rate [and] reduced stress responses."

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• How nature can provide a tonic for loneliness

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A recent study found the sound of rain was most effective within the 40 to 50 decibel range (equivalent to a soft, gentle rainfall), lowering stress levels up to 65%. Heavy rain, which falls into the even lower brown noise frequency, can feel more grounding and immersive, Sarow says, and drown out disruptive sounds to help promote sleep.

Both levels can be calming; Sarow says it often comes down to personal preference. "If someone intentionally listens to these sounds as part of a relaxation routine, the experience can begin to resemble mindfulness or meditation practices, where the sound serves as an anchor for attention and relaxation." 

While my rainstorm didn't exactly put me in a zen state, it did leave me feeling better and more connected to the present moment. Now, every time there's a downpour, I try to spend a little more time immersed in it. When you next see rain in the forecast, consider tuning into the experience too. You might be pleasantly surprised.

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