Rain can make us feel surprisingly good – here's why - BBC

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 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.

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.

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.