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Blue skies have long been linked to a sunny state of mind. But this doesn’t account for gruelling 35 degree Tube journeys crushed into the moist nook of a stranger’s armpit. When pleasant tips into unbearably hot, what are the effects of a heatwave on our psychology?

Linguistically-speaking, we often associate heat with anger. Whether you’re feeling hot under the collar, boiling with rage or just a little hot-tempered, heat metaphors are an easy shorthand for all manner of grumpiness. And there might be a good reason for the link. Research finds that there is a connection between surging mercury and spates of violence. In the US, all kinds of crime increases on hot days, especially violent crime like murder, assault and rape, and this is similar for other countries such as Spain and France – but why?


For the British, where the glimmer of sunshine is mentally bound to the glint of condensation on a cold pint, it might seem obvious why there is more violence during these periods: people are outdoors, congregating in open spaces, and imbibing more alcohol. In 2018, the combined effects of a heatwave and the World Cup prompted Brits to buy a record amount of booze.

But there are other reasons too. Studies suggest an increase in aggression on stickier days might be down to a physiological overlap in how our bodies respond to heat and how it responds to adrenaline: for example, increased heart rate and sweating. This can lead us to misinterpret our own physical sensations, and infer that we’re angry. In turn, physiological arousal can encourage us to act more rashly.

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The mechanism behind heat-induced anger might be even simpler than that. Another theory suggests that discomfort motivates people to lash out, which could be explained by the cognitive phenomenon by which we associate stimuli we encounter with the internal state we’re experiencing. If your shirt sticking to your back is making you uncomfortable, the person who just stepped on your foot might be perceived in harsher terms than usual.

On a larger scale, sporadic rashes of violence like the London riots of 2011 often flare up during hotter weather. Research found the frequency of civil conflicts between 1950 and 2004 in tropical regions was influenced by large-scale shifts in temperature mediated by the El Niño current. Conflicts were twice as likely in the hotter years. However, there were other factors at play too, like the increased likelihood of drought.


The theory linking violence and heat is complicated, however, by the fact that this is not a linear relationship between crime and steadily inching Celsius. Instead, the relationship is an inverted U, meaning that although crime increases up to 30 degrees, after that point it starts to fall again.

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This could be because temperatures higher than 30 degrees are simply too energy-sapping for people to bother getting violent. Work by the psychologists Robert Baron and Paul Bell suggests that discomfort related to moderately high or low temperatures provokes aggression, but at higher temperatures conflicting motives are awoken, like the desire to escape the situation. Namely, at moderately uncomfortable temperatures, we might seek to offload onto others, but when it gets truly unbearable, we might instead direct our mental resources to seeking out an air conditioned haven.

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Heat fuelled aggression takes other forms too. Those taking to the (melting) roads may want to watch out; studies indicate that road rage also ramps up during a hot spell. Researchers found that as the temperature inched from 29 to 42 degrees Celsius, the incidence and intensity of horn honking increased in response to a test situation, where a brave subject sat still at a green light.


But hot weather doesn’t just provoke anger. Across a broad swathe of academic literature, some general truths about weather have emerged: in general, rising temperatures decrease anxiety, and more hours of sun increases optimism. Sunny, high pressure days are associated with better mood and greater ‘openness’, meaning people are more likely to be creative.

However, while moods might be buoyant during a sunny spell, productivity takes a hit - as the urge to dip your feet into a cool body of placid water builds to intolerable levels. One study charted employee productivity in a bank in Japan, an online workforce in the US, and in a laboratory experiment against weather, and found sunshine was associated with lower productivity in all cases. There are physiological reasons that concentration flags during a warmer day. This might be because when the weather is hot, the body becomes less efficient at dispelling heat that is created as a by-product of intense mental activity. This effect can impact on a whole range of cognitive activities and could explain the foggy headed effects of heat.

But given the complexity of the human psyche, it’s hard to conclusively chalk any behaviour up to the weather. “The main reason is that there are individual differences on what sort of weather people like,” says Trevor Harvey, professor of psychology at the University of Dundee and author of The Psychology of Weather. “Many people don’t like hot weather, while some love it.” All of the effects are balanced by the questions of not just weather but seasonality - whether it’s summer or winter – and climate in general.

In today’s world, the psychological effects of hot weather might be changing, as anxiety about climate change intensifies. This summer Europe has been spiked with multiple scorching heatwaves, India’s capital endured its highest ever recorded temperature of 48 degrees, and fires, unprecedented in their latitude and intensity, are currently tearing through arboreal forests in Siberia, Alaska and Greenland. This year, the mental effects triggered by heatwaves might be inextricably linked to the growing catastrophic dread that the world’s climate is in irrevocable freefall.

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Like the freakishly hot spell the UK experienced in February, increasingly we might find something that was once an undisputed good – a warm day – will be tempered by anxiety about the impending threat of ecological collapse. A 2017 report from the American Psychological Association first coined the concept of ‘eco-anxiety’, something that has only increased since then. Meanwhile, research has discovered an increase in suicide and drug abuse rates in areas already hit hard by climate change.

“The potential effects of climate change on people’s behaviour and especially on the likelihood of increased human conflict and violence are of serious concern,” says Anu Realo, professor of psychology at the University of Warwick. “It is possible that if climate change happens slowly and gradually, societies will adapt, but we do not know if the adaptation will make aggressive outbursts and conflicts more or less likely.” In our overheating world, it will become more difficult to say whether psychological effects are down to the weather or simply the paralysing acknowledgement that the world is headed for climate crisis.

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