Temperatures in London this week hit more than 30C. Buses and tubes turned into greenhouses, clothes stuck to skin and tempers frayed. So when a water fight in Hyde Park descended into violence - with three people, including a police officer, stabbed - the news felt as inevitable as it was unwelcome.

From the 1981 riots in Toxteth and Brixton, to those in Oldham in 2001, to the countrywide ones in 2011, hot weather seems to be a catalyst for civil violence. From Shakespeare’s Romeo and Juliet (“For now, these hot days, is the mad blood stirring”) to Spike Lee’s Do the Right Thing (where violence breaks out on the hottest day of the year), it’s a belief echoed through centuries. But is it true?

Psychologist Lance Workman, from the University of South Wales, says there is a relation between rising temperature and civil disturbances. Crime and riots increase between 27C and 32C – but after 32C, “it’s as though people are too hot to bother”. Workman thinks the cause could be both physiological and psychological. Sunshine, he says, leads to an increase of serotonin, sudden boosts of which can lead to increased irritability, while aggression can be caused by frustration and an inability to control things - such as our temperature.

Not everyone agrees. Psychologist Chris Cocking, from the University of Brighton, says: “Describing riots in terms of individuals snapping doesn’t explain why crowds get involved.”

He points out that riots can occur in winter, as did the Russian revolution. Cocking believes the link between disorder and hot weather is much looser - people are just more likely to congregate outside in hot weather. “When the police do briefings before big demonstrations, they pray for rain because fewer people come out,” he explains.

Individual skirmishes can increase when people stay out on summer evenings and drink, but many public summer events are peaceful, and don’t end in attacks on the police. “What is crucial is how the police manage a particular crowd,” he says. “You need a trigger incident - for the police to treat the crowd in a way [that the crowd] perceive to be illegitimate and indiscriminatory.”

He says one problem is that most of the public order tactics the police use – for instance kettling – are, by their very nature, indiscriminate. When they invoke public-order tactics, it unites the crowd, because they feel they have been treated as one - and that treatment is illegitimate.”

Looking at the events in Hyde Park, for instance, Cocking points out that while the water fight spontaneously began at around 3pm, the disorder erupted hours later. Meanwhile the reports that the crowd were chanting “black lives matter”, he says, highlight a wider social angst - from the deaths of black men and women at the hands of the police in the US, to the distrust of the police in London felt by many in ethnic minority communities. Sometimes, it seems, we make our own weather.