Commuters lift temperatures and affect weather patterns in cities
Commuters can increase temperatures in large cities, making them warmer during the week than at weekends, and can also influence cloud, rain and wind patterns according to a study from the University of Melbourne. The researchers suggest that understanding the way in which humans affect meteorological cycles could help cities to respond to extreme weather events. For instance, cars could be banned from cities during heatwaves and white roofs could be used to deflect solar radiation. The research was based on 50 years’ worth of temperature data from Australia’s Bureau of Meteorology.

Hot in the city Asanka Brendon Ratnayake/Getty

Behold humans, the weather-makers. The crowds of commuters that pour into cities during the week can drive up the local air temperature and alter wind, rain and cloud patterns.

Heat generated by human bodies, cars, public transport and operating office buildings causes cities to gradually warm up from Monday through to Friday. The temperature then drops over the weekend.

A recent study led by Nick Earl at the University of Melbourne, Australia, shows that the morning temperature in Melbourne is typically 0.3°C hotter on Thursdays and Fridays than on Sundays. “That’s just the average,” he says. “So some days will heat up more, if for example there isn’t much wind.”

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The temperature jump is caused by heavy traffic and the 250,000 extra people in Melbourne on weekdays compared with weekends, says Earl. “Nothing in nature occurs on a weekly cycle, so it must be due to human activity.”

Earl and his colleagues have shown that three other big Australian cities – Sydney, Brisbane and Adelaide – have similar weekly cycles. But less populated cities like Hobart do not have weekly cycles, most likely because they have smaller workforces.

The findings were based on more than 50 years of temperature data from Australia’s Bureau of Meteorology. They will be presented at the annual conference of the Australian Meteorological and Oceanographic Society in Canberra next month.

Not just temperature

Research has also revealed that central Tokyo and Moscow are hotter on weekdays than weekends.

But temperature is not the only weather variable affected. Wind speed, rainfall and cloud cover can also be greater in urban centres on weekdays, according to studies from several countries.

These are knock-on effects from increases in heat and pollution, says Earl. “For example, warmer temperatures in the city create convection, which can suck in more air from outside, affecting wind speeds and direction,” he says. Pollution particles also seed cloud formation, he says.

Understanding the human impact on meteorological cycles is useful for dealing with extreme weather events, Earl says. “For example, during heatwaves, you could ban cars from the city so that it doesn’t warm up as much.”

This is particularly important in Australia, where heatwaves kill more people than all other natural disasters combined, says Stephanie Jacobs at Monash University in Melbourne.

Another key consideration is urban design, Jacobs says. “One strategy is to use white roofs on city buildings to deflect solar radiation and keep out the heat,” she says.

Read more: Oases of cool: Taking the heat out of urban living