Natalie Theeuwes at the University of Reading in the UK studies the urban heat island, the term for what scientists have known for decades: cities tend to be hotter than surrounding countryside.
She’s looked at street grids, lakes and other features to better understand why. Now her team is saying what people in Paris or London might argue they already know, and it’s that life in the cities isn’t just warmer. It tends to be cloudier too. The clouds are related to the urban heat and the cities are essentially making their own cloudy skies, according to Theeuwes and a research team.
They published the results of their study in Nature npj Climate and Atmospheric Science in May, adding to critical knowledge in the context of climate change and global warming about cities and how heat will shape their future.
Right now there are 33 megacities on the planet, defined by the United Nations as cities or urban regions where more than 10 million people live. That’s expected to grow to 43 by 2030, especially in Asia and Africa. India’s New Delhi, for example, will pass Tokyo as the largest city on the planet by then.
So understanding how urban heat behaves is critical to climate response and adaptation, making the University of Reading research results an interesting find. The scientists looked at satellite data over Paris, where 11.8 million people live, and London with its 12.3 million people, to measure cloud cover. It was between five and 10 percentage points higher in the cities than in surrounding areas.
On the one hand, it would be reasonable to expect cities to be more dry and have fewer clouds because of all the concrete buildings, paved roads and other materials retaining the heat that’s linked to the island effect. Cities have more people, more emissions from buildings and vehicles, and a frequent lack of green space and available tree canopy. That usually means less moisture too.
What the scientists think happens in spring and summer months, though, is that the buildings release their heat during the afternoon and create meteorological turbulence that forces the limited moisture up into the sky. Then the clouds don’t dissipate the same way they might over a rural landscape.
“Therefore, urban areas are seen to directly affect weather phenomena besides temperature, impacting the city’s inhabitants,” they conclude, noting that in some ways the cities oddly behave much as forests do in holding their cloud cover.
What may come with that cloud cover is a layer of thermal protection that traps in the warmth, especially at night, because of a radiative forcing mechanism. Similar patterns have been seen in a number of cities.
Scientists find that overnight low temperatures are rising faster than they are in the daytime, with the hottest night ever recorded on the planet – in Oman at 42.6C – occurring just last June. It adds to concern that cities are getting less of a cool respite, and it’s one more reason why it’s important to understand urban heat, why cities appear to be making their own clouds, and how they contribute to impacts we may expect in the future.