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Large cities may create their own clouds

The Romantic poets may have it right: Life is simply gloomier in the city. Although it’s well-known that concrete-rich cities are often several degrees hotter than the leafy suburbs, scientists have now determined that cities are also capable of prolonging their own cloud cover.

By studying satellite imagery of London and Paris, scientists discovered that during the spring and summer, the modern megacities are persistently cloudier in the afternoon and evening, by several percentage points, than nearby rural areas. The result is surprising: Cities’ lack of vegetation also tends to make them dry, which should lead to less water evaporation and cloud formation.

Using ground-based observations in London, the researchers suggest the heat retained by buildings into the late afternoon drives turbulence in the air, feeding moisture to the clouds, they report this month in npj Climate and Atmospheric Science. Although the study only documented these prolonged clouds in two European cities, there are reasons to believe a similar phenomenon is common in cities from Mexico City to Łódź, Poland.