Convection about the city

Convection - is the heat transfer due to the bulk movement of molecules within fluids such as gases and liquids, including molten rock. Convection includes sub-mechanisms of advection, and diffusion.

Explanation:

Cities become warmer than their rural surroundings due to buildings, roads and other infrastructure replacing open land and vegetation. "Surfaces that were once permeable and moist," writes the"become impermeable and dry."

During the daylight hours, temperatures inside large cities range between 1.8 and 5.4 degrees F warmer than their surrounding areas. At night, city temperatures can be as much as 22 degrees F warmer.

This phenomenon was believed to be the result of concrete and other structures absorbing heat throughout the day and then gradually releasing it at night. But a new study published last week in the journal Nature offers a different explanation: Convection.

How air moves through a city during the day has a greater role in trapping heat, the study claims, than the disappearance of vegetation and the existence of urban structures.