Background

From UCAR's page on Urban Heat Islands:

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• Size and shape of cities—aerodynamically, cities have a very different shape than rural areas. Tall buildings act as obstacles and reduce wind speeds.
• Urban deserts—cities can be thought of as virtual deserts with almost no vegetation and materials that are almost completely impermeable to rain. This combination leads to a lack of evapotranspiration which increases sensible heat.
• Urban canyons—the tall canyons formed by city buildings trap radiant energy in their walls. Comparisons of this "canyon effect" in European and North American cities suggest that areas with denser and taller buildings will more rapidly develop heat islands.
• Humidity effects—although there is little difference in the amount of water that cities and countrysides retain in their atmospheres (absolute humidity), the higher urban temperatures effectively lowers the relative humidity (since warm air can hold more water than cold air).
• Urban haze—The haze of air pollution that hangs over many cities can act as a miniature greenhouse layer, preventing outgoing thermal radiation (heat) from escaping from urban areas.
• Anthropogenic heat—The release of heat from the burning of fossil fuels can also raise urban temperatures. On a typical winter day, Manhattan releases four times more energy from burning fossil fuels than the amount of energy that comes into the urban area from the Sun.

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(UCAR 2019)