On a July afternoon, the black roof of a city building can reach temperatures well above 150 degrees Fahrenheit, hot enough to burn bare skin.
Multiply that by thousands of rooftops, add asphalt streets and the warm air pumped out by countless air conditioners, and you end up with what scientists call an urban heat island: a city that stays noticeably warmer than the countryside around it, especially at night.
One increasingly popular remedy is to cover roofs with plants.
Green roofs, as they are known, come in two broad varieties.
Extensive roofs have only a few inches of lightweight soil and are planted with hardy, low-growing species such as sedum, which can go weeks without rain.
Intensive roofs, by contrast, have deeper soil and can support shrubs, vegetable beds and even small trees, though they demand far more upkeep and a building sturdy enough to bear their weight.
The benefits go well beyond a cooler surface.
A planted roof soaks up much of the rain that falls on it, taking pressure off storm drains during heavy downpours.
It shields the waterproof layer underneath from harsh sunlight and sharp swings in temperature, which can add years to the life of a roof.
It also offers a foothold to butterflies, beetles and birds in neighborhoods where green space is scarce.
None of this comes cheap.
A green roof costs considerably more to install than a conventional one, and the savings on energy and repairs take years to add up.
Moreover, a single planted roof makes little difference on its own.
The real payoff, researchers say, comes when an entire neighborhood follows suit.