City centres are regularly several degrees hotter than the countryside around them.
Concrete and asphalt heat all day and release it slowly at night.
That is why nights no longer cool down, which is when the health damage occurs.
Trees act on the problem twice.
Their shade keeps surfaces from heating, and the water they release cools the air directly.
Measurements in several cities show differences of four to seven degrees under mature .
The benefit, however, is distributed .
Wealthier districts almost always have more tree cover than poorer ones.
Planting programmes often fail for an unglamorous reason: nobody waters the trees.
A young tree needs three summers of care before its roots reach deep water.
Cities that fund rather than planting ceremonies keep far more of what they plant.
Species choice matters too, since the wrong tree damages pipes and pavements.
Shade is cheap , but only if it survives its first three years.