Summer in the city. It’s sweltering. The sun beats down all day. Its heat is absorbed by concrete buildings and blacktop, which hold that heat at night -- and into the next day.

The heat builds over weeks and months, keeping summer city temperatures much higher than their surrounding suburban and rural areas. Cities can be 10 degrees Fahrenheit hotter, or more!

That’s what’s called an urban heat island.

People respond by turning down the thermostat on their air conditioning. Exhaust from AC units and heat from nearby power plants further warm the air.

The increased electric load can lead to brownouts, or blackouts. And if people lose power, and air conditioning – or if they don’t have it to begin with – the high urban temps can be deadly.

Thousands of people in normally temperate cities in Europe and Asia have died over the past years in summer urban heat waves.

So, city planners are working to cool these heat islands. Methods can include so-called ‘cool roofs,’ which use white paint or reflective materials to radiate the sun’s heat, rather than absorb it.

Cool roofs can reduce indoor building temperatures by a few degrees Fahrenheit. Green roofs, planted with gardens or even trees, can double that reduction.

Likewise, creating more urban green space cools cities, as plants give off water vapor to provide evaporative cooling to their leaves, and the air.

Remarkably though, these deadly heat islands may save more lives than they’re endangering – and we’ll talk about that on another EarthDate.