And it's probably easiest to explain why with thermal vision.

It shows the heat all around me and also the coldness of this ice cream cone.

Cities are often hotter than their suburbs due to a phenomenon known as the heat island effect.

All of that asphalt, concrete, dark rooftops and tall buildings, they absorb and store heat.

And there are a lot of people using a lot of energy—driving cars, riding subways, running the AC.

All of these materials and activities either create or retain heat.

And all that stored heat causes steeper nighttime highs when people would otherwise have a chance to cool down.

The way all these heat-storing elements are laid out matters quite a bit.

In cities laid out in grids, like Phoenix, Chicago, or Washington, D.C., the buildings are closer together, like closely packed coals in a fire, keeping heat in.

More space between buildings creates more circulation.

Another way to combat this heat island effect is through green space.

Parks, trees, plants, they don't absorb as much heat.

They actually send water back into the atmosphere.

Between all those hot buildings, that cools things off.

Another solution is color.

Light colors reflect sunlight back into the atmosphere instead of storing it as heat.

It's easiest to see in crosswalks, and it's true for the color of buildings and other urban surfaces, too.

And one long-game way to combat the heat island effect is to avoid using air conditioning when you don't need it.

AC uses a ton of electricity and creates a feedback loop.

When we get that electricity from coal or natural gas, that puts more carbon dioxide into the atmosphere, which warms the planet making cities even hotter.

But this isn't just about comfort.

Today, heat waves kill more people than any other extreme weather event.

More than tornadoes, hurricanes and even floods.

City populations keep growing, and those cities are only getting hotter.

This is Let's Talk, NPR's news explainer show.

Make sure to subscribe to our YouTube channel and check out other shows.

I'm Christopher Joyce, and this is NPR.