Now, it's covered in buildings.

Phoenix, along with Las Vegas, is experiencing a stretch of triple-digit temperatures made worse by a heat dome.

- Record-level temperatures... - ... shows no signs of letting up.

Unrelenting grip on the country.

A lot of our cities have got gotten... gotten bigger in the last 20 years, especially in the southwest.

Here's what happens when heat domes form above cities and how urban growth can make that heat even more deadly.

In Phoenix, over 20 days in July have been at or above 110 degrees.

And space technology company Maxar, which has been tracking the weather, predicts the city will hit at least 30 by the end of the month.

In Las Vegas, 16 days are predicted to hit that high.

That heat is driven, in part, by urban development.

Imagine a city as an island with pavement, roads, surfaces, glass, and steel buildings, all of these surfaces reflect sunlight and that heat energy from the sun back into the air around.

These highly-developed areas can see midday temperatures that are 15 to 20 degrees warmer than surrounding vegetated areas.

We can see how these heat islands formed by looking back at Phoenix⏤here it is 23 years ago.

But after two decades of urban growth, the city has ballooned.

Back in 2000, Las Vegas looked like this.

And, now, its city limits have expanded.

That expansion paralleled their economic growth, but more infrastructure turned up the heat.

These are all places where heat gets trapped, right?

So, in many of these cities, there's not a lot of breezes, there's not a lot of shade or trees.

And, so, as a result, heat can build up in these urban corridors.

It's sometimes hard to grow trees in the west, especially in very arid places, and the native trees that grow there may not be great at providing shade.

So, you have a lot of areas that get a lot... a lot of sun.

The other important thing about plants and trees and vegetation and even grass is they cycle moisture from the ground into the air and help, just a little bit, keep things cooler.

That's driven temps in Phoenix and Las Vegas even higher and drove the creation of these heat islands.

But that's not the only thing fueling the hot weather.

When you have a heat dome on top of this, it really intensifies the temperatures for everyone living in this city.

These domes are a natural weather phenomenon, and for most of July, one has been covering the southwest.

A heat dome is a large ridge of high pressure high in the atmosphere that basically traps a lot of hot air underneath it, and it can last for a long time.

Over time, the heat dome is basically reinforcing itself in the sense that any moisture on the ground or in the soil that may come up into the atmosphere that, perhaps, could cause some rain or rainstorms in the afternoon,

that's going away; the soils are drying out, the land is drying out.

And, so, this hot air is making things worse and worse.

The combination of a heat dome on top of a heat island means that being outside can be even more dangerous.

Burn centers have reported injuries to people who touch hot door handles and walk barefoot on scorched pavement.

These are hot places, right?

And, so, there's often this feeling of, "Well, it's always been hot here; we're used to it, we can deal with it."

But I think that masks some of the really significant risks that are faced by people experiencing homelessness, by people who are elderly, people who work outside,

many of these different groups that are... that are exposed and vulnerable.

In cities already experiencing triple-digit temperatures, the National Oceanic and Atmospheric Administration is preparing officials for increasing heat events.

Everybody is very concerned about... about the future, that with warming and with climate change, we expect more of these events to happen, and when these events happen, they're... they're more intense.

And scientists say the heat is here to stay.

As architects, planners, and engineers look to the future,

they have to take into account the effects of climate change over time, temperatures that are rising, storms and other things that are happening as well.

So, they're taking these things into account with new kinds of technologies, green roofs that can absorb and cool buildings,

and, also, just in the design and shape of buildings that can provide more shade and maybe just have a little bit of resiliency toward a warming climate that we're all facing in the future.