A lush green lawn, a shady tree—when the temperature soars, we gravitate to green. Why are plants so much cooler than, say, a parking lot?

Some plants, like trees, provide shade. But a lawn won’t shade anything bigger than a beetle. So how does it make the air above it cooler? The answer has to do with water.

Everyone knows plants need water to survive. They pull water from the soil through their roots. From there it moves up the stem and into the leaves.

Leaves use water to help make food from sunlight in a process called photosynthesis. Most of the water, however, escapes through tiny pores called stomata. Stomata open in response to sunlight, letting water evaporate into the air.

Evaporation means that a liquid, like water, is changing to a gas. When a substance changes from one state of matter (solid, liquid or gas) into another, it’s called a “phase change.”

All phase changes require energy. When you boil water and make steam, you add the energy from outside using a stove, microwave, or even a fire. When water evaporates naturally, it takes heat energy from the liquid itself to make the change. The liquid that gets left behind is cooler than it was before, because it has lost some of its heat energy.

Chris Martin, a professor of horticulture at Arizona State University, says we’ve all felt this process firsthand.

“It’s like the experience one has in the swimming pool on a hot day. You step out of the pool and the first sensation that you experience is, ‘I’m freezing! It’s 103° out and I’m freezing!’” he says.

Chris Martin places a temperature sensor in a tree to measure its cooling effect.

As the water evaporates from your skin, it uses up heat, leaving you feeling chilly. Your body takes advantage of this cooling process naturally through perspiration, or sweat. Evaporation from plant leaves is called “transpiration.” Transpiration helps pull more water up through the plant against gravity.

At night, the stomata on leaves close up, shutting down transpiration. Yet a golf course at night still feels cooler than a parking lot. Martin says this is because moisture in the soil continues to evaporate.

“The soil is very moist and the air is very dry, so the air is a sink drawing that water out of the soil,” he says.

Taken together, transpiration from leaves and evaporation from the soil are called “evapotranspiration.”

“It’s a longer term to combine both evaporation and transpiration, both of which are technically evaporation. It’s just a question of whether it’s occurring from the soil or the leaves,” Martin explains.

Green in the wallet = green in the neighborhood
Martin is studying how plants influence temperature as part of a larger study on urban heat and its risk to humans. The scientists on the study are looking at different types of land cover, such as grass, dirt or pavement. They want to see how these affect temperatures in different neighborhoods.

They are also studying how land cover and temperature relate to the characteristics of the people living in those areas, such as income level or ethnic group. Martin says there is a link between how green a neighborhood is and the socioeconomic status of the people living there.

“Up until about 1970 or so you didn’t see this. But since about 1970 we’re increasingly seeing that the more wealthy people are, the more vegetation they have,” he says.

Poor people may not be able to afford air conditioning in their homes, either. This makes living in a hot neighborhood even more difficult. Martin says one place people often go to escape the heat is a neighborhood park.

“Even in the heat, there are a lot of people using parks. Lots of homeless people congregate there, also,” he notes. (Read more about how the heat affects homeless people.)

Martin teamed up with Darrel Jenerette, a botanist at the University of California-Riverside. They wanted to see if parks could serve as a heat refuge—a place where people can escape the extreme summer heat. They also wanted to know if there’s a difference in temperatures between parks in wealthy and poor neighborhoods.

Darrel Jenerette records data in Lindo Park in Phoenix.

In July, the researchers placed 100 temperature sensors in 10 Phoenix parks. They selected parks in low-, middle- and high-income neighborhoods, putting sensors in trees and in the ground. The sensors took readings every 30 minutes for 42 days. The data will help the researchers figure out if trees and grass have a cooling effect, and which neighborhoods have the coolest parks.

There are lots of things to consider when you try to put thermometers in public parks. You have to put them in places that will get you the information you want. At the same time, you can’t put them in spots where people might break or steal them.

“Ideally, we want to measure temperatures at the surface and at about 2 meters (6 feet) high,” says Martin. “We are going with 3 meters (10 feet) so we can put them in the canopies of trees. There they will be shaded from the sun. And then we are burying sensors about one inch below the surface. We hope this will give us an accurate approximation of the soil surface temperature.”

Some of the ground sensors were placed in the shade of trees. Others were in areas completely exposed to the sun. The scientists used zip ties to mark where the underground sensors were placed. They also measured each sensor's distance from the nearest tree.

Would you notice this yellow marker in the grass?

The researchers had hoped to compare parks mostly covered in dirt (usually found in low-income neighborhoods) with parks covered with grass. They may not be able to do this, but that’s actually a good thing. For instance, Sherman Park, near Buckeye Road and Interstate 17 in Phoenix, used to be little more than a dirt lot. Now, however, the city has started planting grass there, and it’s becoming greener.

“It’s interesting how parks have changed in Phoenix over the last five years or so,” says Martin. “Phoenix has done an amazing job of upgrading the quality of turf and planting trees, particularly in low socioeconomic neighborhoods. They are dominated by turf grass. It’s the best thing for places to escape heat.”

Teachers: Find lesson plans on evapotranspiration and other topics at ASU’s Ecology Explorers site.