Rocket sets two thermometers side by side on Station Rooftop: one on the dark roof surface, one under the tomato plants.
Ten minutes later he checks. "The roof one is way higher. The plant one is almost comfortable."
Raven writes both numbers down. "What do you notice about the whole city from up here?"
Rocket looks out. Roofs, roads, parking lots, a few parks. "Mostly dark, hard stuff."
Nova hovers over the edge and glows over a green park below. "Where would you stand on the hottest day of summer?" she asks.
"Under a tree," Rocket says instantly. "Or on a white roof."
"Then that is a design idea," Raven says. "And the EPA has numbers on it."
The Environmental Protection Agency explains the cause. Buildings, roads and other structures absorb and re-emit the Sun's heat more than forests and water do.
Where these structures are packed together and greenery is limited, the area becomes an island of higher temperature. That is a heat island.
Heat islands can form in large or small cities, in suburbs, by day or by night, and in any season.
| Measurement (EPA) | Number |
|---|---|
| Daytime air temperature in United States urban areas, compared with outlying areas | About 1 to 7 degrees F higher |
| Nighttime air temperature in urban areas, compared with outlying areas | About 2 to 5 degrees F higher |
| Conventional roof surface on a warm day, compared with the air | Up to 66 degrees F warmer |
| Urban forests, compared with urban areas without greenery (average of 308 studies) | 3.0 degrees F cooler |
| Cool roof effect on the hottest indoor temperature in homes without air conditioning | 2.2 to 5.9 degrees F lower |
The EPA explains that trees and plants lower temperatures in two ways: shade, and evapotranspiration.
Evapotranspiration is when plants pull water up through their roots and release it from their leaves. Evaporating that water uses heat from the air.
Trees that shade a building also reduce how hard its cooling system has to work.
A cool roof reflects more sunlight and sheds the heat it does absorb. Light colors reflect more than dark ones.
The EPA notes that light-colored walls, called cool walls, do a similar job to cool roofs.
The heat index is how hot it really feels when humidity is added to the air temperature.
The National Weather Service gives an example: 96 degrees with 65 percent humidity feels like 121 degrees.
Heat index values assume shade and light wind. Full sunshine can raise them by up to 15 degrees.
The National Weather Service issues an Extreme Heat Watch when extreme heat is possible but uncertain. An Extreme Heat Warning means it is expected or occurring.
A Heat Advisory is for dangerous heat that does not reach warning level. All three say: stay in a cool place during the heat of the day.
| Statement | True or false? |
|---|---|
| Buildings and roads absorb and re-emit more heat than forests and water. | ? |
| Heat islands only form in very large cities. | ? |
| Full sunshine can raise the heat index by up to 15 degrees. | ? |
| An Extreme Heat Watch means dangerous heat is already happening. | ? |
| Shade from trees can reduce how hard a building's cooling system works. | ? |
Strong data reading. Tomorrow you design a weather-ready plan for your home.