Beat Urban Heat!

The physics of staying cool in the summer

When the sun beats down and the sidewalks begin sizzling, how do you stay cool? Run through a sprinkler, take a dip in the pool, start a water gun fight? Do you go to the beach and enjoy the sea breeze, or stay home and turn on an air conditioner?

If you picked any of the above, thank evaporative cooling. Evaporative cooling is how nature and technology keep you from overheating during the year’s hottest days. It’s so simple and so powerful that it can keep entire neighborhoods and cities cool – if we understand how it works and how to use it.

When water evaporates, it’s going from the liquid state to the vapor state. To make this transition, the water has to draw energy from its surroundings; this extra energy helps the water molecules break away from their neighbors so they can float into the air.

If a water droplet sitting on your skin evaporates, it takes a bit of your body heat along with it. Our bodies take advantage of this process by sweating: as sweat evaporates, it cools us down. (High humidity makes us feel hotter than “dry heat” because there’s so much water vapor already in the air that evaporation slows to a halt.)

You use evaporative cooling every time you splash or mist water on yourself. Evaporative cooling is also why air blowing over bodies of water is colder than air blowing over land. It’s the secret process that makes air conditioners work: the refrigerant inside evaporates very quickly and then gets compressed into a liquid again in a loop.

And evaporative cooling is why neighborhoods with many trees and other plants have lower temperatures than neighborhoods without them. Plants contain water; water evaporates through their leaves; evaporation cools the immediate surroundings.

Take a look at the temperature map of Philadelphia, below. It shows the heat exposure index of different neighborhoods (with dark red being the highest).

Why are some parts of the city hotter than others? The answer is complex, involving population density, amount of vegetation, construction material types, and building height ratio to street width. But understanding the physics of heat retention and transfer helps us answer a second important question: how can we alleviate the burden on those sun-baked blocks?

You’ve probably guessed one major tool we can use…evaporative cooling! According to the EPA, adding vegetation to neighborhoods by planting more trees, community gardens, and green roofs helps keep things cool.

Civil engineers are now designing roads made of porous asphalt. This “cool pavement” can absorb water and then cool off as the water evaporates, unlike traditional asphalt. Other smart, non-evaporative choices include using reflective paint on the outsides of buildings to avoid absorbing so much heat from the sun and planning walkable communities to reduce the heat emitted by vehicles.

Summer means long days of hot sunshine. But whether that spells fun or heat stroke depends on how wisely you wield humans’ oldest cooling trick: evaporation.

🌡️🥵 Heat Vulnerability Index 🥵🌡️
The map below shows how heat vulnerability varies across the city.

Northwest Philadelphia
This area has lower temperatures in the summer and many street trees and green spaces. Its population is not very sensitive to extreme heat. Overall, this area is not especially vulnerable to heat.

South Philadelphia
South Philly is one of the most densely populated areas in the US. It has high temperatures in the day compared to other neighborhoods and its population is very sensitive to extreme heat. Overall, this area has a very high heat vulnerability score.

North Philadelphia
This area is located north of downtown and has a high population density. Many people who live here have chronic health conditions. This area is hotter than other areas of the city in the summer. Overall, this area has a very high heat vulnerability score.

Remember: very hot weather can make people sick, especially the very young and old, and those with contributing physical issues (including pregnancy). In extreme conditions, the City will declare a “Heat Health Emergency,” during which time cooling centers extend their hours and mobile medical teams may be dispatched as needed.

Philly also maintains a special “Heatline” you can call for information, safety tips, and even medical advice from health department nurses. Call 215-765-9040 (if someone’s in crisis, call 911). The city also has a “code red” program for unhoused people and animals which offers a shelter spot during these heat emergencies (215) 232-1984 📱

Happy sweating! 💦👍😎

About Anne Hylden 3 Articles
Anne Hylden holds an M.S. in Inorganic Chemistry and is pursuing an M.A. in Science Writing. She taught math and chemistry for twelve years before turning to freelance writing. When she's not thinking about science, she enjoys drinking tea, walking her dog around Northwest Philly, and laughing with her partner Bob.

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