WATCH: Why It's Usually Hotter In A City

In the summer, the temperature in New York City is about 7 degrees Fahrenheit higher than in its surrounding areas, according to NASA. That is not unusual. Cities are often warmer than their suburbs because of a phenomenon called "the heat island effect." The way a city is designed — the building materials used, the way streets are arranged, the lack of canopy — can actually sequester heat. More than half of the world's population (and growing) live in cities, so interest in figuring out how to cool them down might be growing, too.