Compared to their surroundings, cities can be hot -- hot enough to influence the weather. Industrial, domestic, and transportation-related activities constantly release heat, and after a warm day, concrete surfaces radiate stored heat long into the night. These phenomena can be strong enough to drive thunderstorms off course. But it isn't only about the heat cities release; it's also about their spatial layout. By channeling winds and generating turbulence hundreds of meters into the atmosphere, the presence and organization of buildings also affect weather and air quality.