Liz, cities do tend to be warmer than the surrounding countryside. Cities hold heat partly because concrete and pavement absorb sunlight. But the major reason cities are warmer is that they can't easily cool down. Consider that when a dog gets hot, it pants. When you get hot, you sweat to help cool your body.
Forests also sweat.
But a city can't do that. When you cut down a city's trees, pave over its soil surfaces and funnel all rainfall and runoff into sewers, there's no way for the city to soak in water so that it can later be released. Special thanks today to the RGK Foundation of Austin, Texas. We're Block and Byrd for Earth and Sky.