Streetsblog posted this satellite image of NYC earlier today (courtesy of NASA's Earth Observatory). They remarked on the obvious difference in temperatures between parks and urbanized areas. I'd also like to point out the streets (like 125th St. in Harlem and Flatbush in Brooklyn) that show up clearly as white-hot lines through neighborhoods. Thank internal combustion engines for that phenomenon: cars, trucks, and buses can heat up the air immediately around them by dozens of degrees on a hot day if they're idling in traffic with the a/c on.
I rode my bike to work in Central Park on Wednesday, day one of the recent heat wave. Busy streets full of traffic, like Park Avenue in Midtown, were substantially warmer than adjacent streets with less traffic, and Belvedere Castle in Central Park was the least hot of all (the weather station there still reported 101 degrees of Fahrenheit yesterday).
Air conditioning hurts the situation as much as it feels good, since a/c units blast waste heat outside in order to make a building or a vehicle cooler. I was therefore not overjoyed when I rode past a large department store whose open doors were refrigerating an entire city block on my way home that afternoon. I'm pretty sure you can make out that building's HVAC unit as the bright-white pixel west of Roosevelt Island on the map above.