The temperature data was collected via thermocouples — sensitive instruments that measure the electric voltage created by temperature differences — attached to the windows of cars driven by more than a dozen volunteers sent across the city. The researchers were instructed to zig-zag across a specific corner of the city from 3 p.m. to 4 p.m. Aug. 29, again from 7 p.m. to 8 p.m. that night, and then from 6 a.m. to 7 a.m. the next day.