It’s on days like today – sunny, hot and humid – that make me very thankful for the shade that trees provide. I’ve got it down to a science how best to navigate the trip from the bus stop to my home. I know exactly which route 
has the most shade, when I need to cross the street, when I know I’ll be in the hot sun for a few minutes. The difference in temperature between shade and sun is amazing. By shading surfaces like asphalt, roofs, and concrete sidewalks, those surfaces don’t heat up and trap the heat, keeping temperatures lower.
I read about an experiment that NASA did in Huntsville, Alabama, recording the difference in temperature around a mall parking lot. The experiment showed that wherever trees were planted, temperatures were reduced by 17.2° C . Pretty amazing. There’s also a web site that has a slide show showing the changes in temperature over a period of nearly 20 years for Atlanta, Georgia, as trees were removed. It’s a very telling story.
I only know that I feel much cooler in the shade than in the sun!
Until next time!
Lower Olentangy Urban Arboretum 
I think that the ‘experiment’ that you may be referencing is described here.
Although your numbers don’t exactly match up, the website does tell of a 31 degree F differential in the daytime and a 12 degree F differential at night. Sixty-three, 31, 12, whatever the number… trees do reduce the heat island effect.
It should be noted that asphalt absorbs heat and can radiate that heat back into the atmosphere. It can get quite hot and the treed areas stay cooler.
The site I used was http://www.ghcc.msfc.nasa.gov/land/heatisl/heatisl.htm. It mentions:
“A spot check of day temperatures around the Mall shows that in the middle of the parking lot temperatures reach about 48.8 °C . However, a “tree island”, a small planter containing a couple of trees in the parking lot is only 31.6 °C. So, even a small area of tree coverage surrounded by a very hot parking lot reduced temperatures by a significant 17.2 °C !! ”
Not being a math major, I might have messed up on the conversion…but like you said, trees make it cooler!