Here's some anecdata. My training partner and I have a standard out-and-back 2x30' tempo ride we do often. It's on a bike trail that parallels a river so it can be quite cool and damp in the winter. I looked at a ride from February and compared it with a ride in June.

2/9/16
46F
266w avg. power
19.7mph

6/6/16
67F
263w avg. power
21.1mph

So, 21 degrees warmer meant that for the same power, I went 7% or nearly a mile and a half per hour faster.