I read a Greencast Tech Note that said:
"Given adequate moisture is present, and no air movement, the turf canopy temperature is 15 F higher than the air temperature in full sun."
I was surprised by that, because I've been measuring air temperature and surface temperature occasionally since 2014 and I have not measured a single turf canopy in which there was a 15 F difference. I make my measurements in the C scale, and 15 F is 8.3°C. The highest increase I've measured in canopy temperature over air temperature is 6.6°C.
Here's the data I'm working with from multiple days in 2014, 2015, and 2016. These are 50 measurements from putting greens in Thailand and Japan, sometimes in full sun and sometimes cloudy, with these temperatures, humidity, and wind. Grasses are bermuda, seashore paspalum, and korai.
The range in air temperatures is 25.9 to 35.1°C (78.6 to 95.2°F).
The range in humidity is 44.3 to 81.9%.
The range in surface temperature is 24.3 to 39.5°C (75.7 to 103.1°F).
For the data I've measured, it doesn't seem like wind speed has a huge effect. The wind has ranged from 0 to 4 m/s (0 to 8.9 mph). I've only got wind speed for 23 of the 50 measurements.
Here's the difference between air temperature and surface temperature.
These aren't all in full sun, because there was sometimes cloud cover, but many are in full sun, and there is nothing that reaches an 8.3°C difference, which is what one would expect if the Tech Note was correct that "the turf canopy temperature is 15 F higher than the air temperature in full sun."
I looked at the example air and surface temperature from Turf-Vu for July 28.
Then I plotted the difference.
On a celsius scale, compared to my data by time of day, this shows that almost all my measurements have a smaller difference between surface and air temperature.
Either my thermometers are off by a few degrees, or there are often situations in which the turf canopy temperature is less than 15 F higher than the air temperature in full sun.
Species effect, perhaps? Or cool-season versus warm-season grass?
This video from PACE Turf has more info about temperature, air movement, and cool-season grasses.