Today was an especially hot day at Bangkok. I went out for lunch, and when I was driving home I noticed that the thermometer on my dashboard was indicating an outside temperature of 42°C. Upon arriving home, I checked the official temperature, found it to be 38°C, and I promptly went outside with my infrared thermometer to measure the surface temperature of concrete and of grass. See the above image at full size here.
These measurements were made at 14:00. The concrete measured 53.6°C, about 15°C above the air temperature. Manilagrass (Zoysia matrella) wilting in the sun had a similar temperature, 48.8°C, more than 10°C above the air temperature. Meanwhile, manilagrass and carpetgrass (Axonopus compressus) in the same lawn, in areas with adequate water so the grass could transpire, had leaf temperatures ranging from 1.8°C below to 1.2°C above the ambient air temperature.
As part of a putting green performance data research project, I've collected leaf temperature data from hundreds of greens across multiple grass species in many countries. As of today, I've measured the surface temperature 802 times. If you are interested in reading more, these data are summarized beginning on page 21 of this report. It has been my observation that in conditions of full sun, minimal wind, and adequate plant water status, meaning the grass leaves are not wilting, no matter how hight the ambient air temperature, the leaf temperature will generally be within 1°C or 2°C of the air temperature. Have you ever measured anything different?
For those more familiar with °F than °C:
38°C = 100.4°F
53.6°C = 128.5°F