Uncut grass keeps the ground at a static 19.5°C; a grass cut at 10cm keeps the ground temperature at 24.5°C; bare ground in the middle of the summer rises up to more than 40°C.
Keep the grass long and you change the ambient air temperature, and enable carbon to remain in the ground. The more grass/tree coverage the less capped earth you have exposed. Anything longer then 10cm helps to keep moisture in the grass.
