I'm not even addressing the fact that doing any physical labor when it's that hot outside is dangerous here. I'm just addressing a question I get about cleaning gravestones during extreme weather.
The type of gravestone you should avoid cleaning on hot, sunny days are the black, granite ones. These are the grave markers that are hot to the touch. If this is the case, you should NOT clean them. Why? According to Jason Church, materials conservator, for the National Park Service (NCPTT), the liquids you are using to clean may react with the surface finish of the granite, permanently damaging it. No one wants that to happen. Remember our guiding principle when cleaning gravestones? Do no harm to yourself or the grave marker.
What about light colored gravestones or those which are non-granite? I would suggest avoiding cleaning those too if they are hot to the touch. Just wait for a cooler time of day or for a more mild day.