Some people very much believe in it while some other mets think it is nominal. I think this blurb I grabbed off the Googles explains the science theory but would be interested to see if anyone has done real testing on it. I would think on a short term basis it would only amount in a change of a degree or so unless it rained that morning and the ground was actively wet. Heavy rain 3 days ago would have long ago been absorbed and evaporated in the area of the soil the sun gets to. The article this came from discusses long term drought and its effects on heat waves.
This can also be explained as follows: when the ground is dry, heat from the sun does not have to expend energy to evaporate the already-dry soil, and instead heats the air to a more intense degree, causing air temperatures to rise. This relationship suggests that dry soils can drive a rise in temperature, like a heat wave.