Sun causes direct heating.
I have spent a LOT of time gettintg wood to 6-8% moisture content (MC) for bow making. The humidity of the air will determine the ultimate %MC. At 30% RH air, the wood will stabilize at 6% MC. Heat and airflow both effect this. One of the best ways to drive out some serious bulk moisture in bow staves is to leave them in your car on a sunny day. It doesn't take too much wood to actually fog your windows.
Think about how this works- moisture is taken from the surface of the wood. Without airflow there's a stagnant layer of more humid air around the wood- effective higher %RH in the air, less driving force for water to go into the air and reach equilibrium.
Heat does 2 things- higher energy in water pushes it to evaporate. Also- hot air will hold more water- so even at the same %RH, hot wood/air will reach equilibrium quicker. If you put a woodpile in the sun in the summer- the surface of the wood can get quite hot! If the air isn't moving, then hot wood will actually heat the air around it and create a convective airflow.
Look up Prof. Gary Settles at the Penn State University Gas Dynamics Lab for some incredible Schlieren photography of the airflow coming off of humans just because they're warmer than their environment. Imagine the airflow off a log at say 120F in the sun!