They found that some of the laws that apply in the physics of real skies also apply in the artists depiction.
Since you cant really see air, Id go with a slightly different explanation. Being a gardener and being so connected to my plants, I think he connected with nature on a very deep but subconscious level. 
How does this express itself, like what Van Gogh did? In my case I was telling students about a particular kind of pest and how to spot it when it lay its eggs. A student said he wished he could see what it looked like.
Without even thinking, I said sure and walked to one of hundreds of plants in this garden. I flipped the leaf over and there were the eggs. I showed the students.
Later a student asked how I knew where to find the pest and its eggs. I didnt know how to answer.
Similarly, I think Van Gogh felt out the air currents.Just in tune with nature.
          
          
          
        
         
        
         
   
   
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