In less than a month thousands of square miles of the central plains states, TX north to ND, go throughthis progression starting in the south and moving north.
1) Tall green (grass) wheat shading the earth and giving off water vapor helping to cool the atmosphere.
2) Wheat matures, plant dies and grain berries dry, the earth is still shaded from sun light by the standing wheat.
3) Harvesters cut the plant save the berries leaving short stubble and straw, with almost all shade removed the soil and air temperatures in the field rise.
4) To speed the composting of the straw and prevent weeds, plowing and disking of the fields soon follows, leaving soil fully exposed to the sun, temps increase further.
This is the farming method for wheat in the central plains states. So far as I know there is no fault in this process, it's what my family has participated in for a century, I explain it only to inform you of something few people are aware of. When hundreds upon hundreds of square miles of farm land goes from respiration of cooling water vapor to bare earth the temperature of the region begins to rise. It is a seasonal solar produced rise for sure as days grow longer accelerated by our farming of wheat on a massive scale. But as barefoot kids and dogs know well the sunny day rule, walking in grass and walking on blacktop are not the same.
But, 2012 could be a scorcher, my theory, we had a warm winter, record spring highs, high soil temps, we are on pace to harvest wheat 3 weeks earlier than ever before. Kansas will be cutting wheat in mid May. As the pictures show examples of the 8 million acres will soon be without shade in Kansas alone. The temperature feed back loop will have a 3 week jump start to a long hot summer in the plains states.
When I was a kid we cut wheat at the very end of June, now it's 45 days earlier, and the summers are hotter.
Temperature variations: grass over blacktop or white roof over black roof, in the range of 40°f.