The featured chart is of daily solar radiation (red line) and average 4 inch soil temperature (black line) from the Ames ISUAG site. As you probably can recall, air temperatures have been well below freezing recently, so why have soil temperatures been warming recently? The reason is that warming blanket of snow sitting on the ground. The blanket traps the heat from deeper depths in the soil allowing the top soil to slowly warm. When there is no snow, the top soil loose heat accumulated during the day at night when air temperatures are colder.

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