The layer of air comprising the lower few hundred feet above the ground is known in meteorological circles as the "boundary layer."

It is this lower layer of the atmosphere that is most affected by the interaction between sunshine and the Earth's surface. In winter, when the ground is covered in snow, a great deal of the sunlight is reflected back into space, allowing the boundary layer to get and stay cool.

Temperature inversions, when a layer of warm air aloft covers cooler air below, are much more common during winter. When the boundary layer is particularly cold, it is not unusual to see the plume of steam coming off the American Crystal Sugar Co. beet plant to reveal a change in direction at some point a few hundred feet up.

This change is occurring at the top of the boundary layer. The observer can usually assume that the temperature of the air above this wind shift is warmer than the air down at the level of the observer.

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