1.   During summer when thermal contrast is minimal, weather systems move half as fast.

2.   During the autumn, the thermal contrast between the north pole and the equator grows.

3.   During summer when thermal contrasts are minimal, weather systems move half as fast.

4.   During summer, when thermal contrast is minimal, weather systems move half as fast.

5.   Now, as the sun gets higher in the sky, the thermal contrasts that fuel large storms diminish.

6.   The contrasting records in the northern tier indicate a strengthening jet stream, which is driven by such thermal contrasts.

7.   The jet stream, a ribbon of winds in the upper atmosphere that drives weather systems across the country, draws energy from the large thermal contrasts.

8.   These large thermal contrasts help to drive the jet stream.

9.   Winter storms thrive on large temperature differences, gaining potency as thermal contrasts increase.