How can we explain the instability of the weather on the plains of the continent of North America?

The nature of the relief and the winds coming from three sides are to blame: the northeast trade winds, the western and northeast winds.
The flat character of the relief forms of the middle part of the continent leads to the unhindered movement of air masses from south to north and from north to south. This is the reason that cold air from the subarctic zone in winter often reaches the south – the Gulf of Mexico, causing a sharp decrease in temperature. However, tropical air masses extend far to the north. Such free movement of the masses is the cause of the instability of the weather on the plains. Here in winter, the thaw often replaces frost.

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