The "dry line" is responsible for some of the most violent thunderstorms in the world. And, it occurs in the south-central plains of the United States.
Arizona plays a role in this violent setup.
During the northern hemisphere spring months, areas of low pressure frequently develop along the front range of the Rocky Mountains (east of the mountain chain.)
With its associated counter-clockwise flow, low pressure in this position cycles cold air southward from Canada, desert air eastward from the southwest deserts (including Arizona) and moist air northward out of the Gulf of Mexico.
The violent division between the dry air from the deserts and the moist air from the Gulf of Mexico is called the "dry line."
Along this line, moist air is forced upward in the atmosphere as the dry air plows underneath it.
When moist air rises, it cools. Cool air has less capacity to hold onto moisture. So, the invisible moisture (humidity) condenses into cloud droplets. As this process continues, the clouds grow larger and larger with the end result being violent thunderstorms and even tornados.
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