This week is so cold that much of the U.S. – Michigan included – is in a deep freeze called a polar vortex.

So, what exactly is a polar vortex?

According to CNN meteorologist Brandon Miller, a polar vortex is a “circulation of strong, upper-level winds that normally surround the northern pole in a counterclockwise direction -- a polar low-pressure system.” The frigid winds, Miller says, keep the “bitter cold air locked in the Arctic regions of the Northern Hemisphere. It is not a single storm.”

He added, “On occasion, this vortex can become distorted and dip much farther south than you would normally find it, allowing cold air to spill southward.” In other words, arctic temperatures and winds are in the U.S.

Although a polar vortex isn’t a “storm,” Miller has some advice for anyone caught in the frigid temperatures.

“When faced with significant cold temperatures, you should stay inside whenever possible, layer clothing if you must be outside, winterize your home and car, etc.,” he said.