Around 40 million people in the U.S. are under a winter storm warning from an unusual blast of severe winter weather.

A rare winter storm charging through Texas and the northern Gulf Coast left New Orleans and Houston frozen Tuesday.

"It's called 'black ice' because it tends to look like the rest of the pavement on the road, although in reality, it's actually clear," the US Forest Service notes. "Black ice forms without creating bubbles, which allows it to blend in with any surface it forms over. Black ice is dangerous precisely because it's hard to detect."