"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."

With freezing temperatures holding firm, much of Houston remains at a standstill, as schools, businesses and many roads remained closed Wednesday morning. State of play: Driving on roads is still not advised as melted snow has refrozen.