This week has given us some uncommonly harsh cold weather in parts of the US. That's given us the chance to trot out some uncommon weather terms.
Frost quakes in Chicago, anyone? Some people reportedly woke up Wednesday to a series of booms -- and frost quakes may have been the culprit.
Here's what a frost quake is, along with definitions of other terms we learned this week.
Frost quakes: Boom, indeed. A frost quake describes the cracking of soil and rock that occurs when underground water has suddenly frozen and expanded. That cracking causes booming or banging sounds. This natural phenomenon, called a cryoseism, can happen especially when there is a sudden drop in temperature.
Flash freeze: This is pretty much what it sounds like: Freezing that takes place in a flash when temperatures plunge rapidly.
Snow squall: A short but ferocious burst of heavy snow that's going to make it very difficult to see -- *like the brief, near-whiteout conditions we saw in New York, Philadelphia and other cities. It usually comes with strong winds.
Steam fog: A fog produced over lakes -- any lake, really, but it's common on the Great Lakes. When cold, dry air moves over a relatively warmer lake, the warm, moist air of the lake mixes with the air above producing fog.
Polar vortex: This has received more attention in recent years, but it's not new, and it's not even intermittent. The polar vortex always exists -- always swirling counterclockwise around the North and South poles. In the Northern Hemisphere, its winds keep bitter cold air locked in the Arctic regions.
*But come winter, it expands. And sometimes, it dips much farther south than you would normally find it. When that happens, you get an outbreak of especially cold air.