It's really cold in much of the U.S. right now, so aiming to put a cheerier spin on winter we offer 2 questions from nature's Did You Know? file:

1. What is hoarfrost?

2. How do snowflakes form, and is every flake unique?

First, hoarfrost. It's unusual, fast-melting stuff—elongated, delicate ice crystals that appear to have been jolted by a fritz of electricity. This photo from Yosemite National Park provides a neat look at the stuff:

Yosemite also supplies an explanation for how hoarfrost forms—any cold place with lots of water vapor in the atmosphere but no wind. Check out the full explanation from an installment of Yosemite Ranger Notes.

Next, snowflakes. The following video from BytesizeScience.com, affiliated with the American Chemical Society, explains that snow begins as cloud dust that collects water vapor, then freezes, warms, freezes again and develops into a 6-sided work of art.

The longer a falling snowflake gets knocked around in the atmosphere, the more time its branches can evolve into wacky shapes.

So is every snowflake unique? Quoting the video: “The old saying that no 2 snowflakes are alike may be true for larger snowflakes, but not for smaller, simpler crystals that fall out of the sky at earlier stages, before they’ve had a chance to fully develop.”

Low temperatures Wednesday: -8°F in Duluth, Minn., -2°F in Fargo, N.D., 1°F in Boise and Grand Junction, Colo., 4°F in Minneapolis, Milwaukee, Pittsburgh and Salt Lake City, 5°F in Detroit, 6°F in Hartford, Conn. 9°F in Indianapolis and 11°F in New York City. Slight temperature rises are forecast for most cities today.

In case you missed it, check out our Wednesday post about ice climbing on silos, ice fall in Yosemite and guarding against frostbite.

Footnote: In Los Angeles, which hit 78°F Wednesday and on Monday reached 81°F (a record high for the date). Jimmy Kimmel reported that temperatures in Southern California are required to stay above 60°F. "Otherwise," he said, "the collagen in our celebrities' faces will freeze."