At just the right temperature and humidity levels, frost will form in hexagonal towers. Here’s a more descriptive photo of what this looks like (click on the photo for a larger version). These towers were about 1cm tall.

Hoar frost is a beautiful phenomenon that can cause problems if you’re in the mountains in winter. The hoar frost forms on the snow, and when fresh snow falls on top of it, it forms a weak layer in the snowpack. This can make avalanches a lot more likely.

Taken in Yosemite National Park.