Google has tons of supported rich snippet markup options including reviews, people, products, businesses and organizations. recipes, events, music, videos, and more. But I've never seen Google show a weather rich snippet before.
Patrick Altoft spotted Google showing rich snippets for snow and weather conditions. Here is a picture:
I've never seen this before and as I look at the source code on that page, I don't see anything that special about the markup to imply rich snippet support.
Maybe there is some sort of tool being used here or a beta with this site for Google to test this out.
Personally, I find this to be a great use of rich snippets in Google.
Forum discussion at Twitter.
Update: You know you've been covering search too much when you forget what you wrote about just two years before. We actually noticed this in January 2011 and it was a feature Google announced back then.