Facebook is helping people know how to dress for an event, a new feature in its seemingly unending evolution that is puzzling because it offers users the chance to hop off the social networking site.

Now when you set up an event on Facebook you can let guests know whether they should wear a jacket, sunglasses or snow boots thanks to an integration with the online weather service Weather Underground, a property of The Weather Channel located at wunderground.com.

You might wonder why Facebook is doing this, since it makes big bucks by keeping users on its own site and looking at and clicking on ads. Is it merely a generous addition in the spirit of simply making its site more helpful and useful? It's doubtful.

In reality, dropping weather info into its Events function is a product of a hackathon, TechCrunch reports. A hackathon, which is a big tradition at Facebook, involves computer programmers and others in the field collaborating on software projects.

Regardless, for anybody not familiar with Weather Underground, this new Facebook feature is a good chance to snoop around on the crowdsourced weather site, which makes great use of a community of weather enthusiasts who like to blog about snowdrifts, droughts, floods and whatnot. And in addition to more weather data that you'd probably ever need, you can peruse millions of uploaded photos of things like blizzards, auroras and cool-looking clouds.

To see the new weather information via Facebook, click on Events under Favorites on the left side of your Facebook page, then +Create Event on the top right. You won't see any weather options quite yet. First, you need to let Facebook know where and when you're holding your gathering, obviously.

Oddly, event organizers then see a line item that details current weather conditions at the stated location -- puzzling information considering odds are any event you're setting up isn't being held right now. However, once you send your invitation out to friends (or anybody if you make it public) people who view it will see forecasted weather data for the day of the event, although they might not know it unless they click on the link.

If you're a power Facebook user who really digs weather, you might also check out the Weather Underground Facebook app, which will do things such as post severe weather alerts to your wall and let you see what kind of weather your friends are experiencing in real-time.

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