Portland, Oregon has long had a Weather Ball -- a series of lights that tell you, in rough terms, a weather forecast. It's located downtown, on top of a building. From many locations in Portland you can spot the ball and get a general sense of upcoming weather. In Portland, the lights are coded like so (thanks to Lyza Gardner for the explanation):

It can tell you one of six things, that is:

1. It's going to get hotter (steady red)
2. It's going to get colder (steady white)
3. It's going to stay about the same (steady green)
4. (and 5 and 6) It's going to precipitate (blinking [plus a color above])

While this isn't much use for someone like myself who's, ahem, color confused, it's a neat idea, and surprisingly little-known among newer residents of the city. I rarely care about the specifics of the forecast: for me it's enough to know it'll be "hotter"; "colder"; or, say, "colder and raining." That's exactly what the Portland Weather Ball does well -- and does it for free, all the time. Just look to the horizon, locate the light, and you've got your next day's weather.

It turns out that such weather beacons exist in many cities around the world. Pictured above left is the "WZZM 13 Weatherball" in Grand Rapids, Michigan (I couldn't find pictures of Portland's rather less grandiose ball). To see the Grand Rapids ball in action, check out this Flickr search.

Wikipedia explains the origins of weather beacons as advertisements:

The first attempt to create a weather beacon as a form of advertising was from Douglas Leigh, who, in 1941, arranged a lighting scheme for the Empire State Building to display a weather forecast code with the decoder packaged with Coca-Cola bottles. The plan was abandoned following the attack on Pearl Harbor later that year. Mr. Leigh resurrected his idea in Minneapolis in October 1949 with the Northwestern National Bank Weatherball.

Is there a weather ball in your neighborhood? If so, do people know about it, or is it just one more light on the horizon?