Various locations
In the not-too-distant past, one had only to look up to know the weather forecast (in the right areas of the city).
First National Banks once had brilliant weather beacons atop their many locations. Since the late 50s or early 60s, these white plastic balls with colored lights within would show a different color depending on the predicted temperature. There was a collection of verse to help with decoding the balls' predictions:

When the Weatherball is red, warmer weather is ahead.
When the Weatherball is white, colder weather is in sight.
When the Weatherball is green, no change in weather is foreseen.
When colors blink by day or night, precipitation is in sight.
This verse, or something similar to it was used in a jingle in the ads for the First Bank System. The jingle was performed by local musician Jo Austin, who accompanied herself on the Hammond Organ. Others may remember the jingle a bit differently, but I will defer to my wife whose brain has been wired from an early age to absorb any and all information about weather.
In the 1990s, the autumn of the Weatherballs, KELO radio announcers would precede their regular weather breaks with: "The Keloland color radar Weatherballs are White (or Red or Green)", and follow with more specific information. The Weatherballs were controlled at KELO am by use of a control panel in their studios. I'm not sure if they were hard-wired to the studio, controlled by radio, or perhaps weatherman Dave Dedrick's powerful mind waves.
It started on the top of the old Citizen's bank building (currently Great Western) on the Northeast corner of 9th and Phillips. In this picture, you can see the ball above the giant KELO call letters.
Another shot, lower angle. This ball was made up of horizontal rings of neon tubes, presumably of different color to match weather conditions. If anyone knows were this old weatherball is, I'd love to get it for my wife. I'm sure the neighbors would be thrilled to see it on our house.
Here's a betterh shot from atop the Citizen's Bank Building.

By the looks of this orb, it really seems like it's capable of controlling the weather.

Photo Courtesy Midcontinent Media/Ken Mills.
Only two of the four second-generation Weatherballs remain in Sioux Falls, though they are dormant. One Weatherball is here at 141 N. Main (8th & Main).
I wonder what it would take to get these babies going again. New bulbs? I happen to have about six red light bulbs left over from my bachelor days. These I'd gladly donate.
The other dormant, but existent Weatherball is here at 33rd & Minnesota. Though they don't work, these things are a welcome sight to me. I never quite appreciated them when they were working. For sure they were of limited use when they did work, but what an excellent service to offer your customers and fellow citizens. Free checking is nice, too.
This former First National location at 10th and Omaha no longer bears the Weatherball on its sign.

The fourth location at 41st and Louise no longer exists. It was replaced by a jewelry store. They took the sign down rather than reshape it into a weather-predicting diamond.
The Weatherballs were a nice feature to have in Sioux Falls. They stopped their helpful predictions in the mid to late 90s. There's something about their warm glow that was a comfort to those of us who experienced them. These days it wouldn't take much to automate them so that humans need not even set them. All it would take is someone at US Bank with enough vision to get the ball rolling.

Update: Greg Allis of Sioux Falls announced in 2008 that he was looking to get the balls rolling again. There hasn't been an update about this since.
Mark was kind enough to lend me the Argus Leader supplement containing the First National Bank location pictures.
Check out the other Weatherballs and beacons of the world here.
Thanks to Roger Blair and Jeff Richards for additional information.