So I'm on the way home tonight, just getting out of Rochester. Like any normal person, I'm flipping through the radio presets trying to find that perfect song to listen to. Everyone has presets for their radio, right? So I'm starting to cycle through. Nothing good there. Biebs... absolutely not! Nope, overplayed. Put me to sleep. A song about a tractor a dog and the wife leaving, heard that before. Alright, I surf the stations quite a bit.
Each button has a genre. When I hit that little number "1" I'm perplexed to hear Kool and the Gang - Celebration starts to play, when I'm expecting Top 40. Unless I jumped in the time machine and went back 35 years, something is most certainly not right.
The button I pushed is programmed to pick up WIZM - 93.3 FM, or, more commonly known as Z93 out of La Crosse, WI. Being from Iowa, I instantly knew what was happening. I picked up KIOA out of Des Moines, IA which plays more of an oldies genre and shares the same 93.3 frequency.
The signal was crystal clear. Even the Radio Data System (RDS) was displaying station and song information. How could this be possible? Des Moines is 170 miles away from Rochester as the birds fly. La Crosse is closer to 70 miles.
The answer lies with a phenomenon called Atmospheric Ducting. Think of a radio signal leaving the transmitter. Normally it is set to broadcast a certain distance out away from the antenna. Some of those radio waves continue to head on a trajectory into the voids of space. But in a ducting scenario, sometimes these waves can interact with a temperature inversion. That is a warmer layer of air above a cooler layer near the surface. This warm layer helps refract the radio waves and sends some of the energy back down towards earth.
The radio waves that normally would be limited due to line of sight and the curvature of the earth are allowed to propagate longer distances due to the ducting.
This is exactly what was happening tonight. A look at three atmospheric soundings, or a vertical profile of the atmosphere, reveal there was an inversion over Iowa and Minnesota. The red line on the following soundings is the temperature at any given point in the atmosphere. Generally speaking, if the line moves to the right as it rises, the temperature is warming.
The soundings from Davenport, Omaha and Minneapolis (In order below) are the closest sounding locations to the area. One can extrapolate the readings from each site to form an average for our area. Note the inversion on July 1st at 7PM , noted with the arrow at each location. Each inversion is located around 3k to 5k feet high.
It's that subtle 3°C change that made all the difference. The KIOA signal normally reaches out a radius of about 80 to 90 miles from it's tower near Des Moines. For me to hear it as I got up to speed on I90, that distance was doubled and it overcame a signal originating from a much closer radio station.
Atmospheric ducting doesn't just impact radio waves either. The visible spectrum (mirages), over air television frequencies and even sound waves can be altered by this phenomenon. It's really impressive what our atmosphere can do!
While Atmospheric ducting allowed me to hear a radio station from very far away tonight, in more populous areas, it can make listening to the radio a nightmare. With more radio stations in closer proximity to each other with the same frequencies, the ducting can cause significant interference, even at close range to a transmitter.
So the next time you hear some odd static or a radio station that you've never heard before in a particular area, it could be caused by something caused atmospheric ducting.
In case you're wondering my favorite genres... Top 40 and Country. Not your most likely combo, is it?
Storm Tracker 6 Chief Meteorologist