Intro: Using a Radio to Detect Lightning
Small radios can be used for more than listening to music or sports. All radios (even cheap AM only radios) can be used to detect lightning and other atmospheric phenomena. With a trained ear, one can even determine if the lightning is moving toward or away from you.
Step 1: Technical Details
Radio noise is all around us and can be heard on an AM radio. The loud buzz that you can hear on an AM radio when you turn on a vacuum cleaner is basically radio noise caused by the sparking of the commutator of the electric motor. If you have trolley buses or tramcars in your city, you can hear the sound of the electricity sparking from the 600-volt lines to the brushes and the hum of the DC motor in your car radio. This is one of the reasons why FM radio is preferred over AM radio. The radio shown in the picture can tune from approximately 100 kHz to 30 MHz which is all AM plus the regular 87 to 108 MHz FM band. The reason that AM is much more prone to such noise or static is that the FM receiver has a circuit that allows only signals through that change in frequency but not those that change in amplitude to be detected. When you have a lightning strike, you have a discharge of millions of volts which produce intense discharges of radio waves (called Sferics or Spherics) at random frequencies which are more prevalent below 1 MHz. This is why a basic AM radio or even better a radio like the Tecsun PL-380 (which can tune below the standard broadcast band ) is good for this purpose. Almost all modern AM radios have a highly directional loopstick or ferrite rode antenna which can be pointed in the direction of the lightning storm.
Step 2: Finding a "Radio Quiet" Location
The above spectrum analyzer screen capture shows the radio noise that would be found in a typical modern dwelling. It shows a range of frequencies from 1 hertz to 500 kilohertz. The vertical scale is logarithmic and the horizontal scale is linear. The vertical scale is in voltage and the horizontal scale is in frequency. The "peaks" show frequencies of higher intensity than the background noise. Note how the amplitude of the background noise decreases with an increase in frequency. If you are located in a city with lots of radio noise from computers, power lines, and what-have-you, it will be easier to find a location out in the country where the detection of lightning storms will be easier.
Step 3: Listening for Lightning
Once you have found a suitable location, it's not necessary for there to be lightning your area. The radio is sensitive enough to detect lightning induced static from hundreds or thousands of miles away. Just put the radio on the lowest frequency of the AM band that doesn't have a station around 550 to 600 kHz and if the radio has the capability for tuning below 550 kHz, tune as low as you can. Even down to 100 kHz if possible. Turn the volume up to a listenable level and turn the radio around in a 360 pattern. Listen for sharp clicks. The direction that the sharp clicks are coming from the loudest will be the general direction the storm is in. The loopstick antenna in the radio is usually placed in the radio lengthwise so that's the direction the strongest signal comes from. You can experiment with this by tuning into a station that you know what direction their transmitter is from you. This is best done in the day and it's best to tune into a station that's not too close so that the radio's circuitry isn't overloaded.
Step 4: Conclusion
With practice, you should be able to get pretty good at determining what direction lightning storms are located and comparing this to weather maps and forecasts from other locations. If you live in a very lightning prone area like Florida you will find much more activity than you would in an area like the Pacific Northwest where lightning storms are, relatively rare.