|MadSci Network: Environment & Ecology|
The two weather phenomena that have the most noticeable, yet very different, effects on AM radio transmissions are lightning and temperature inversions. Lightning causes interference in the AM signal, whereas temperature inversions help the AM signal travel greater distances. If you've ever listened to an AM radio with thunderstorms close by, you've no doubt heard the cracking and popping of lightning interference. On the other hand, if you've ever listened to your AM radio at night, you may have noticed that you can pick up radio stations from other states, and sometimes other countries! This is the effect caused by a temperature inversion (when the earth's surface cools down such that the temperature increases with height jus t above the ground). It helps to know that typically during the day, the temperature decreases with height in this layer. When an inversion is present, the airmass close to the ground is cooler and therefore more dense, than the air just above the inversi on. As a radio wave travels through the 'more dense' air and encounters the 'less dense' air just above the inversion, the wave is refracted, or bent, back down towards the ground. It then bounces back and forth between the ground and the top of th e inversion, staying closer to the earth's curved surface than it otherwise would, and therefore it can be received at greater distances from the antenna.
Listening to AM radio would be a more useful way to detect lightning than to predict local weather. You may consider modifying the scope of your project to include this aspect. Another suggestion would be to listen to your AM radio at night, scanning the entire AM band, and document the locations of the many different stations you pick up. You could try to correlate the distance of the stations that you hear with the strength of the inversion on the night you heard that station. A good way to determine th e strength of the inversion is by using an atmospheric sounding from a nearby National Weather Service station that launches weather balloons.
If any of this interests you, and you'd like help in obtaining atmospheric soundings, just drop me an email message! My email address is firstname.lastname@example.org I'm currently moving to a new office, so my email may not be working again until September 30 or so.
Thanks for a great question! Good luck!
Carl Morgan, Meteorologist
NWS, Wilmington, NC
Carl Morgan later provided the following further inforamtion:
I would like to add additional information contributed by a longtime HAM radio operator. This information pertains to the "skip" of radio waves. My previous answer attributed this phenomenon to low-level temperature inversions, however the ionosphere, a layer of charged atoms roughly 60- 1000 km above the earth's surface, is the primary cause of "skip".
According to Steve Sarasohn, a HAM radio operator for 40+ years... "The low altitude and highly absorbing D layer prevents medium frequency radio signals from penetrating and no skip is possible. Radio propagation is limited to ground wave. At night, the D layer disappears and the signals reflect off the F2 layer and create skip. I've noticed the change in propagation, from skip to ground wave, at sunrise or a few minutes after, when the air temperature has not yet changed. Also, reflecting the signals off of a higher layer would seem to yield longer skip than reflecting them off of a lower layer."
I hope this clears things up!
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