[paulddraper]

> (think mountain pass conditions) are broadcast over AM radio

This isn't the scientific theory, but I drive the mountains a lot and AM consistently dies much sooner than FM.

Can't explain it, but 100% true.

EDIT: I live in Salt Like City area and drive through the Wasatch Range on a near weekly basis. # of receivable AM stations goes to zero nearly immediately.

[tcmart14]

AM can be untrustworthy in mountains too. Many factors play in such as how the landscape is actually laid out, the make-up of the rock in the mountain. How signals bounce off the rock, etc. It for sure isn't fool proof. I wouldn't think that much better than AM unless your close to the tower. AM's big advantage is being able to travel farther on less power, but the low frequencies don't have good penetration power. This is why the Navy uses radios that runs on 900+ MHz for shipboard radio, 900 MHz (UHF range) has less distance, but better penetrating power through steel decks. Compared to frequencies that AM operates (HF range). Then there are atmospheric conditions and the cycle of the sun. At a solar maximum sure, but at a solar minimum, probably not.

[hguant]

Generally AM is used in mountainous areas because it's MUCH cheaper than FM, and those areas don't have the population density to make FM stations viable.

It's also used for natural parks, and anywhere you might need updated, local, information, again, because it's so cheap and because receiving is so easy.

[cf100clunk]

Day time vs night time? In the evening while camping in Canada's north in flat or mountainous terrain I've sat beside a fire with a portable radio and tuned to certain high power U.S. stations on the AM radio band. The locals will tell you it is not uncommon. As for reception in a moving vehicle in the daytime, any number of fields or other conditions may be causing the dropouts but in general AM radio propagates vastly farther than FM for a given Effective Radiated Power range specified in the station's license.