(originally posted to sci.astro, Jan 21, 2003, by David Knisely)
Someone asked why some AM radio stations fade out at night. I tried to answer the question, but didn't do very well... Fortunately, David Knisely was able to provide the correct answer:
By "AM radio", we will assume for the moment that the original poster means the signals from 540 kHz to 1700 kHz, the so called "AM Band", which is more properly known as "medium wave" (as opposed to the so-called "FM Band" which runs from 88 MHz to 108 MHz in North America). Skywave signals (those heading upwards rather than along the Earth's surface) at night in that frequency range can often be bent back down towards the Earth's surface by the F-layer of the Ionosphere, thus enabling some signals to propagate for longer distances than they would via just the ground wave or line-of-sight paths. During daylight hours, the Sun ionizes and creates the D-layer low in the Ionosphere, a layer which usually *absorbs* radio frequencies much below 10 Mhz to varying degrees. Those in the 540-1700 kHz "AM Band" heading upwards (skywave signals) are almost completely absorbed by the D-layer, and thus daylight propagation is merely local or "ground wave" (usually less than 200 miles or so). At night, the D-layer vanishes due to lack of sunlight, but the F-layer remains, allowing long distance signals to be heard, sometimes for over 1,000 miles from their transmitters. Local stations (less than 10 to 15 miles away) are sometimes overwhelmed by the multiple skywave signals at night from the large number of stations on the same frequency, vanishing into a mass of jumbled signals and noise after sunset, unless these locals are either very close to the listener (within 10 miles) or are very high power stations (ie: 10 kW or greater).
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