AM, the older brother of AM & FM radio, has been a staple in our day-to-day lives since its invention in 1906. Today, it’s typically home to news and talk radio stations. While AM has grown in tandem with society over the past century and a quarter, it hasn't come without its shortcomings. AM’s sound quality lags behind FM, and while technology can usually be to blame, weather can also be a factor. To fully understand Mother Nature’s impacts, let’s first get used to the mechanics of radio waves. 

We can hear AM radio due to the distortion of radio waves coming from a tower. The name “AM” is an acronym meaning “amplitude modification”. Waves are sent out at different frequencies, or the rate at which a wave repeats itself, from various radio stations. The “channel number” of a radio station corresponds to the frequency the waves are being outputted from, so 660 AM would correspond to a wave frequency of 660 kilohertz (kHz).

These radio waves are part of the electromagnetic spectrum, a classification of energy that gets sent out in waves. Visible light, like radio waves, is part of the spectrum, as well. This can lead to some forms of powerful light interfering with radio waves. For example, strong solar storms with high X-ray and UV light output can disrupt AM radio signals and cause interruptions. Another more common interrupter is lightning.

Lightning strikes can emit an incredible amount of energy. A single strike can radiate waves that bounce around the world several times before dissipating, with hundreds of megahertz of energy (MHz, or 1000 times a kHz). The strongest strikes can lead to static bursts, called sferics. These bursts can contain energy in the low kilohertz range. AM bands have a much higher frequency, close to a megahertz, but bursts this large can still easily disrupt the radio’s signal. This can cause the familiar “static” sound to play for a couple of seconds as energy from the waves ripples. As FM’s wave frequency is around 100 times higher than AM, this is not a problem with this type of radio.

While AM radio can be interfered with by lightning, another part of the day-to-day life can enhance its quality. To figure out what it is, we need to look higher up in the atmosphere, towards Earth's ionosphere. This is a layer far above our heads, starting around 40 miles aloft. This layer is known for atoms ionizing, or exchanging electrons, with one another. The main driver for this is energy from the sun, which 

Three layers exist. From top to bottom, the F-layer, E-layer, and D-layer. During the daytime, waves sent up into the air get absorbed by the D-layer, which limits the range that radio waves can be picked up in. Since the ionosphere's existence is due to radiation from the sun striking the atmosphere, it decreases in density overnight. The lowest layer, the D-Layer, basically disappears.

Without the D-Layer to absorb radiation, radio waves can act freely and “bounce” between Earth’s surface and the base of the E-Layer, which does not absorb the radiation. As a result, AM waves can travel thousands of miles before they eventually lose energy. This has a dual effect; while AM stations can now increase coverage overnight, this would lead to massive amounts of overlap in coverage across the country. To combat this, local AM stations either dim or completely shut off power overnight and leave just a few stations on that frequency to broadcast uninterrupted.