AM radio waves range from 535 to 1735 kHz (kilohertz). The distance these waves travel depend on the weather conditions, transmitter wattage, conductivity of the soil, and ionosphere refraction.
FM Radio waves range from 88 to 108 MHz (megahertz). These waves that are sent out go in a straight line and don't get refracted in the ionosphere. The keep traveling into space once the travel so far.
Why did FM radio come to exist?
Armstrong knew the limitations of AM radio and he wanted to develop a new type of radio that could have better quality, no static interference from anything, and no nighttime interference from ionosphere refraction.
What stopped it from developing at first?
Armstrong pitched his idea to David Sarnof, who was the head of RCA, an AM radio empire. They bought patents and helped Armstrong build a FM radio, but they then shut down the project to protect their AM radio empire. Once they realized that the FM radio industry was making money, they invested again.