Clear reception of radio signals depends upon the ability to exclude unwanted noise. Noise from natural sources takes two forms, atmospheric and extraterrestrial. Atmospherics result from the electromagnetic disturbances caused by lightning in thunderstorms and are at a maximum at night and in summer and in the tropical latitudes. Extraterrestrial noise interferes with radio communication in the VHF band (and down to 20 mc). It is due to electromagnetic phenomena of galactic or solar origin. Solar noise increases during the daytime and during peak periods in the 11-year sunspot cycle. Solar flares and geomagnetic storms may produce radio communication blackouts, particularly in the polar regions.