Long?range, short?wave radio transmissions depend on conditions in the ionosphere, an electrified region of the atmosphere extending from approximately 45 to 250 mi above the earth's surface. The ionosphere contains several semi?permanent ionized layers which reflect most radio waves having wavelengths between 10 m and 10 km. Multiple reflection of a radio signal between ionospheric layers and the earth makes longdistance short?wave transmission possible. The ionospheric layers are extremely variable with time and space and are extremely sensitive to the mood of the sun. The waxing and waning of splar activity during the eleven?year solar cycle is accompanied by changes in the reflectivity and other characteristics of the ionospheric layers. These changes may cause absorption of signals or propagation in unexpected directions or to unusual distances. They can be studied by analysis of a large number of radio contacts between distant points.