Definitions for "Long wave"
A large wave in the polar jet stream and the westerlies that extends from the middle to the upper troposphere. Often associated with the formation of a mid-latitude cyclone at the ground surface. Contrasts with short waves. Also called Rossby waves.
a radio wave with a wavelength longer than a kilometer (a frequency below 300 kiloHertz)
With regard to atmospheric circulation, a wave in the major belt of the westerlies, which is characterized by large length and significant amplitude.