Zonal wavenumber
In meteorological applications, a zonal wavenumber or hemispheric wavenumber is the dimensionless number of wavelengths fitting within a full circle around the globe at a given latitude: k = 2 π r cos φ λ , {\displaystyle k={\frac {2\pi r\cos \varphi }{\lambda }},} where λ is the wavelength, r = 6378 km is the Earth's radius, and φ {\displaystyle \varphi } is the latitude. Zonal wavenumbers are typically counted on the upper level (say 500-millibar) geopotential maps by identifying troughs and ridges of the waves.