Atmospheric tides are diurnal and annual variations as well as higher harmonics of atmospheric parameters like pressure, temperature, or winds. They are generated mainly thermally by the regular solar heat input into the system atmosphere-earth's surface (solar tides). The daily variations of the solar tides have a basic period of one solar day. The semidiurnal component reaches maximum amplitudes of its ground pressure of the order of about 1 hPa, a value just above the meteorological noise. The pressure amplitudes of the zonally averaged seasonal waves with the basic period of one tropical year, however, have amplitudes of the order of 20 hPa and are therefore prominent global-scale atmospheric wave structures. The gravitationally generated lunar semidiurnal atmospheric tides have maximum pressure amplitudes on the ground that are about a factor 20 smaller than those of the solar tides. In order to detect such small a signal, it must be filtered out of the meteorological noise by a statistical analysis spanning several decades. Observations, excitation mechanisms, and the theory of atmospheric tides are reviewed in this chapter.
