An atmospheric tide due to the gravitational attraction of the moon. The only detectable components are the 12-lunar-hour or semidiurnal, as in the oceanic tides, and two others of very nearly the same period. The amplitude of this atmospheric tide is so small, about 0. 06 mb in the Tropics and 0. 02 mb in middle latitudes, that it is detected only by careful statistical analysis of a long record. See tide.