An atmospheric tide due to the thermal or gravitational action of the sun. Six- and eight-hour components of small amplitude have been observed. They are primarily thermal in origin. The 12-hour component has by many times the greatest amplitude of any atmospheric tidal component, about 1. 5 mb at the equator and 0. 5 mb in mid-latitudes. This relatively large amplitude is often explained as a resonance effect. The 24-hour component is a thermal tide with great local variability.