The development of fronts and frontal wave forms. Frontogenesis occurs in well-defined areas; for example, off the east coast of North America (see Bell and Bossart, (1989) QJ Royal Met. Soc. 115, 498). Fronts are less common in the tropics, where contrasts between air masses are less marked, but see Chen et al. (2007) Monthly Weather Rev. 136, 1.