Baroclinic leaf

an elongated cloud pattern formed within the jet stream zone associated with marked baroclinicity (i.e. strong thermal contrast). The boundary (in satellite imagery) on the polar air-mass side of the development is well defined, and has the look of a 'stretched-out' "S" shape. The downstream / warm air-mass edge is less distinct. This feature represents the initial (or frontogenetic) stage of a system development, certainly in the mid-troposphere, and often (but not always) at the surface. Not all baroclinic leaves lead to marked cyclogenesis although they will be the first stage of such. (See also Dry Intrusion).