Basically non-frontal depression, comprising cold air surrounded by warmer air at a higher pressure. It is typically associated with circulation in cold air masses in the mid-troposphere over north-eastern USA and north-eastern Siberia, though sometimes occurring also over the oceans in air that has emerged from the arctic, and it is often persistent. Such depressions are marked by more or less concentric isotherms around the core of the low. Such lows may originate along the arctic coast as a result of strong vertical uplift and adiabatic cooling in occlusions; they do not necessarily influence surface weather conditions, but when they occur over warmer surfaces in middle latitudes, convection develops strongly. See also cut-off low.