Which meteorological phenomenon is characterized by a rapidly descending cold air mass?

The phenomenon characterized by a rapidly descending cold air mass is a microburst. A microburst is a type of intense downdraft that occurs during thunderstorms, where cold air descends suddenly and spreads out upon reaching the ground. This strong and localized downdraft can result in wind gusts that can exceed 100 mph and are particularly hazardous for aircraft during takeoff and landing phases. The rapid descent of the cold air creates a significant difference in pressure and can lead to violent changes in wind direction and speed, making it vital for pilots to recognize and avoid areas where these phenomena are likely to occur.

Other phenomena mentioned do not involve the same rapid descending cold air mass. A heatwave reflects extended periods of excessively high temperatures but does not relate to cold air masses. Frontal passages refer to the movement of weather fronts, where air masses collide and can lead to various weather conditions, but they do not involve the intense downward movement characteristic of microbursts. Cold fronts, while they do represent a boundary where cold air replaces warmer air, do not necessarily exhibit the extremely rapid and localized downdraft that defines a microburst.