Which characteristic defines a cold front?

A cold front is defined by the advance of colder air pushing into warmer air. This characteristic is significant because it results in a variety of weather phenomena. As the colder air mass moves in, it forces the warmer, lighter air to rise. This rising air can lead to the formation of clouds and precipitation, often producing severe weather conditions such as thunderstorms and a noticeable drop in temperature.

In contrast, the advance of warmer air into colder air typically describes a warm front, which has different weather implications. High humidity levels can be present in various atmospheric conditions and are not uniquely associated with cold fronts. The reduction of atmospheric pressure can occur with various weather phenomena and is not exclusive to cold fronts; in fact, pressure changes occur throughout different types of weather systems, but they are not definitive characteristics that define a cold front. Thus, the defining characteristic of a cold front is indeed the movement of colder air into warmer air, leading to characteristic weather changes.