What is the main impact of a "cold front"?

The main impact of a cold front is characterized by severe weather conditions and a rapid drop in temperature. When a cold front moves into an area, it replaces the warmer air mass with much cooler air. This shifting air results in drastic changes in weather conditions, often leading to storms, heavy precipitation, and strong winds. The temperature can drop significantly, sometimes even within minutes, as the cooler air mass takes over.

Cold fronts typically lead to the formation of cumulonimbus clouds, which are associated with thunderstorms and severe weather events. The cold air pushes the warm, moist air upwards, causing condensation and precipitation along the frontal boundary, contributing to intense weather phenomena.

The other options do not accurately represent the behavior of cold fronts. For instance, a gradual rise in temperatures is more associated with a warm front, while increased humidity leading to fog is typical of stable, warm conditions rather than the instability brought by a cold front. Extended periods of calm weather are more indicative of high-pressure systems rather than the turbulence brought about by a cold front. Thus, severe weather and rapid drops in temperature are the hallmark effects of a cold front.