What is a common visual sign that indicates an approaching cold front?

A common visual sign of an approaching cold front includes the presence of cumulus clouds. As a cold front moves into an area, the leading edge of the cold air forces warmer air to rise rapidly. This rising process leads to the formation of clouds, typically cumulus clouds, which often develop into cumulonimbus clouds if the atmospheric conditions are favorable for continued vertical development.

Cumulus clouds indicate that the air is unstable, which is a characteristic often associated with the lifting action provided by a cold front. When the moisture in the rising air cools and condenses, it forms these cloud types, which can develop into thunderstorms and precipitation, indicating the impacts of the cold front.

Clear blue skies do not represent an approaching cold front, as stable and warm conditions are usually present before the front arrives. Low-lying fog is typically associated with stable air and may be seen in the warm air ahead of the front, not indicative of the front itself. Heavy fog is likewise a signal of moisture present in stable conditions rather than the turbulent weather associated with a cold front. Thus, the presence of cumulus clouds serves as a strong visual cue for pilots and meteorologists that a cold front is on its way.