What occurs when humidity is high and temperatures drop rapidly?

When humidity is high and temperatures drop rapidly, fog develops as the correct phenomenon. This occurs because high humidity means that the air is saturated with moisture. When the temperature decreases, the capacity of the air to hold moisture is reduced, which can lead to condensation.

As the air cools, it can reach its dew point—the temperature at which air becomes saturated with moisture—resulting in tiny water droplets forming in the air. This aggregation of moisture appears as fog. Fog is essentially a cloud that forms at or near the ground, and it reduces visibility due to the water droplets suspended in the air.

Other scenarios are less relevant in this context. For example, while snow can form under certain conditions of humidity and cold temperatures, it typically requires specific criteria that involve a continuous process of cooling rather than the immediate drop described in the question. Dry air prevailing is contrary to the premise of high humidity. Heat waves are associated with prolonged high temperatures and not relevant to the conditions of cooling described.