Fog Is Just a Cloud at Ground Level

Have you ever wondered what it would be like to walk through a cloud? On a foggy morning, that's precisely what you are doing. The transformation from clear air to a thick, ghostly blanket begins when the air cools to its dew point—the temperature at which it becomes fully saturated and can no longer hold all of its water vapor. This excess moisture must go somewhere, so it condenses from an invisible gas into billions of microscopic liquid water droplets. For this to happen, the droplets need a surface to form on, latching onto tiny, invisible particles in the air like dust, pollen, or sea salt.

This cooling process can occur in several ways, creating different types of fog. Radiation fog, for instance, often forms on clear, calm nights as the ground rapidly loses heat and chills the layer of air directly above it. Advection fog, famous for shrouding San Francisco's Golden Gate Bridge, happens when warm, moist air drifts over a cooler surface, like the cold ocean current. Historically, the disorienting nature of dense fog was a major hazard for navigation. This very real danger spurred the invention of tools like the foghorn, whose powerful, low-frequency blast was designed to cut through the opaque air and warn mariners of unseen coastlines and other vessels.