How does fog differ from a cloud?

• A. Fog is a cloud at high altitude.
• B. Fog is a cloud that has no moisture.
• C. Fog is a cloud based at the surface.
• D. Fog is always thicker than a cloud.

Answer: (C)

Fog is a cloud that forms at the surface. When the air right at the ground becomes saturated—often by cooling to the dew point or adding moisture—tiny water droplets become suspended at ground level, creating the visible haze that reduces visibility. Clouds, by contrast, form higher up in the atmosphere where rising air cools and condenses into droplets, so their bases are above the surface. That location difference—fog being at the ground while clouds are aloft—is what sets fog apart. The other statements don’t fit because fog does contain moisture, isn’t defined by being at high altitude, and its “thickness” isn’t a defining characteristic.