Which atmospheric phenomenon primarily indicates high moisture levels?

Fog is a phenomenon that primarily indicates high moisture levels in the atmosphere. It is formed when water vapor condenses into tiny water droplets suspended in the air at or near the ground level, creating a cloud that reduces visibility. This process occurs when the air temperature meets the dew point, allowing the moisture in the air to reach saturation.

While clouds also indicate moisture in the atmosphere, they are typically found at higher altitudes and represent larger volumes of water vapor in various states, including liquid or ice. Thus, although clouds signify moisture, fog is a more localized indicator of high moisture levels as it directly relates to ground-level humidity.

On the other hand, smog is a type of air pollution that combines fog with smoke or other pollutants, which does not directly indicate moisture levels. Snow, while also a form of precipitation that indicates cold temperatures and humidity, occurs under specific temperature conditions and does not represent moisture levels in the same way as fog does.