___ is a visible aggregate of minute water droplets at the Earth's surface that reduce horizontal visibility to less than 5/8 SM.

Fog is defined as a visible aggregation of tiny water droplets that forms at the Earth’s surface, and it significantly reduces horizontal visibility to less than 5/8 statute mile (SM). This phenomenon occurs when the air temperature drops to the dew point, causing water vapor to condense into small droplets suspended in the air.

Understanding the characteristics of fog is essential for pilots and air traffic controllers, since it can greatly affect flight operations and visibility conditions. In contrast, while clouds are also comprised of water droplets, they exist at higher altitudes and do not necessarily impede surface visibility in the same way. Mist, while similar to fog, typically allows for greater visibility, exceeding the 5/8 SM threshold. Smog, on the other hand, is a combination of smoke and fog, often associated with pollution, but it does not specifically refer to reduced visibility due to water droplets alone. Therefore, fog is the precise term that describes the condition outlined in the question.