Which of these is not a form of hydrometeor?

Soil moisture is not classified as a hydrometeor because hydrometeors specifically refer to forms of water or ice that are suspended in the atmosphere. This includes precipitation (like rain and snow), as well as clouds and fog, which consist of tiny water droplets or ice crystals that form in the air.

Rain is a direct form of precipitation, while clouds are aggregated hydrometeors that can lead to precipitation. Fog is a type of stratus cloud that forms near the ground due to condensation. Soil moisture, on the other hand, refers to the water that is held in the soil and is part of the ground water system, rather than being suspended in the atmosphere. Therefore, it does not fit the definition of a hydrometeor.