Fog

Fog is the suspension of fine droplets of water in the air, as water vapour. Fog can be considered a type of low-laying cloud. Its moisture is heavely influenced by local factors (e.g.) nearby bodies of water, topography and climate.

Fog forms when the difference between the air and dew point is less than 2.5°C. (Dew point is the temperature to which air is cooled to become water vapour).

It begins to form when water vapour condensates into tiny water droplets that are suspended in the air. This can occur in many forms: Precipitation, wind convergence, stratification of the air, warming of bodies of water, transpiration from plants, wet lands and movement of cool or dry air over bodies of water.

Fog normally happens at a relative humidity (ratio of how much water vapour is in the air and how much water vapour the air could potentially contain at a given temperature) near 100%.

At 100% relative humidity, the air cannot hold moisture, becoming supersaturated. The higher the water vapour, the less of oxygen molecules are present, which gives a sensation of breathlessness when walking through fog.

Fog can also form at lower humidity's.