Cloud fog, often simply referred to as fog, is measured primarily by visibility. The density of the cloud determines whether it's categorized as mist or fog.
Visibility as the Key Metric
The crucial factor in differentiating mist from fog is how far you can see through it.
- Mist: If you can see beyond 1000 meters (1 kilometer) through the cloud, it's considered mist.
- Fog: If visibility drops below 1000 meters, it's classified as fog. According to the reference, "If you can see more than 1000 meters through the cloud it is mist, but if visibility drops below 1000 meters it's called fog."
Tools for Measuring Visibility
While human observation plays a role, specialized instruments provide more precise measurements:
-
Transmissometers: These devices measure the amount of light transmitted through the air. Lower light transmission indicates poorer visibility and denser fog.
-
Visibility Sensors: These sensors use various technologies (such as forward scatter or backscatter) to determine the scattering of light by particles in the air, which is directly related to visibility.
Practical Implications
Understanding fog measurement is vital for:
- Transportation: Airports, harbors, and roadways rely on visibility data to ensure safe operations.
- Weather Forecasting: Fog prediction models use visibility as a key input parameter.
- Public Safety: Fog conditions can impact outdoor activities and emergency response.
