Fog is essentially a cloud touching the ground. It consists of tiny water droplets—or minute ice particles in very cold conditions—that reduce horizontal visibility.
Under the WMO definition, fog is present when surface visibility falls below 1,000 metres. Above that threshold, mist or haze may be used depending on moisture and suspended particles.
The basic process
Air reaches saturation when it cools to its dew point or gains enough moisture. Water vapour then condenses onto microscopic particles, producing fog droplets.
Light wind helps mix a shallow air layer. Complete calm may confine the phenomenon very close to the surface, while strong wind usually disperses fog or lifts it into low cloud.
Common types in Greece
Radiation fog often develops in valleys and plains after a clear, calm night. Advection fog forms when moist air crosses a colder surface, while upslope fog forms as air is forced over rising terrain.
Near the sea or a lake, evaporation fog may appear when cold air passes over relatively warm water.
Why forecasting is difficult
Small differences in cloud cover, soil moisture and wind can change the outcome completely. A grid model may indicate good visibility while a sheltered valley only a few kilometres away has dense fog.
Drivers need direct observation, lower speed and official guidance; forecast visibility is only an indication.
These explanations and calculations are educational. For safety, occupational, health, marine or aviation decisions, follow the appropriate official services.