With fog causing transport disruption and delays across the UK this week, there is a lot of interest in what causes this weather phenomenon.
So what is fog? The definition of fog is water droplets suspended in air – much the same as a cloud, but obviously at a lower level. The water droplets must be thick enough to reduce visibility to 1,000 metres or less. If visibility is higher than that, then it’s officially mist (there’s no upper limit for visibility for mist).
Fog forms when relatively moist and mild air close to the ground cools quickly, causing the moisture in the air to condense (at which point it becomes visible to the human eye). This normally happens in autumn and winter under clear skies, which allows heat from the ground to escape quickly to cause rapid temperature drops. Winds need to be light too, otherwise the fog will be dispersed.
Fog can also be formed in other ways, such as when warm, moist air moves over a cold ocean – something that’s more common in the UK spring when air temperatures increase but the ocean is still cold. It’s exactly the same process, but the cooling is effected in a different way.
The thickness of fog (or mist) will be determined by the amount of moisture (humidity) of the air and the amount of time the air cools for – the more time, the thicker the fog. Obviously fog can get very thick, and anything less than 100 metres would be considered thick fog.