The ethereal sight that greets you when you open the curtains early in the morning, the ground seemingly covered in cloud can be quite breathtaking. It’s as if you’ve been elevated to a higher plane and you’re now looking down at the clouds from a mountain top. Yet early morning fog is a common sight that most of us have seen at some point during the year – so what causes fog in the morning ?
During the hours of darkness the ground will be cool and this affects the air close to the ground, cooling it down. At the same time moisture will collect on the ground – known as dew.
When the sun begins to rise, the ground begins to get warmer and consequently the air near to the ground starts to warm up too. This makes the air close to the surface less dense and unstable and it begins to rise carrying the water vapour with it. As this warm air meets the cooler stable air air the water vapour cools to form tiny water droplets, clinging to tiny particles in the air. This is known as the point of temperature inversion. This is fog and it’s formed by the same process that makes clouds in sky far above. So we are actually looking down at the clouds.
As the sun gets higher in the sky it warms the lower air more and more, which rises up further and further. The point of temperature inversion also rises and so does the fog, eventually disappearing.
In cities full of air pollution, this process is carried out the same way. However dust particles, toxins and gases are trapped in the lower layers of air rather than moisture. This is called smog.
You may notice that when fog appears, it’s usually relatively calm conditions. You won’t see the same kind of fog on a windy day. This is because the wind will mix the cool and warm layers of air together. When the cool and warm layers mix, fog doesn’t get a chance to develop.
We often see another kind of fog on lakes, rivers and the sea. We call this evaporation fog. This happens when cool air moves over much warmer water leading to condensation and convection. Sometimes this looks like steam. We call this steam fog or sea smoke. You sometimes see this lying above a heated swimming pool and hot tubs.
On a cold morning it can seem that you’re breathing fog. This happens because when we breathe out we’re exhaling water vapour (along with CO2 and other gases). When the water vapour leaves your mouth it loses energy and turns into liquid forming a little fog.
What’s the difference between fog and mist ?
Fog and Mist are really the same thing, but the density of each is different. Mist is a less dense version of fog. With fog you won’t be able to see further than 1km. With mist you’d be able to see between 1 and 2km.