You know those early morning dog walks, the ones with a mist in the air, or is it fog? The look of the landscape can be transformed by a layer of mist and fog sometimes creating a scene of beauty, especially when the sunlight is filtering through.
But what are the differences between these weather phenomena and what causes them? Why is haze completely different to fog and mist and the origin of some popular sayings and phrases.
Let us take a closer look.
Mist and Fog
Basic mist and fog are the same thing: water droplets suspended in the air. They are basically terms for clouds at ground or low level.
The name for it changes depending on how far you can see through it, based on how dense the water droplets are.
Officially, for aviation purposes, if you can see more than 1000 metres (3280 feet) ahead then it is mist. If you can see less than 1000 metres ahead then it is called fog.
That is the official and technical answer, but what about our weather forecasts that call it all mist more often than not? That’s because the met office doesn’t call it fog on a weather forecast unless visibility is reduced to less than 180 metres (590 feet).
If the visibility drops below 50 metres then it is called a ‘dense fog’.
Because mist is less dense, that is why it clears quicker with a breeze or as the sun rises. A fog is more dense so will linger for longer.
Types Of Fog
The many causes of fog are what give the types of fog their name. Not all these clouds that touch the ground are created in the same way. Here are a few examples.
Radiation Fog
Those cold winter days where you have heard that it will be a clear blue sky, and yet you wake up early to a fog. This is more than likely a morning radiation fog.
What generally happens is that during a clear cold evening and night, the ground is cooled by infrared thermal radiation. This then conducts to make the air above the ground very cool and creates a thick dew because the air cannot hold it. A mist or fog gets created by condensation.
You will find that when the sun rises, and as the ground starts to warm, the fog or mist clears rather quickly.
Advection Fog and Coastal Fog
In the other warmer parts of the year we have advection fog which is formed when warm moist air passes over a cool surface with the wind.
In the UK the best example of advection fog is along the coast, especially the east coast. The warm moist air comes in from the North Sea, passes over the cold water and cools until it cannot hold the vapour anymore forming droplets, thus fog, or what we know as a coastal fog. When it meets the warm land it starts to dissipate.
An extreme form of advection fog can be seen on the coast of California in the warmer months. You must have seen the ubiquitous photos of the Golden Gate Bridge in San Francisco shrouded in fog.
Cloud Inversion or Valley Fog
There is nothing quite like waking up on a mountain top after a night wild camping, to see the valleys below you filled with cloud whilst you are above it all in the clear skies. This is commonly called a cloud inversion.
As we know, the higher you go, for example when climbing a mountain, the colder it gets. The colder air is usually higher in the atmosphere.
On a cold windless night and during a period of high pressure the cold moist air forms a cold dew at ground level.
Because of the high pressure there is a layer of warm dry air that falls to a low altitude and hovers above the cold moisture filled valleys, effectively trapping it in.
This anomaly means that the norms are reversed creating an ‘inversion’ where at ground level it is cold and as you climb the mountain you enter warmer air.
A cloud inversion is one of my favourite sights. Always worth getting up early or camping out up high to see them.
Haze
We all know hazy days. We go out and walk on what we think is a warm sunny day. We climb somewhere were we think we will be able to see for miles and miles, only to find there is haze in the air that reduces that distant view somewhat.
Haze is also a hinderance to photographers as the detail of landscape photos get lost behind a sheen of white in the sky.
It is quite a different thing to mist and fog and, alas, haze is caused by pollution, mainly man-made but it can also be naturally forming. It is caused by really tiny, dry, pollutant particles that are too small to see individually with our eyes. But as a whole mass what we see is a sheen of haze.
Man made pollutants that cause haze include that from car exhaust, factories, wildfires and so on. Natural particles can be caused by sand and dust being blown in the air or the after effects of a volcano eruption. This is why desert areas can have a thick haze when sand gets stuck up in the atmosphere.
Red Sky at Night and Morning
This colourful effect is caused by haze and its effect on the light spectrum.
When the spectrum of light from the low sun hits the haze the shortwave blue light gets scattered. The longer wave red light penetrates the haze much more easily and thus a hazy clear day gives way to those gloriously red sunsets and deep red sunrises.
The reason for the common saying, “Red sky at night, shepherds’ delight; red sky in the morning, shepherds take warning,” is due to the probability of what these phenomena actually mean.
We, in the UK, get most of our weather fronts coming in off the Atlantic in the west, and people in the past have used the red sky to guess the weather for the next day, predicting high pressure. Also remember that the sun sets in the west and rises in the east.
So if the sky is red at night in the west, then that could be a sign that high pressure with trapped particles is incoming and may lead to nice calm weather the next day.
If the sky is red in the morning, then the high pressure has passed and is over to the east, meaning a low front could be incoming with rain.
Smog
Looking at the word smog you would assume that it means a combination of smoke and fog. We often hear it combined with mention of the big industrial times and the cities consumed by a thick smog.
Smog is not a fog, actually, but a very thick haze. When we burnt a lot of coal for industry and had dirtier cars, you can imagine how many polluting particles were being produced in the atmosphere. When combined with high pressure you get a huge thick smog that can linger for a very long time.
A Pea Souper
Yes, that old phrase means a thick fog, haze or smog.
In industrial times the smog had elements of sulphur from burning coal. This caused many deaths to the vulnerable as well as respiratory problems. London had it bad in Victorian times and the smog became known as ‘London Particular’. Dickens used the phrase himself in Bleak House to describe such fog.
London Particular then became the nickname for a very thick split pea and ham soup that you could stand a spoon in.
So now the names have been transposed: the name of the fog gave rise to the name of a soup. But the soup gave rise to the name of a fog! A bit confusing, isn’t it?
Further Reading: Lake, Mere, Tarn and Water – The Difference
