Looking over a misty valley from height

Weather Photographer of the Year 2021: Setting the scene - mist and fog

Continuing our series of posts looking at some of the weather themes from the past five years of Weather Photographer of the Year, regular guest writer Frank Barrow takes a look at mist and fog.

The difference between mist and fog

Mist and fog are common phenomena in the UK. They are both formed in the same way. The difference is set by an essentially arbitrary measure of the visibility within the mist or fog. If visibility is less than 10km it can be called mist, if less than 1km it is called fog. These figures were chosen with safety at sea and in the air in mind. Nowadays the importance of road transport means different limits are used in public service forecasts. The definition of mist remains the same, but fog requires visibility to be 200m or less.

This image shows mist rather than fog.

Thinking about images like this may be the inspiration for John Keats’ famous line in his poem 'To Autumn'.

‘Days of mist and mellow fruitfulness.’



So fog and mist are phenomena that lead to a decrease in visibility.

What do we mean by visibility?

In meteorology, we define visibility as 'the greatest distance at which an object can be seen and recognised in daylight.' The definition of visibility at night is probably the least useful in all meteorology, and a favourite of mine. The definition of visibility at night is 'the greatest distance at which an object can be seen and recognised if it was in daylight.' Honestly!

How do they form?

Mist and fog form due to condensation onto microscopic particles known as condensation nuclei. Condensation nuclei are all around us. We breathe them in and out without noticing, but without them, it is difficult if not impossible for condensation to occur.

In the UK it is very rare to see fog made of ice particles, our temperatures do not get low enough. When temperatures are below 0°C, the droplets become what is known as ‘supercooled’, below 0°C but still liquid. Supercooling is another of the myriad of strange properties of water. When supercooled droplets strike a surface they instantly freeze and build up as 'hoar frost' This is what we see on a ‘frosty morning’

In this image, hoar frost has been and is being deposited on the vegetation in the foreground. The fog clearly has no great depth as the rising sun can already be seen.


Incidentally, the observed visibility in this direction would be the distance from the observer to the windmill.


The process of fog and mist formation is useful for identifying areas where mist and fog are most likely to occur. Two things are needed, moisture and some way of cooling the air until condensation occurs.

Sources of moisture are available in many locations. The limiting factor is usually cooling. The easiest way of cooling air in the atmosphere is to make it rise. Rising air forms cloud. However, except for air forced to rise over hills, this will not necessarily reduce surface visibility. Fog and mist are produced by cooling of the ground that spreads to no more than a few hundred feet above ground level. Surface cooling is due to the escape of longwave radiation to space.

Optimum conditions for the formation of ‘radiation’ fog:

Clear sky or just thin high cloud - Allows radiation to escape to space

Moist ground - Sources of moisture

Moist air in the lowest 100m or so - Sources of moisture

Light surface winds - A small amount of turbulence is useful

Favourable local topography - Cold air draining into hollows and valleys concentrates the surface cooling


The two images below illustrate these conditions,