As we head into the cooler months fog will become a more common sight, but what exactly is it? 

In its simplest definition, fog is simply a cloud at ground level. More specifically it is microscopic water droplets suspended in the air near to the surface of the earth, reducing visibility to less than 1000m. 

There are several different types of fog, with different mechanisms of formation, with radiation fog being one of the most common. Radiation fog is caused by radiational cooling overnight at and near the surface of the earth and requires a few key elements for its formation.

Before we take a look at these key elements, we should first take a closer look at radiational cooling. Simply put, it is the cooling of the earth's surface, and near surface air, due to the loss of longwave radiation. At night more longwave radiation escapes to space than incoming shortwave radiation to the surface, resulting in a net cooling effect.

The first of the key elements mentioned above is high relative humidity, providing the moisture needed to form fog droplets. The second and third elements are calm-to-light winds and rapid cooling. Look out for surface high pressure systems as these generally bring light winds, in addition to enhancing radiational cooling at the surface – ticking two of the key elements.  

Clear skies are also a key factor in the formation of radiation fog. Cloud acts like a blanket, preventing long wave radiation from escaping to space and leading to a warmer surface. Warmer air can hold more moisture than cooler air and as such condensation is more unlikely to take place. 

So how exactly does radiation fog form? 

As daytime heating ceases, the earth's surface and the layer of air just above it begin to cool via a process called conduction. If there is enough water vapour in the air, and sufficient cooling at the surface, this low-level air will reach saturation.  

It should be noted here that air is an extremely poor conductor of heat and as such only a very shallow layer near the ground is cooled. This is where light winds come into play. A slight breeze will help mix the air a little, with cooler air then being distributed through a deeper layer. 

As cooling continues, near-surface air will condense onto objects as water droplets (known as dew). If the winds are too light, this is where the process ends, with only a few centimetres of the near-surface air cooled sufficiently. Conversely, if winds are too strong, warmer and drier air is mixed down, effectively stopping any condensation taking place. 

If cooling continues however, with just the right amount of wind, excess water vapour in the saturated layer just above the surface condenses, forming fog droplets. As radiational cooling continues, the fog layer deepens and can reach a couple of metres deep. 

Radiation fog is usually relatively short lived once the sun rises. As the sun slowly warms the ground throughout the morning, it warms the surface just above it, causing the water vapour (fog) to evaporate from the ground up. Whilst it is common to hear the phrase ‘burning off’ when referring to fog disappearing, in actual fact, it is more dissipating.