Our island can see Mist and Fog at any time of the year and there are many different types that we experience as we move through each season.
One thing that all Fogs have in common is that warm air can hold more invisible water vapour than cool air, so when warm air is forced to cool, the invisible water vapour condenses into visible droplets which is Fog.
SEA FOG (advection fog)
This type of fog develops most frequently during the spring and early summer, although it can develop at any time of the year.
It develops when a warm moist airflow moves across a cool body of water (such as the English Channel).
The warm air is rapidly cooled by the water below it, the moisture condenses and produces fog.
Sometimes this type of fog remains very localised, only affecting a small area along the coast, but occasionally it can become widespread and can spread right across the island, especially if there is a moderate or fresh breeze. However if the winds are very light, the fog will only hug the windward coasts, although on rare occasions the fog can combine with inland radiation fog, but this only happens at night or during the first part of the morning.
This type of fog isn’t quite as dramatic as it sounds!
During clear evenings the land cools, this is because the heat of the day is radiated out into space. The air close to the ground cools especially quick and sometimes causes fog to develop. Low lying locations tend to see the densest Mist and Fog, this is simply because cold air will always sink and try to find the lowest point. Higher levels of moisture tend to produce more extensive Radiation Fog and if the air temperature is below 0’c it is called “Freezing Fog” which freeze onto surfaces and objects, creating a white crust of ice that can sometimes look like snow, although it is important to remember that the Fog in the air will remain as water droplets until the air temperature is well below 0’c (sometimes needing to be as low as -20’c).
When air is forced to rise up over a hill due to the prevailing wind, the air begins to cool as it increases in elevation and this cooling means that the invisible water vapour starts to condense into visible droplets (Fog).
This type of fog develops most commonly over Tennyson Down and St Catherine’s Down, although can develop over any undulating landscape where the air is forced to rise up and over a hillside.