Fog is a complex meteorological phenomenon that occurs under particular conditions of temperature and saturation of the water vapour suspended in the air.
You get fog whenever the water present in the air in the form of water vapour condenses. Condensation is the phenomenon by which the rarefied water molecules suspended in the air join together, forming tiny droplets of water.
In particular this change from gas to liquid (condensation) happens when the temperature reaches what is normally called the “dew point”.
Depending on the physical phenomenon that generates it, we know various types of fog:
- Transported fog;
- Advection fog;
- Evaporation fog;
- Upslope fog;
- Precipitation fog;
- Freezing fog;
- Arctic fog;
Let's look at each of these phenomena one by one.
Transported fog is the kind of fog that already exists and that, having formed in favourable areas, is carried to the spot by the wind.
Advection fog occurs when a warm air mass saturated with moisture reaches colder ground or large colder stretches of water. A typical example is the fog that forms over the sea when masses of tropical air move towards colder latitudes, making the water they contain condense.
Radiation fog, which occurs because of thermal inversion, at night or at dawn, when the ground, through radiation (loss of heat by infrared rays), gives off large amounts of heat and becomes particularly cold. The warm air brushing against the ground, cooled in this way, cools down too, condensing the water suspended in it.
Sometimes it can happen that ground covered in frost or dew manages to warm up and evaporate the dew, which, as it rises, comes into contact with a layer of freezing, still air above, so it condenses back into fog. This is the classic bank of evaporation fog, just a few metres high, with the tops of the trees poking out above it.
Upslope fog is the one that catches the mountaineer by surprise, and it occurs when a mass of stable, moist air is forced to climb a slope, meeting ever lower pressure (because of the increasing altitude). As the pressure drops, this air mass expands and its temperature falls. As it cools, the water reaches dew point and starts to form microscopic droplets.
The main feature of fog is that it creates a particular phenomenon of uniform diffusion of light through refraction inside the tiny droplets. It is precisely this uniform refraction of light that causes the poor visibility of the surrounding landscape: the trees and the outline of the mountains look blurred and distances are flattened.
To predict the formation of fog you need to be an excellent connoisseur of the local microclimate, and also of meteorological phenomena, and to know how to read weather charts correctly.
For the mountaineer fog can mean big problems. You are no longer able to find clear geographical reference points on the ground (the landscape is completely unreadable), you can no longer judge distances, how far you have walked and whether you are heading in the right direction, and this makes knowing how to use a compass essential.