Fog in the mountains is a frequent phenomenon. It is both beautiful and dangerous. Fog can severely deteriorate visibility and make navigation very difficult. Today we will talk about where it comes from.
As we have already found out, mountain and valley winds blow in the mountains. Due to them, the air rises up the slopes during the day and flows down the slopes into the valley at night. When the air rises, it is forced against the direction of gravity. Gravity pulls the air down, and it still goes up, because it is forced out by the cold air. The air has to give up some of its energy, that is, some of its heat. Thus the air is cooled (this is called adiabatic cooling).
Note. The air also exchanges heat with the environment. Therefore, it can also be cooled due to contact with colder air at higher altitudes. The smaller the volume of the air, the more it can be cooled due to contact with the surrounding cold air.
As it cools, condensation — the transformation of water vapor into droplets — can begin. Whether or not droplets form depends on how humid the air is and how much time it has to cool. This is described in detail in the text about the dew point. If the air is very humid, it quickly reaches condensation when it cools, and if the air is dry, it needs to cool significantly to get the droplets. So, the air in the mountains rises during the day, cools down in the process, and if it is humid enough, moisture condenses in it. From the sea, it will look like a cloud, but for those living on the slopes, it will be fog.
We have learned how mountain and valley winds cause fog in the mountains. However, this is not the only mechanism for fog formation in the mountains. There are several universal mechanisms that work for both mountains and plains. We will look at them in one of our future letters.
Text: Windy.app team
Cover photo: Sergey-pesterev / Unsplash