I grew up in a famously foggy place. Usually, it’s just a nuisance, but if you know how it works, at least you can add a little poetry to it.

The first thing you need to know is that warm air keeps moisture better than colder one. Which is to say, water remains a vapor more easily in warm air.

When a mass of warm and moist air cools rapidly, water is forced to condensate in small droplets. That’s what happens to the very moist air coming out of our how shower when it hits the bathroom mirror. And also to the warm summer evening breeze that grazes our ice-cold cocktail.


CC-BY jenny downing, via Flickr.

But it can also happen if air has no actual surface to hang on to. In that case, the droplets just float in the air.

Particularly in winter, the ground can cool down faster than the air around it—especially around dawn and dusk, when the sun rapidly warms the air. So humidity in the air closer to the ground condensates and, layer by layer, the fog comes up.

As the day goes by, the ground warms up and the zone where fog can form moves upwards. So fog moves up to go away too. Some areas are more subject to the phenomenon, particularly those with little wind and humid air. If you lived there, you’ll have noticed that in foggy times it’s also very often cloudy. The reason is that those clouds are just the moved-up fog.

Indeed, fog forms the same way clouds do: condensation of moist warm air cooling down. Which is where you can find solace to your foggy days—just think of it as a very low cloud.


Cover photo: walking the embankment, CC-BY-NC-ND palmasco, via Flickr. Some rights reserved.

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