What is fog, and how does it form?

DW (English)

Fog coalesces when water vapor condenses. The resulting droplets reflect light and obscure vision. This week's viewer question comes from Dobrila B. from Bosnia and Herzegovina.

Transcript

00:00 What is fog and how does it form?

00:04 In simple terms, fog is nothing more than a cloud floating close to the surface of the Earth.

00:15 For it to form, the air either has to grow cooler or moister.

00:22 When the water vapour in air reaches a critical point, it starts to condense.

00:29 The resulting water droplets reflect the light and blanket the landscape.

00:34 That visible vapour qualifies as fog when it reduces visibility to less than one kilometre.

00:43 Air can absorb differing amounts of water vapour.

00:51 The warmer it is, the more moisture it can absorb.

00:58 Fog always looks the same, but it can come about for a range of different reasons.

01:03 It often forms in valleys, for example, because the ground there cools down quickly,

01:09 thereby lowering the temperature of the air directly above it.

01:13 And because that colder air can hold less moisture, the water vapour in it condenses into droplets.

01:21 Over lakes and rivers, water evaporating from the surface into the cold air can raise humidity to the saturation point, and then it turns foggy.

01:30 The Grand Banks of Newfoundland in the North Atlantic are considered the foggiest place on Earth.

01:37 As cold and warm masses of air collide and mix there, the warm, moist air cools down, condensing its vapour into fog.

01:45 Something similar happens when your moist, warm breath hits cold air.

01:51 Another type of fog can be witnessed in mountains.

01:54 When humid air rises up the side of a mountain, air pressure and temperature both drop.

02:00 At some point, its water vapour condenses, leading to the formation of what's called mountain fog.

02:08 [Music]

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