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.
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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]