00:00 What are condensation trails?
00:03 Contrails are artificial clouds that form from exhaust emitted by jets.
00:11 They appear as long streaks in the sky when airplanes fly at a certain altitude.
00:16 The perfect contrail-forming zone is around 10 km up in the lowest layer of the atmosphere.
00:22 Our atmosphere comprises five layers in all.
00:29 The lowest is the troposphere, which stretches 7 km up at the poles, and 17 km above the tropics.
00:37 The temperature drops by about 6 degrees Celsius every kilometre of altitude.
00:42 At the upper edge of the troposphere, it can get as cold as -60 degrees.
00:47 Ideal conditions for contrails to form.
00:51 When an aircraft burns fuel, it emits carbon dioxide and tiny particles of other compounds,
00:57 but also large quantities of water vapour, H2O.
01:01 These water molecules attach to the particles and immediately freeze in the cold to form ice crystals.
01:09 If humidity is also very high in the area, the condensation trails form even faster and remain visible for longer.
01:25 The composition of contrails is very similar to that of natural cirrus clouds.
01:29 Both are made up of ice crystals, and under favourable conditions, they can just keep on growing.
01:36 In some situations, they can create streaks of cloud that are several kilometres wide.
01:43 Called contrail cirrus clouds, these can temporarily cover up to 10% of the sky over regions that see heavy air traffic.
01:53 Research teams from various space agencies have spent years studying the effects of contrails on the climate.
02:00 On the one hand, clouds cool the Earth's surface by reflecting some of the sun's rays.
02:09 But at the same time, they prevent heat absorbed by the Earth's surface from radiating back into space.
02:15 The research indicates the second effect is stronger, so contrails also further heat up the atmosphere.
02:22 [MUSIC PLAYING]
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