What are contrails and how do they form?
Condensation trails are linear artificial clouds that form from the exhaust of airplanes. Do they have an effect on the climate? This week's viewer question comes from Al Ben M. in Panama.
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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.
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