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Stratosphere: a new look explanation for typical events

Why high speed jets/ aircraft prefer Stratosphere?

  • Commercial airlines typically fly at altitudes of 9–12 km (30,000–39,000 ft) which is in the lower reaches of the stratosphere in temperate latitudes. This optimizes fuel efficiency, mostly due to
  1. The low temperatures encountered near the tropopause
  2. Low air density reduces parasitic drag on the airframe. It allows the airliner to fly faster for the same amount of drag.
  • Because of the temperature stratification or specifically, temperature inversion, there is little convection and mixing in the stratosphere, so the layers of air are quite stable. Commercial jet aircraft fly in the lower stratosphere to avoid the turbulence which is common in the troposphere below.
  • Moreover, the stratosphere is very dry; air there contains little water vapor. With little convection and absence of water vapour, only few clouds are formed in this layer. This again increases the visibility for high speed jets.

What is Polar Stratospheric Cloud?

Also known as nacreous clouds or mother of Pearl Clouds, are clouds produced in the lower stratosphere near poles during winter at an altitudes of 15-25km. They are best observed during civil twilight when the sun is between 1 and 6 degrees below the horizon as well as in winter and in more northerly latitudes.

They are implicated in the formation of ozone holes. The effects on ozone depletion arise because

  1. They support chemical reactions that produce active chlorine which catalyzes ozone destruction
  2. They Remove gaseous nitric acid, perturbing nitrogen and chlorine cycles in a way which increases ozone destruction

Types of PSCs

PSCs are classified into three types Ia, Ib and II according to their chemical composition which can be measured using LIDAR. The technique also determines the height and ambient temperature of the cloud.

  • Type I clouds contain water, nitric acid and/or sulphuric acid and they are a source of polar ozone depletion.
  1. Type Ia clouds consist of large, aspherical particles, consisting of nitric acid trihydrate (NAT)
  2. Type Ib clouds contain small, spherical particles (non-depolarising), of a liquid supercooled ternary solution (STS) of sulfuric acid, nitric acid and water.
  • Type II clouds, which are very rarely observed in the Arctic, consist of water ice only.
  • Only Type II clouds are necessarily nacreous whereas Type I clouds can be iridescent under certain conditions, just as any other cloud.

Why ozone depletion mainly occur over Poles?

  • Presence of Polar Stratospheric Clouds that has great vertical extent.
  • The ice crystals of PSCs form stable compounds with the halogens.
  • This stable compound reach the ozone layer during winter season
  • Polar Jet Stream also speed up its arrival near ozone layer
  • After winter, during spring the sun rays starts arriving at poles, thus melting ice crystals.
  • Melting ice crystals release halogens, which have great affinity for oxygen atoms.
  • They convert ozone into diatomic oxygen which lacks the capability to absorb UV Rays, resulting in ozone depletion.

Why no cloud formation in stratosphere?

  • Almost absence of water vapour in the stratosphere
  • Constant temperature condition in the lower stratosphere
  • Lack of vertical convection in the stratosphere due to temperature inversion.
  • There is little convection and mixing in the stratosphere, so the layers of air there are quite stable.

Why materials in the stratosphere stay for long?

  • Due to the lack of vertical convection in the stratosphere, materials that get into the stratosphere can stay there for long times. Such is the case for the ozone-destroying chemicals called CFCs (chlorofluorocarbons).
  • Large volcanic eruptions and major meteorite impacts can fling aerosol particles up into the stratosphere where they may linger for months or years, sometimes altering Earth’s global climate.
  • Rocket launches inject exhaust gases into the stratosphere, producing uncertain consequences.
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