POLAR STRATOSPHERIC CLOUDS (Syllabus: GS Paper 1 – Geography)

Context: Recently, weather watchers have captured images of rare Polar stratospheric clouds over Scotland, northern England and the West Midland.

Polar Stratospheric clouds (PSC)

• Known as: Nacreous clouds or mother of pearl
• Formation: The stratosphere is very dry; unlike the troposphere, it rarely allows clouds to form.
• Characteristics: Due to their high altitude and the curvature of the surface of the Earth, these clouds will receive sunlight from below the horizon and reflect it to the ground, shining brightly well before dawn or after dusk.

oPSCs form at very low temperatures, below −78 °C (−108 °F). These temperatures can occur in the lower stratosphere in polar winter.

• It causes ozone depletion: PSCs play important roles in stratospheric ozone depletion at high latitudes during winter and spring (e.g., the Antarctic ozone hole). PSC particles serve as sites for heterogeneous reactions that convert stable chlorine reservoir species to radicals that catalytically destroy ozone.

Types of PSC

• Type I Clouds contain water, nitric acid and/or sulphuric acid and they are a source of polar ozone depletion. They form when temperatures drop to -78°C or lower.

oType Ia Clouds consist of large, aspherical particles, consisting of nitric acid trihydrate (NAT).

oType Ib Clouds contain small, spherical particles (non-depolarising), of a liquid supercooled ternary solution (STS) of sulphuric acid, nitric acid and water.

oType Ic Clouds consist of metastable water-rich nitric acid in a solid phase.

• Type II Clouds, which are very rarely observed in the Arctic, consist of water ice only. They need temperatures of -83°C or lower to form.