Nacreous and Polar Stratospheric Clouds

Nacreous and Polar Stratospheric Clouds: A Mesmerizing Atmospheric Phenomenon

Nacreous clouds, also known as mother-of-pearl clouds, are a captivating type of wave cloud that can be observed downwind of mountain ranges. These ethereal formations are characterized by their sheet-like appearance, which slowly undulates and stretches as gravity waves evolve in the lower stratosphere. They often accompany very high surface winds, indicating the presence of, or inducing, winds and waves in the stratosphere.

These extraordinary clouds form at frigid temperatures of around minus 85ºC, significantly colder than the average lower stratospheric temperatures. Comprised of ice particles approximately 10µm across, nacreous clouds must consist of crystals of similar sizes to create their distinctive bright iridescent colors through diffraction and interference.

It is important to note that nacreous clouds fall under the category of Polar Stratospheric Clouds (PSCs). PSCs can be classified into two types, each possessing unique characteristics and compositions.

Type II PSCs

Nacreous clouds belong to this category, characterized by their ice crystal composition and extremely low temperatures of approximately minus 85ºC. These clouds exhibit mesmerizing displays of vibrant colors due to the diffraction and interference of light passing through the ice crystals.

Type I PSCs

Type I PSCs are less visually striking compared to nacreous clouds. They appear more diffuse and display less intense colors. However, it is not uncommon to find nacreous clouds embedded within this type. Type I PSCs are slightly warmer than Type II, with temperatures hovering around minus 78ºC. They are composed of exotic solids or liquid droplets.

Within Type I PSCs, further subcategories exist, each with its own distinct composition:

  • Type Ia: Crystalline compounds of water and nitric acid, particularly nitric acid trihydrate (HNO3.3H2O).
  • Type Ib: Small spherical droplets of a solution consisting of nitric and sulfuric acids.
  • Type Ic: Small non-spherical particles of a metastable nitric acid-water phase.

For a long time, Polar Stratospheric Clouds were considered mere curiosities with no significant impact. However, it is now known that Type I clouds play a detrimental role in the destruction of stratospheric ozone over the Antarctic and Arctic regions. The surfaces of these clouds act as catalysts, converting benign forms of man-made chlorine into active free radicals like chlorine monoxide (ClO). When Spring sunlight returns, these radicals engage in a series of chain reactions, ultimately leading to the destruction of numerous ozone molecules. Additionally, cloud formation exacerbates the situation by removing gaseous nitric acid from the stratosphere. This removal prevents its combination with ClO, which would otherwise form less reactive forms of chlorine.

In conclusion, nacreous and Polar Stratospheric Clouds present a mesmerizing atmospheric phenomenon. These wave clouds, with their undulating sheet-like forms and vibrant iridescent colors, offer a captivating sight. Understanding their composition and the harmful effects of certain types of Polar Stratospheric Clouds on the ozone layer highlights the delicate balance within our atmosphere. As we continue to study and appreciate these captivating formations, we gain further insight into the intricate workings of our planet's atmospheric dynamics.

Nacreous clouds are wave clouds. They are often found downwind of mountain ranges which induce gravity waves in the lower stratosphere. Their sheet-like forms slowly undulate and stretch as the waves evolve. The clouds can also be associated with very high surface winds which may indicate the presence of, or induce, winds and waves in the stratosphere.

They form at temperatures of around minus 85ºC, colder than average lower stratophere temperatures, and are comprised of ice particles ~10µm across. The clouds must be composed of similar sized crystals to produce the characteristic bright iridescent colours by diffraction and interference.

Nacreous clouds are a type of Polar Stratospheric Cloud (PSC).

POLAR STRATOSPHERIC CLOUDS

Type II

Nacreous clouds composed of ice crystals with temperatures of ~minus 85ºC.

Type I

Less spectacular than nacreous clouds, more diffuse and less bright colours. Sometimes nacreous clouds are embedded in them. Type I clouds are slightly warmer (~ minus 78ºC) than Type II and are composed of exotic solids or liquid droplets.

Type Ia

Crystalline compounds of water and nitric acid - especially NAT, nitric acid trihydrate HNO3.3H2O

Type Ib

Small spherical droplets of a solution of nitric and sulphuric acids.

Type Ic

Small non spherical particles of a metastable nitric acid - water phase

PSCs were long regarded as curiosities and of no real consequence. However, Type I clouds are now known as sites of harmful destruction of stratospheric ozone over the Antarctic and Arctic. Their surfaces act as catalysts which convert more benign forms of man-made chlorine into active free radicals (for example ClO, chlorine monoxide). During the return of Spring sunlight these radicals destroy many ozone molecules in a series of chain reactions. Cloud formation is doubly harmful because it also removes gaseous nitric acid from the stratosphere which would otherwise combine with ClO to form less reactive forms of chlorine.

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