science

Polar stratospheric cloud

Polar stratospheric cloud is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Polar stratospheric cloud rather than just read about it. In short: A polar stratospheric cloud (PSC) is a cloud that forms in the winter polar stratosphere at altitudes from 15,000 to 25,000 m (49,000 to 82,000 ft). They are best observed during civil twilight, when the Sun is between 1° and 6° below the horizon, as well as in winter and in more northerly latitudes.

Key takeaways

Polar stratospheric cloud belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Polar stratospheric cloud to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Polar stratospheric cloud from memory before moving on to harder problems.

Reference excerpt

A polar stratospheric cloud (PSC) is a cloud that forms in the winter polar stratosphere at altitudes from 15,000 to 25,000 m (49,000 to 82,000 ft). They are best observed during civil twilight, when the Sun is between 1° and 6° below the horizon, as well as in winter and in more northerly latitudes. One main type of PSC is composed of mostly supercooled droplets of water and nitric acid and is implicated in the formation of ozone holes. The other main type consists only of ice crystals, which are not harmful. This type of PSC is also called nacreous cloud (; from nacre, or mother of pearl), due to its iridescence.

Formation

The stratosphere is very dry; unlike the troposphere, it rarely allows clouds to form. In the extreme cold of the polar winter, however, stratospheric clouds of different types may form, which are classified according to their physical state (super-cooled liquid or ice) and chemical composition. 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. PSCs form at very low temperatures, below −78 °C (−108 °F). These temperatures can occur in the lower stratosphere in polar winter. In the Antarctic, temperatures below −88 °C (−126 °F) frequently cause type II PSCs. Such low temperatures are rarer in the Arctic. In the Northern Hemisphere, the generation of lee waves by mountains may locally cool the lower stratosphere and lead to the formation of lenticular (lens-shaped) PSCs.

Forward scattering of sunlight within the clouds produces a pearly-white appearance. Particles within the optically thin clouds cause coloured interference fringes by diffraction. The visibility of the colours may be enhanced with a polarising filter.

Types

PSCs are classified into two main types, each of which consists of several subtypes.

Type I clouds have a generally stratiform appearance resembling cirrostratus or haze. They are sometimes sub-classified according to their chemical composition which can be measured using LIDAR. The technique also determines the height and ambient temperature of the cloud. They contain water, nitric acid and/or sulfuric acid and are a source of polar ozone depletion. The effects on ozone depletion arise because they support chemical reactions that produce active chlorine which catalyzes ozone destruction, and also because they remove gaseous nitric acid, perturbing nitrogen and chlorine cycles in a way which increases ozone depletion. Type Ia clouds consist of large, aspherical particles, consisting of nitric acid trihydrate (NAT). Type Ib clouds contain small, spherical particles (non-depolarising), of a liquid supercooled ternary solution (STS) of sulfuric acid, nitric acid, and water. Type Ic clouds consist of metastable water-rich nitric acid in a solid phase. Type II clouds, which are very rarely observed in the Arctic, have cirriform and lenticular sub-types and 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. The World Meteorological Organization no longer uses the alpha-numeric nomenclature seen in this article, and distinguishes only between super-cooled stratiform acid-water PSCs and cirriform-lenticular water ice nacreous PSCs.

See also Aurora Circumhorizontal arc Cloud iridescence Noctilucent clouds

References

External links

Research Nacreous Clouds at atoptics.co.uk Polar Stratospheric Clouds Above Spitsbergen at Alfred Wegener Institute

News reports "Rare cloud spotted". Sydney Morning Herald. AAP. 1 August 2006. https://news.yahoo.com/s/ap/20060801/ap_on_sc/antarctica_clouds_4 http://www.cnn.com/2006/TECH/science/08/01/antarctica.clouds.ap/index.html

Illustrations

Worked examples

Example 1 — a first encounter with Polar stratospheric cloud

Start with the simplest possible case. Write down what Polar stratospheric cloud claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Polar stratospheric cloud before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Polar stratospheric cloud ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Polar stratospheric cloud

In research: Polar stratospheric cloud appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Polar stratospheric cloud in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Polar stratospheric cloud is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cloud types, Ozone depletion, so understanding it makes those chapters shorter.
In everyday life: Look for Polar stratospheric cloud outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Polar stratospheric cloud in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Polar stratospheric cloud means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Polar stratospheric cloud out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Polar stratospheric cloud in simple terms?

Why does Polar stratospheric cloud matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Polar stratospheric cloud?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Polar stratospheric cloud.