physics

Parry arc

Parry arc is a physics 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 Parry arc rather than just read about it. In short: A Parry arc is a rare halo, an optical phenomenon which occasionally appears over a 22° halo together with an upper tangent arc. Discovery The halo was first described by Sir William Edward Parry (1790–1855) in 1820 during one of his Arctic expeditions in search for the Northwest Passage.

Key takeaways

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

Reference excerpt

A Parry arc is a rare halo, an optical phenomenon which occasionally appears over a 22° halo together with an upper tangent arc.

Discovery The halo was first described by Sir William Edward Parry (1790–1855) in 1820 during one of his Arctic expeditions in search for the Northwest Passage. On April 8, under harsh conditions while his two ships were trapped by ice forcing him to winter over at Melville Island in the northern Canadian Arctic Archipelago, he made a drawing of the phenomenon. The drawing accurately renders the parhelic circle, a 22° halo, a pair of sun dogs, a lower tangent arc, a 46° halo, and a circumzenithal arc. He did, however, get the upper tangent arc slightly wrong. On the other hand, he added two arcs extending laterally from the bases of the 46° halo, for long interpreted as incorrectly drawn infralateral arcs, but were probably correctly drawn subhelic arcs (both produced by the same crystal orientation but with light passing through different faces of the crystals).

Formation Parry arcs are generated by double-oriented hexagonal column ice crystals, i.e. a so-called Parry orientation, where both the central main axis of the prism and the top and bottom prism side faces are oriented horizontally. This orientation is responsible for several rare haloes. Parry arcs are the result of light passing through two side faces forming a 60° angle. The shape of Parry arcs changes with the elevation of the sun and are subsequently called upper or lower arcs to indicate they are found above or under the sun, and sunvex or suncave depending on their orientation. The mechanism by which column crystals adopt this special Parry orientation has been subject to much speculation – recent laboratory experiments have shown that it is the presence of crystals with a scalene hexagonal cross-section which are likely to be the cause. Parry arcs can be confused with either upper tangent arcs, Lowitz arcs, and any of the odd radius halos produced by pyramidal crystals.

Illustrations

Worked examples

Example 1 — a first encounter with Parry arc

Start with the simplest possible case. Write down what Parry arc claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, 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 Parry arc 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 Parry arc 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 Parry arc

In research: Parry arc appears in physics 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 Parry arc 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: Parry arc is common in secondary-school and first-year university syllabi. It links to neighbouring topics Atmospheric optical phenomena, so understanding it makes those chapters shorter.
In everyday life: Look for Parry arc 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 Parry arc in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Parry arc 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 Parry arc out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Parry arc in simple terms?

A Parry arc is a rare halo, an optical phenomenon which occasionally appears over a 22° halo together with an upper tangent arc. Discovery The halo was first described by Sir William Edward Parry (1790–1855) in 1820 during one of his Arctic expeditions in search for the Northwest Passage.

Why does Parry arc matter?

Because it connects several physics 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 Parry arc?

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 Parry arc.