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Tangent arc

Tangent 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 Tangent arc rather than just read about it. In short: Tangent arcs are a type of halo, an atmospheric optical phenomenon, which appears above and below the observed Sun or Moon, tangent to the 22° halo. To produce these arcs, rod-shaped hexagonal ice crystals need to have their long axis aligned horizontally.

Tangent arc — main illustration
Tangent arc — illustration

Key takeaways

  • Tangent 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 Tangent arc to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Tangent arc from memory before moving on to harder problems.

Reference excerpt

Tangent arcs are a type of halo, an atmospheric optical phenomenon, which appears above and below the observed Sun or Moon, tangent to the 22° halo. To produce these arcs, rod-shaped hexagonal ice crystals need to have their long axis aligned horizontally.

Description

Upper arc The shape of an upper tangent arc varies with the elevation of the Sun; while the Sun is low (less than 29–32°) it appears as an arc over the observed Sun forming a sharp angle. As the Sun is seen to rise above the Earth's horizon, the curved wings of the arc lower towards the 22° halo while gradually becoming longer. As the Sun rises over 29–32°, the upper tangent arc unites with the lower tangent arc to form the circumscribed halo.

Lower arc

The lower tangent arc is rarely observable, appearing under and tangent to a 22° halo centred on the Sun. Just like upper tangent arcs, the shape of a lower arc is dependent on the altitude of the Sun. As the Sun is observed slipping over Earth's horizon the lower tangent arc forms a sharp, wing-shaped angle below the Sun. As the Sun rises over Earth's horizon, the arc first folds upon itself and then takes the shape of a wide arc. As the Sun reaches 29-32° over the horizon, it finally begins to widen and merge with the upper tangent arc to form the circumscribed halo. Since by definition, the Sun elevation must exceed 22° above the horizon, most observations are from elevated observation points such as mountains and planes.

Origin Both the upper and lower tangent arc form when hexagonal rod-shaped ice crystals in cirrus clouds have their long axis oriented horizontally. Each crystal can have its long axis oriented in a different horizontal direction, and can rotate around the long axis. Such a crystal configuration also produces other halos, including 22° halos and sun dogs; a predominant horizontal orientation is required to produce a crisp upper tangent arc. Like all colored halos, tangent arcs grade from red towards the Sun (i.e., downwards) to blue away from it, because red light is refracted less strongly than blue light.

See also

Circumhorizontal arc Circumzenithal arc Kern arc Subsun

References

External links www.paraselene.de - Tangent Arcs (including several HaloSim simulations.) Atmospheric Optics - Alaska Lower Tangent Arc - Great photo by Ryan Skorecki. Atmospheric Optics - Lower Tangent arc - A photo taken from an aeroplane. Lower Tangent Arc Sep 25, 2005 - Photos of a lower tangent arc.

Illustrations

Tangent arc: An upper tangent arc appearing above San Francisco Bay and Marin County in California, at sunset on April 9, 2023.
An upper tangent arc appearing above San Francisco Bay and Marin County in California, at sunset on April 9, 2023.
Tangent arc: An upper tangent arc with the Sun at an observed low altitude.
An upper tangent arc with the Sun at an observed low altitude.
Tangent arc: An upper tangent arc seen over the setting Sun at Santa Rosa Beach, Florida.
An upper tangent arc seen over the setting Sun at Santa Rosa Beach, Florida.
Tangent arc: A halo display observed over the South Pole. Featured in the photo are several distinct phenomena: A parhelic circle (horizontal line), a 22° halo (circle) with a sundog (bright spot), and an upper tangent arc.Photo: Cindy McFee, NOAA, December 1980.[1]
A halo display observed over the South Pole. Featured in the photo are several distinct phenomena: A parhelic circle (horizontal line), a 22° halo (circle) with a sundog (bright spot), and an upper tangent arc.Photo: Cindy McFee, NOAA, December 1980.[1]
Tangent arc: A 22° halo featuring two sun dogs on both sides, as well as an upper and a lower tangent arcs at the top and the bottom.
A 22° halo featuring two sun dogs on both sides, as well as an upper and a lower tangent arcs at the top and the bottom.

Worked examples

Example 1 — a first encounter with Tangent arc

Start with the simplest possible case. Write down what Tangent 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 Tangent 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 Tangent 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 Tangent arc

In research
Tangent 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 Tangent 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
Tangent 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 Tangent 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 Tangent arc in 20 minutes

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

Frequently asked questions

What is Tangent arc in simple terms?

Tangent arcs are a type of halo, an atmospheric optical phenomenon, which appears above and below the observed Sun or Moon, tangent to the 22° halo. To produce these arcs, rod-shaped hexagonal ice crystals need to have their long axis aligned horizontally.

Why does Tangent 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 Tangent 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 Tangent arc.

Tags

  • Atmospheric optical phenomena

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