OPOD - Dogs in the Sky

Last updated on September 16, 2023

OPOD - Dogs in the Sky: Exploring the Phenomenon in Detail

Have you ever looked up at the sky and seen a large dog seemingly about to devour the sun? This intriguing optical phenomenon, known as a 'hole punch,' has captured the attention of skywatchers around the world. In this article, we will delve into the details of this mesmerizing spectacle and explore the science behind its formation.

The sight of a sundog nestled within the stomach of a hole punch is truly a sight to behold. To understand this phenomenon, we need to examine the role of plate-shaped ice crystals descending from the hole punch, which are visible as dark drifts. These ice crystals play a crucial role in generating the sundog, creating a captivating interplay of light and shadow in the sky.

But what causes the appearance of the dark dog arising from an instability in the cloud? While the exact details and triggers of this phenomenon remain uncertain, scientists have pieced together a broader understanding of its mechanisms.

To comprehend the formation of hole punches and their associated dogs, we must first explore the concept of supercooled water droplets. Contrary to popular belief, water does not always freeze at 0 degrees Celsius. In certain conditions, if no nuclei are present on which ice crystals can grow (a process known as heterogeneous nucleation), water can remain at sub-zero temperatures, a state known as supercooling. At intermediate temperatures, clouds can be predominantly composed of these tiny supercooled water droplets.

From a thermodynamic perspective, this supercooled state is considered metastable since ice represents a more stable condition. However, introduce enough nuclei or disturb the cloud in some way, and ice crystals will rapidly grow. The equilibrium vapor pressure of water over supercooled droplets is greater than that over ice at the same temperature, and the process of ice crystallization releases heat. As a result, once ice crystallization begins, it proceeds swiftly, with the necessary water vapor supplied by the evaporation of surrounding water droplets. The cloud soon transforms from one composed of supercooled water drops into a cloud predominantly made up of ice crystals.

This phenomenon is not exclusive to hole punches and dogs in the sky. In fact, it is one of the major mechanisms by which rain forms. Typically, water droplets in clouds struggle to collide and coalesce effectively, hindering the formation of large raindrops. However, ice crystals have the ability to grow and cluster into intricate snowflakes. Consequently, the cloud either precipitates snow or, if temperatures are higher, the snowflakes melt into raindrops.

What sets hole punches and their accompanying dogs apart is the sudden transformation of a localized region within a cloud from supercooled droplets into large halo-forming ice crystals, leaving behind a distinctive hole. While several hypotheses exist to explain the necessary localized disturbance, no definitive answer has been found. Some propose that falling ice crystals from higher cloud fragments may initiate the process, while others suggest disturbances caused by aircraft or natural high and medium-level turbulence and air currents.

In conclusion, the enigmatic appearance of dogs in the sky, nestled within hole punches, continues to captivate our imagination. By understanding the underlying science behind these optical phenomena, we gain a deeper appreciation for the intricacies of our atmosphere. So, the next time you gaze up at the heavens and witness this celestial spectacle, take a moment to marvel at the complex interplay between temperature, ice crystals, and the delicate balance that shapes our atmospheric wonders.

Dogs in the Sky - 1

A sighting from Hungary by Alexandra Farkas. ©Alexandra Farkas, shown with permission.

The large dog about to swallow the sun is a 'hole punch'. The sundog already in its stomach is generated by plate shaped ice crystals descending from the hole punch in fall streaks visible as dark drifts.

More Hungarian dogs in the next OPOD.

The dark dog arises from an instability in the cloud. The details and what triggers it are uncertain but the broader picture is known.

Water does not always freeze at 0 degrees Celsius. If no nuclei are present on which ice crystals can grow (heterogeneous nucleation) the water can remain at sub-zero temperatures - it is supercooled. At sufficiently low temperatures spontaneous nucleation occurs but at intermediate temperatures clouds can be composed largely of tiny supercooled water droplets.

Thermodynamically this is a metastable condition because ice is the more stable state. Introduce enough nuclei or otherwise disturb the cloud and ice crystals grow very rapidly.

This happens because the equilibrium vapour pressure of water over supercooled droplets is greater than that over ice at the same temperature and because ice crystallisation releases heat. Once ice crystallisation starts it continues rapidly with the necessary water vapour supplied by evaporation of the surrounding water drops. Very soon we have a cloud largely of ice crystals rather than one of supercooled water drops.

This is not special. It is one of the major mechanisms by which rain forms. The water drops in clouds are not usually able to collide and coalesce sufficiently to form large raindrops. Ice crystals can, however, easily grow and cluster into snowflakes. The cloud then snows, or if the temperatures are high enough, the flakes melt into raindrops.

What is unusual is that a relatively small (several hundred metres) region of a cloud should suddenly transform from supercooled droplets into large halo forming ice crystals leaving a hole.

Several hypotheses exist to explain the necessary localised disturbance. Falling ice crystals from higher cloud fragments might initiate the process. Disturbances by aircraft have been suggested. Natural high and medium level turbulence and air currents might also disturb the delicate metastable state.

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Reference Atmospheric Optics

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