Physics Photo of the Week - September 22, 2023

Physics Photo of the Week

September 22, 2023

Sun Halo over the Arizona Desert - Photos by Dr. Betsy Arnold, University of Arizona

Warren Wilson Professor Dr. Amy Boyd and her research colleagues:
Margrit McIntosh, Betsy Arnold, and Lucinda McDade (director of the California Botanic Garden) were  observing environmental parameters near Tucson, Arizona when they noticed this magnificent atmospheric halo visible above the desert.  Lucinda McDade can be seen in the  photo making observations of desert plants.  Click on the photo and notice the ruler next to the rock on her right side - one of several tools being used.  

Regarding the ongoing research in the desert of Arizona Dr Boyd writes:

Several of us (Amy Boyd, Margrit McIntosh, Betsy Arnold and Lucinda McDade) study the population biology of an endangered Sonoran Desert cactus, the Nichol's Turk's head cactus, Echinocactus horizonthalonius var. nicholii, in the Waterman Mountains of Arizona.  We've been studying the cactus for over 27 years now, following the changes in population size and structure and increasing our understanding of the species' ecological needs and reasons for decline.

We appreciate the ecological research these scientists are doing to learn more about the desert environments.  We also thank them for taking the break to marvel at the unusual halo in the sky formed by particular clouds - high altitude cirrus clouds composed of ice crystals.

The cirrus clouds form when warm moist air passes over a cold clear-air mass.  This occurs at very high altitudes where the air is very cold, less than about -40 deg C.  Instead of being formed from water droplets these clouds are formed from tiny ice crystals.  Sometimes the tiny ice crystals are small hexagonal rods - less than 1 mm long and less than 0.1 mm wide.  These needles resemble old straight wooden hexagonal pencils, which act as small prisms. The small needles are so small that they fall slowly in the atmosphere due to the air friction. The aerodynamics of small things (dust, pollen, Styrofoam packing chips, …) fall with a constant velocity (air friction cancels the force of gravity). If small things are not small spheres, but flat disks or needles, they fall oriented such that the long axes are horizontal. Other orientations are unstable. Imagine confetti flakes – they tend to fall fluttering down in primarily horizontal orientations. Thus the needle-like ice-crystals of a cirrus cloud tend to be oriented horizontally as they drift slowly downward.

A hexagon ice crystal refracts the light similar to a familiar triangular prism – with the corners cut off as shown in the drawing below.  The beam of light comes from the Sun.

The crystals all deflect the Sun's rays by various angles - indicated by the angle D above.  However, due to the geometry of a triangular (or hexagonal) prism, all deflections of the light are at least 22 degrees.  The angle depends on the 60 degree geometry of the prisms and the refractive index of ice.  No matter the orientation of the hexagon crystal the deflection angle (labeled D in the picture above) is always at least 22 degrees.  Thus the scattered light from the crystals tends to be brighter at the outside the 22 degrees.  The halo's angular radius is 22 degrees and the sky is darker within the halo's circle.

The photo at right is another photo made by Dr. Arnold in which the halo is centered on the photograph and shows less distortion of the perfect circular shape of the halo.  The apparent oval shape of the top photo is due to geometric distortion of off-center circles with a wide-angle camera lens.

 

 

 

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 Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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