Sun dogs!

Sun dogs, illustrated above, are an atmospheric phenomenon resulting from tiny ice crystals in the air. In fact, they are part of a halo or parhelion around the Sun (or Moon), at an angle of 22° from it. This halo is typically intensified to the left and right of the Sun, forming the two “dogs” (and sometimes also intensified above and below). An explanation must therefore come in two parts – why the halo, and why the intensification?

For the first question, the tiny ice crystals floating in the air over wintry landscapes act as tiny prisms, deflecting the light by about 22° (see the diagram below, superimposed on a microscope image of an Antarctic ice crystal, by Hannes Grobe):

Because prisms deflect different colours slightly differently (as shown below), a slight rainbow effect is sometimes visible in the 22° halo, with red on the inside. However, because the deflection actually occurs over a range of angles from 22° upwards (depending on the orientation of the crystals), the blue and green colours are usually completely washed out.

The second question was: why the intensification on the two sides of the sun? This occurs because flat ice crystals have a tendency to fall through the air like paper plates, with the hexagonal surfaces at the top and bottom. Intensification above and below the sun is sometimes also seen, forming a kind of cross. This results from other, rod-shaped, ice crystals which are oriented with their long axis horizontal.

The diagram below shows the two kinds of ice crystal:

I have not been fortunate enough to see sun dogs myself, but it’s on the bucket list.

Advertisements

The Field Guide to Natural Phenomena: a book review

    
The Field Guide to Natural Phenomena [Wonders] by Keith Heidorn and Ian Whitelaw (2010)

The Field Guide to Natural Phenomena [Wonders], subtitled The Secret World of Optical, Atmospheric and Celestial Phenomena (this book by Keith Heidorn and Ian Whitelaw seems to have two alternate main titles) describes and explains a range of natural phenomena that one is likely to meet. One might meet them while taking a long drive, for example. Included in the book are brief discussions of geysers, tides, clouds, mirages, meteors, waterspouts, rainbows, sundogs, aurorae, and much more.


Waterspout, moon, aurora, double rainbow, and sundogs (image credits: 1, 2, 3, 4, 5)

This is a great little book, and well-illustrated. Families interested in teaching children about science might like to keep a copy in the family vehicle.

* * * *
The Field Guide to Natural Phenomena [Wonders] by Keith Heidorn and Ian Whitelaw: 4 stars

Aurora chasers


Aurora Borealis (photo: Chad Blakley, lightsoverlapland.com)

The Aurora Australis and Aurora Borealis make for spectacular images, like the ones above and below. It is not surprising that some keen photographers are on the lookout for the perfect viewing opportunity.


Aurora Borealis seen from Alaska (photo: SrA Joshua Strang, USAF)

The aurorae become more visible during geomagnetic storms, so that the planetary Kp index of geomagnetic activity is a rough guide to visibility (with location-based visibility thresholds shown in NOAA maps for Australia & NZ, Eurasia, and North America). Aurora forecasts based on Kp and other space weather factors are available for Australia & NZ, Europe, and North America.

Finally, let me mention two handbooks for Aurora chasers – one new (aimed at the South) and one from 1992 (aimed at the North):

The Aurora Chaser's Handbook, by Margaret Sonnemann (2013)            The Aurora Watcher's Handbook, by Neil Davis (1992)