The Puzzles of Icicles

Desperately needing some fresh air to cure cabin fever after being holed up for over a week providing hospice care for my dying 17-year-old cat, “Ping,” I went for a late afternoon walk.  Cloaked in a blanket of fresh white snow with frigid temps, my walk was refreshing.  It was just what I needed.

Long icicles in the sunlight caught my attention.  These icicles had patterns or ripples on them.  All the icicles had these equally spaced ripples.

What causes these ripples?

The explanation must be simple, I self-assured myself.

A little research revealed that we do not know why these ripples form on icicles.  What?  Come on, man, we can fly a person to the moon and back and nobody has figured out this puzzle of icicles?   Didn’t Newton, Galileo, or one of those old-timers solve this?

Resorting my priorities for the day, also known as “procrastinating”, I read and read about the mysteries of these icicle ripples.

Here is what we do know about  them:

  1. Icicle ripples do not form on icicles formed from distilled water — an important finding!
  2. Icicle ripples do form on icicles made of water that contains impurities (salt, or other elements).
  3. Ripples form on icicles when the water has a very low concentration of impurities.
  4. The amount of impurities in the water only slightly affects the ripple size.
  5. The wavelength of the icicle ripples is nearly independent of the concentration of impurities in the water.
  6. The wavelength of approximately one centimeter (cm) appears to be universal.

Even though I am surprised that we do not yet have a theory to explain icicle ripples, it is kind of cool that we don’t.  It would be boring if we could explain all the physical phenomena we observe.

The first icicle that caught my attention to the ripple pattern is shown here with a falling pendant drop.  Note the regular spacing of the ripples.

Drip, drip, drip… icicles in an area sheltered from the wind.

The sunlight backlighting the icicles is what first caught my attention as a photographer.  The regular occurring pattern of ripples, which appear as white stripes with the same spacing  (~ 1.0 cm) caught my attention.

The small sunbursts seen in this photo are real.  They are caused by the diffraction of light in the smaller aperture of my camera lens.

The Overall Shape of Icicles

Icicles form when water drips from an overhanging structure and the air temperature is below freezing—the initial drips of water freeze into a tiny column of ice.  Subsequent drops of water flow down the surface of this initial icicle.  Some of that water flowing down the icicle will freeze, and if the water supply is sufficient, some of the flowing water will reach the tip of the icicle and form a “pendant drop.”  Only then can the icicle grow in length.  The pendant water drop grows in size and then falls.  Another pendant drop of water soon replaces it—drip, drip, drip.  The drop of water on the tip of the icicle is essential to the growth of an icicle.

When water freezes into ice, latent heat is removed from the water.  The energy released during the phase transition from water to ice at 0° C equals the energy required to melt ice into water at 0° C.  This energy is the latent heat of fusion.  I like to think of this as the energy required to melt ice into water is “stored” in the water and released as heat when water is frozen.

For the dripping water to transform into ice, the latent heat must have a place to go.  The freezing process requires the transfer of latent heat from the water.

More latent heat is released from the water drop at the tip of the icicle than at the base since more air is in contact with the pendant water drop.  The result is more cooling at the tip — causing the length of the icicle to grow faster than its diameter.

Icicles grow in length about 20-60 times faster than they grow in diameter — because the heat transfer at the tip of an icicle is 20-60 times greater than at the base(1)Makkonen, L. (1988).  A Model of Icicle GrowthJournal of Glaciology, 34(116), 64-70.  doi:10.3189/S0022143000009072.

Icicles Have a Hollow Tube Full of Water Inside Them

The tips of icicles are hollow.

A hollow tube, full of water, runs up the middle of the icicle.  This tube forms due to the most efficient mechanism for water on the tip of the icicle to lose its latent and turn to ice(2)Makkonen, L. (1988).  A Model of Icicle GrowthJournal of Glaciology, 34(116), 64-70.  doi:10.3189/S0022143000009072. The hollow tube of water sometimes reaches from the tip of the icicle almost to its base.

The ends of icicles are hollow.  Here is a pine needle pushed up inside the hollow tip of an icicle.

I was surprised that it appears we may know more about how snowflakes form than we do about icicle formation.

It looks as if the leading researcher, Steven Morris at the University of Toronto, studying ripples on icicles has made recent progress in understanding the causes of the ripples.  By introducing a fluorescent dye in the feed water they are finding that the surface of the icicle is not uniformly wetted by dripping water and the concentration of impurities in the feed water is more strongly related to the ripple effect than previously thought(3)https://iopscience.iop.org/article/10.1088/1367-2630/ac3cf4/metahttps://iopscience.iop.org/article/10.1088/1367-2630/ac3cf4/meta.

You can view the data and images of over 230,000 icicles carefully grown in a laboratory at the University of Toronto by visiting the Icicle Atlas.  The goal of these data is to involve other scientists to help solve the mysteries of icicles.

I will never look at icicles the same anymore.  It was a fun and exciting personal discovery for me, an accidental one, to see these ripples on icicles and find out more about them.

Photographing the cold of winter reveals interesting puzzles of nature to me — like seeing mountains apparently lifted up into the sky.

A drop of water,
freezing cold air all around,
gives up its heat to form ice

This conglomeration of icicles formed from the rain gutter spout that flows into one of my cisterns.  It was about 4 feet long and 5 inches in diameter at the top.  You can see the ripples with their universal 1 cm spacing.

The scientist does not study nature because it is useful; he studies it because he delights in it, and he delights in it because it is beautiful.

— Henri Poincare
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An interesting atmospheric optical effect.  The shadow of the mountain range projected onto sheets of virga.