Thursday, August 15, 2019

How the Alta Snowflake Is Really Created

It may be hard to believe, but not everything you read or see on social media is true.  Often, the fallacy is cleverly hidden because it is distributed by what appears to be a reputable source.  Even experts and professional groups can be fooled or make mistakes.

Several days ago, @ZonePhysics, issued the tweet below showing how a drop of water turns into a snowflake.

That certainly looks like a convincing video and a reliable source.   Some people suggested the video showed the creation of the Alta Snowflake depicted on the Alta blue-dot logo below.

It's a wonderful story, but unfortunately it's not true.

The video above doesn't show the formation of a snowflake, it shows the melting of a snowflake.  The video is run in reverse, so it appears the water turns into a snowflake.  It's convincing, but misleading and inaccurate.

The Alta Snowflake is a rendition of a snowflake known as a stellar dendrite.  Stellar dendrites have six arms shaped like a tree (dendrite means tree like) and a star-like appearance.  Hence the name.  Ken Libbrect's great guide to snowflakes includes photos of many types of stellar dendrites.    

Snowflakes of this type do not form from water turning into ice.  When a large water droplet, say a raindrop freezes, you get a ball of ice, often referred to in the United States as sleet.  Sleet is rare in Utah, but a common feature of the so called "transition zone" between rain and snow found in the warm-frontal region of cyclones over the central and eastern United States.

Instead, stellar dendrites form primarily from water vapor condensing directly to ice through a process known as deposition. Liquid water is not involved at all.  It is possible to demonstrate the process in a cold chamber under the right conditions.  Here's an example produced by Ken in his lab.  

Ken has many more videos of snowflake growth available at  Knock yourself out.  

All of this is not to say that water turning into ice doesn't contribute to the growth of real-world snow. Most snow-producing clouds consist of a mixture of ice particles (including snowflakes) and liquid cloud droplets.  The liquid cloud droplets are colder than 0˚C (32˚F), but have not yet frozen.  Scientists call this supercooled.  Under the right conditions, these droplets collide with and freeze onto snowflakes and other ice crystals through a process known as accretion or riming.  Below are images, reproduced from my book and taken by the Electron and Confocal Microscopy Laboratory of the U. S. Department of Agriculture, of a lightly rimed ice crystal known as a plate (top right), moderately rimed dendrite that has been broken up some (center right), and a heavily rimed ice particle in which the original ice particle or snowflake is indistinguishable (lower right).  The latter is called graupel.  

Source: Secrets of the Greatest Snow on Earth
Stellar dendrites, because of their long branches and arms, typically produce low-density (i.e., dry) snow because of all the gaps and pores within the crystal structure.  Real storms are comprised of many different types of snowflakes and ice particles, but if you want blower pow, stellar dendrites are going to be abundant.  Thus, the Alta marketing people knew what they were doing.  On the other hand, the Alta skiing cognoscenti know that graupel, despite its high density, also produces amazing skiing.  It just wouldn't look that good as the basis for a logo.  

1 comment:

  1. I showed my daughters the two videos without giving any info and asked which was the video of a growing snowflake. They got it right due to "snow needing to start on a crystal...I read it in a book" and "I saw an episode on Magic School Bus about it". I guess they don't like listening to me but glad they're learning it somewhere!