|Photomicrograph of a snowflake by Wilson A. Bentley. (Source: http://snowflakebentley.com/WBpopmech.htm)|
Birth of an Ice Crystal
As discussed in the previous post about general precipitation types, water comes in three forms- ice (solid), water (liquid), and water vapor (gas). Water changes between these three phases in the atmosphere depending mainly on the environmental temperature. An interesting feature about water is that it doesn't automatically freeze when the temperature drops below 0 degrees Celsius (C) or 32 degrees Fahrenheit (F) but actually can remain in liquid form until the temperature gets to -40 C (-40 F). The reverse is not true, however, and ice always melts when the temperature gets above 0 C (32 F). The water droplets that exist between -40 to 0 C (-40 to 32 F) are called "supercooled liquid water droplets" because they are just so hip and with-it, right? Just kidding, but they do allow for a lot of interesting details allowing for snowflake growth.
So let's say the temperature within a cloud is about -10 C (14 F) and there are a whole bunch of supercooled liquid water droplets hanging around. If the temperature doesn't decrease by much, how do you get ice growth since all of the droplets won't spontaneously freeze at that warm temperature (which is known as homogeneous nucleation)? The answer is heterogeneous nucleation which can take three forms: growth by deposition, growth by aggregation and growth by accretion. We'll get to those methods in the next section. To understand each of these methods of growth we need to discuss one more thing-- water vapor pressure.
Water and ice behave differently when interacting with water vapor. We know that with liquid water there are the processes of evaporation (changing phase from liquid to gas) and condensation (going from gas to liquid) and those processes occur simultaneously and constantly when the air is saturated with respect to liquid water. If the air is sub-saturated then more evaporation occurs than condensation versus when the air is super-saturated in which case more condensation occurs (and that's linked to how raindrops form!). Similar processes occur for ice, or solid phase water, and are called sublimation and deposition. Sublimation is like evaporation because water goes from the ice phase to vapor phase (skipping the middleman or the liquid phase). Deposition is like condensation or when water vapor changes phase straight to ice (again skipping the liquid phase). So water vapor interacts with both liquid water and ice, but how does it choose which one to interact with when they are mixed together? That is where vapor pressure comes into play. There is an equilibrium state for both liquid water and ice for it to exist in a saturated environment which would mean the rate of evaporation = condensation and the rate of sublimation = deposition. The pressure exerted by the vapor molecules in each of those separate phase scenarios is known as the saturation vapor pressure and they are different values for the liquid phase and the solid or ice phase. The saturation vapor pressure with respect to ice is less than that of liquid water which means that if given the choice, water vapor will preferentially react with the ice and help the ice to grow instead of reacting with the liquid water. Deposition wins over condensation in those cases which is good news for snowflake growth! Below is an idealized schematic for how the different phases of water may exist in the atmosphere. Notice how the temperature decreases with height so there are more ice crystals at higher altitudes.
A snowflake is born when supercooled water droplets freeze, most likely onto an ice nuclei which is a small piece of ice that can be found within a cloud around temperatures of -10 C (14 F) that may have been a piece of salt or even dirt that water vapor can freeze upon. That ice nuclei can smash into supercooled water droplets and cause them to freeze on contact. The best freezing occurs when an ice nucleus becomes completely submerged within a supercooled water droplet and cause it to completely freeze over, symmetrically. Due to the shape of the water molecule itself, an ice crystal will grow to usually mimic that shape and form into a hexagon, or six-sided polygon. Once there exists an ice crystal, a snowflake is born!
Growing Up (or Radially Outward)
The snowflakes you've seen likely aren't little tiny hexagons but rather the beautiful six-cornered geometric masterpieces that make for wonderful wintertime decorations. But how do snowflakes grow from tiny ice nuclei to what you've seen on your coat? The answer is just like how we grow up- with time and travel, of course!
The three methods of snowflake growth that I introduced above include the following: growth by deposition, growth by aggregation and growth by accretion. Let's start with growth by deposition.
As stated above, deposition means water vapor freezing directly into ice, or in this case onto an ice crystal. When an ice crystal is present in a cloud with a bunch of supercooled water droplets, water will actually be transferred from the droplet to the ice crystal because of the difference in saturation vapor pressure between liquid and ice. The image below depicts that process and was borrowed from the UCAR COMET module "Topics in Precipitation Type Forecasting."
Snowflakes only lose their symmetry if they crash into other snowflakes and break like what can happen during growth by aggregation. Growth by aggregation is a process by which snowflakes collide and stick together. They can become physically intertwined (like how your paperclips always do) or pieces can actually melt at the edges to fuse snowflakes together (usually around 0 C (32 F)). The diagram shown above of the layers of the atmosphere illustrates how ice crystals may grow aloft and then fall down through layers containing other types of snowflakes, so they are likely to interact.
Growth by accretion is also known as growth by riming. This occurs when a snowflake falls through supercooled liquid water and the droplets actually freeze upon its surface. This process can be seen in the diagram below.
The snowflakes that reach the ground sometimes grow by all three processes described above. In fierce winter storms, known as blizzards, intense updrafts of vertically-moving air and gusts of wind can take snowflakes on such an incredible journey that they don't even know who they are anymore when they reach the ground. They have melted, refrozen, gathered more snowflakes and sometimes even remained boring liquid rain. The previous post described all of the precipitation types we are so familiar with on Long Island (including sleet, yuck)! While the most gentle snowfall of flakes growing slowly within a cloud and falling straight downwards towards the ground may yield the "prettiest" dendritic snowflakes, it's important to appreciate the story that even the "ugly" snowflakes tell us upon reaching the ground. And that is just what some scientists are doing.
Reaching the Ground and Catching the Curiosity of Scientists
The history of snowflake observations goes back not quite to Ancient Greece (given their warm, Mediterranean climate so they missed out on some great times with nature) but to about the middle of the 13th century. For example, curious minds such as Albertus Magnus pondered their shape and later Johannes Kepler wrote a paper trying to understand why snowflakes have six corners in 1610. More and more scientists and travelers wanted to understand why snowflakes were so artistic and tried to sketch their melting, fleeting forms by hand without the best success. It was actually a non-scientist, bachelor farmer in Vermont who begun the pioneering work of photographing them through a microscope.
Wilson A. Bentley lived from 1865-1931 in Jericho, Vermont and spent his lifetime taking photomicrographs, or photos through a microscope of snowflakes that fell during most storms that hit his farm. He had no weather charts (initially) nor training in meteorology but did have some standard atmospheric measurement equipment (with a thermometer being the most important!) and a microscope, blackboard, and a camera. He was humble, curious and diligent with his photographs and likely struggled a little bit financially because he sold copies of his images for only 5 cents! An incredible book written by an incredible scientist who worked in cloud microphysics and cloud seeding, Duncan C. Blanchard, is called "The Snowflake Man: A Biography of Wilson A. Bentley" and is a highly recommended read. If you ever travel up north to Vermont, they have a museum in Jericho with some of his equipment and snowflake images. Check out this youtube video for a short documentary if you just can't wait to see more but can't travel up there anytime soon! The Snowflake Man is truly a gem to meteorology and shows how anyone, as long as they have the curiosity, can do great things for science.
There has been a lot of research on snowflakes and snow crystal growth since, both through observations and using computer models. There are still a ton of unanswered questions in this relatively new science and this PBS article highlights a few of them. On a local note, Dr. Brian Colle and members of the Coastal Meteorology and Atmospheric Prediction Group are continuing to look into the stories that snowflakes tell us when they reach the ground and if a weather model can even try to get that story right. He is currently working with Ruyi Yu to compare weather model results of snowflake growth with observations taken from aircraft data from a flight right through a snowband. That research is still being completed and is supercool! (Pun intended.)
A Special Look into Snowflake Research on Long Island
Dr. Brian Colle has been working with a few of his graduate students for a number of years to understand how computer models calculate snow crystal growth. He has been using observations on the ground from a vertically pointing radar located on the roof of a building at Stony Brook University's School of Marine and Atmospheric Sciences, the local NOAA/National Weather Service NYC Radar located in Upton, NY, a particle distrometer that can differentiate between rain and snow and drizzle that is also up on the roof at SBU, and his own photomicrographs. Yes, Long Island has it's own Snowflake Man!
He most recently published a paper with a recent graduate student (David Stark now at the NOAA/NWS NYC Weather Forecast Office) and a collaborator (Dr. Sandra Yuter of NCState) on the observed snowflake habits during two East Coast winter storms. During a number of storms while you were likely relaxing with a cup of cocoa after frantically buying some bread and milk (or at least that's what I was doing!) Dr. Colle and David would stay up all night if necessary to take snowfall measurements and photomicrographs of what was falling to the ground. They would then go back and analyze recorded atmospheric conditions such as temperature and vertical air motion to really understand what was going on above our heads that would lead to the distinct snowflake habits. Below is a snapshot of some photomicrographs during a snowstorm on December 19th, 2009. The images are not as picturesque as those of Wilson A. Bentley because they wanted to capture the whole complicated story of what was falling to the ground whereas Bentley would take a feather to his slide and push away all of the "unwanted" flakes. The diversity of the snow habits on their slides really provides insight into the complex storm dynamics and temperature structure that were occurring.
|Stark et al. 2013: Fig. 7.|
Would you like more information? Check out the following resources:
- For more information about ice nucleation, refer to this College of Dupage webpage: http://weather.cod.edu/sirvatka/bergeron.html
- Sign up with a free account from the University Corporation for Atmospheric Research (UCAR) Meteorological Education (MetEd) COMET Program and learn all about precipitation type at this site: http://www.meted.ucar.edu/norlat/snow/preciptype/
- Scientific American Article by Charles A. Knight: http://www.scientificamerican.com/article.cfm?id=why-do-snowflakes-crystal
- Stark et al. 2013 article about the microphysics within two East Coast winter storms: http://journals.ametsoc.org/doi/abs/10.1175/MWR-D-12-00276.1?prevSearch=[Contrib%3A+david+stark]&searchHistoryKey=
- All other sources of information are found as embedded links within the above article.