Monday, December 23, 2013

A Snowflake's Journey is Deciphered on the Ground

It is now officially the winter season and we've already had a few snow events on Long Island in the late fall of 2013. Although we likely won't be able to look outside and see more snow before the Holiday Season is over, there are plenty of snowflake decorations adorning buildings, shops and lawns. There is a rich history of science associated with snowflakes, all of which started with observations using the human eye like you've likely done when snow falls on your coat or gloves. You've heard the saying that no two snowflakes are alike but sometimes the ones that fall at one time are very similar. Therefore, snowflakes tell a story about the atmospheric environment above our heads that they fell through in order to reach us on the ground that's definitely worth investigating.

Photomicrograph of a snowflake by Wilson A. Bentley. (Source:

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." 

It takes less energy for water vapor to freeze onto rough and jagged edges than over smooth, even surfaces of the ice crystal so that is why each of the six snowflake "arms" get to grow to such lengths radially outward from the center of the ice nucleus at a rapid rate. Each arm grows the same way at the same time which is why snowflakes are so symmetric. However, how they grow is dependent on the temperature through which they are traveling. That's how a snowflake can have six arms that all tell the same complex story of how they formed. Check out this video of crystal growth to see what I'm talking about! Since it's highly unlikely that two snowflakes took the same exact journey through the same exact path in the atmosphere-- that's why Wilson A. Bentley coined the conjecture that "no two snowflakes are alike!" Snowflakes have been classified into types, or habits, that are very dependent on temperature. The following chart shows the favored snow habits according to the atmospheric temperature in which they grow. The most "ideal" six-pronged snowflakes are called dendrites and typically form when the atmospheric temperature is between -12 and -16 C (3.2 to 10.4 F) in a layer appropriately known as the dendtritic growth zone.


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.

As the snowflake travels towards the ground (as indicated by the black arrow) it falls faster than the water droplets around it so it encounters many and they freeze onto its surface. This process of riming makes for less "idealistic" snowflakes but allows for great snowman-making snow on the ground!

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.
By taking such observations through many winter storms, they were able to match certain snow habits to certain phases of a cyclone or certain distances between Long Island and the surface cyclone center. Be on the lookout for more of their published results! Although David has graduated with his Masters degree, Dr. Colle soldiers on and continues to be Long Island's own Snowflake Man to increase the understanding and prediction of what will fall on the ground of Long Island by understanding each flake's incredible journey.

Would you like more information? Check out the following resources:

Sunday, December 8, 2013

Rain, Sleet or Snow? Wouldn't You Just Love To Know?!

If you've lived on Long Island for at least one winter then you'll know from experience that the precipitation type associated with some storms can be quite mixed up. Take the February Blizzard of 2013, for example. Most of the Island saw rain to start which transitioned to sleet in some places before becoming snow, while other locations along the South Shore just received rain. The processes that are responsible for the precipitation type that you receive on the ground are fairly well-understood, but require really high resolution data to be able to predict it. Temperature and moisture data for the atmosphere are most important in determining what type of precipitation will fall on the ground.

Background Information
A fundamental piece of knowledge is that water comes in three forms-- solid (ice), liquid (water) and gas (water vapor). When water changes phase from one form to another it either absorbs a small amount of energy in the form of heat which cools the surrounding environment or it releases that heat energy which slightly warms the surrounding environment. For meteorology, a phase change is only really dependent on the environmental temperature, or the air temperature surrounding the precipitation particle (aka hydrometeor). Air that is below freezing or below 32F/0C will support snow and ice but air that is above freezing which is above 32F/0C only supports liquid precipitation or rain. For more information, check out some of the chemistry lessons from the Khan Academy. The air above the surface can be above freezing, but if the surface is at or below freezing then the rain can freeze on contact with the surface; this is known as freezing rain.

If you know the temperature and how it changes with height (called a vertical profile of temperature) then shouldn't you be able to predict what type of precipitation will fall on the ground? The environmental temperature changes a lot due to the motion of the winds moving air from surrounding regions around and from the precipitation itself through reactions. An important reaction within areas of precipitation are when ice and snow melt into rain that absorbs heat energy which slightly cools the environment. The reverse is true, so when rain freezes into sleet that process actually releases a small amount of heat into the environment. Therefore, the environmental temperature is quite dynamic, but tends to mostly be regulated by advection of surrounding temperature values by the wind. The National Weather Service New York Office created a phenomenal video describing how hydrometeors change depending on the environmental temperature. Take a moment to check it out!

NOAA/NWS New York created a great video and are active on Facebook to discuss mixed precip.

 Moisture is also important because if the air is not saturated then water (liquid or ice) will evaporate into the air to try to allow it to reach saturation. This is related to a temperature-dependent quantity known as relative humidity (RH). If the relative humidity is < 100% then any rain or snow that falls into that layer of low RH will partially evaporate and evaporation, like snow melting, absorbs heat from the environment which results in a slight cooling of the air. The temperature and moisture as given by the dew point temperature near the surface can provide some clues about whether to expect warm or cold precipitation. If your surface temperature is above freezing, but your dew point temperature is a lot lower (i.e. you are pretty dry at the surface with a low RH) then you'd expect your surface temperature to cool due to evaporation at the onset of precipitation when the rain or snow falls into that dry layer and works hard to saturate it. That temperature can even be readily calculated from temperature, RH and surface pressure and is known as the wet-bulb temperature.

Now that you've been briefed on how important knowing the atmospheric temperature and moisture is during times when temperatures are near freezing we can discuss the observations and forecasting of the event of 8-9 December 2013 that is currently underway!

Surface temperature values are readily available through a relatively dense network of observations that are monitored by the National Weather Service. Check out this cool interactive display (click on observations to see surface temperature, dew point and wind). What about understanding the temperature profiles with height? That's where weather balloons come in handy. The balloons provide temperature, dew point and wind data at various points extending throughout the troposphere, or the lowest layer of the atmosphere that all of our weather is confined to. The plots can be a little confusing to look at but once you get used to them, they provide a ton of useful information.

12 UTC (7 AM EST) OKX Sounding 8 Dec.
 Balloons are launched twice a day at 12 UTC (7 AM EST) and (7 PM EST) from over 100 stations across the U.S. including the NWS New York City Office located in Upton, NY (abbreviated as OKX) which is about 20 miles east of Stony Brook University. The sounding plot to the left shows two curves, the red temperature curve and a green dew point curve. They are plotted with pressure and height on the y-axis with the surface at the bottom. Highlighted on that blue bar is an isotherm, or a line of constant temperature that slope at an angle. That is the 0C isotherm or 32F which shows that the temperatures contoured to the left of the blue line are below freezing and to the right of the blue line are above freezing. At around 7 AM EST this morning, all of the air above our heads was below freezing, which would support snow as the dominant precipitation type.

While these are useful, there are only soundings available two times a day. What if the precipitation falls in between those times like it most often does? For that, forecasters must rely on model data and even model soundings, or temperature and moisture profiles that are output from the weather models. Here's where the forecasting challenge truly lies, in my opinion. We've discussed how sensitive the precipitation type is to temperature, especially whether the environment is above or below freezing. What if the favored model is off by a few degrees and those degrees mean the difference between above or below freezing and therefore rain, snow or a mix? That's where the uncertainty lies.

Forecasters monitor real-time data such as radar, satellite and surface observations to compare what is actually happening with what the models showed might happen. Updated model output is available more frequently during the day than weather balloons, but it's important to check whether the models are on the right track with reality. Forecasters take their knowledge and observations and use it to scrutinize the next model data coming in. For example if one model (e.g. ECMWF) had the temperature and precipitation all wrong for the 1 PM forecast, then at 1 PM a forecaster would take the ECMWF with a grain of salt.

A new useful tool for precipitation observations is called mPING which uses crowd-sourcing to get data. Do you want to submit your own precipitation observation? Just download the iPhone or Android app and submit something today! This tool was developed by scientists at the National Severe Storms Laboratory (NSSL), University of Oklahoma (OU), and the Cooperative Institute for Mesoscale Meteorological Studies (CIMMS) and is definitely used by forecasters and scientists so help us out and if you see something falling-- mPING it and check out other observations near you!

Event Overview and Model Forecast
As of 3 PM EST on 8 December, a moderate snow band was located across Philadelphia and stretched eastward over the Atlantic Ocean but translated eastward staying well south of Long Island. There is high pressure in place over New England that is slowly slipping eastward and as it does, low-level winds should move from out of the north to more out of the east and southeast which will advect in warmer temperatures at low-levels.

As the evening progresses, NYC and Long Island could see some light snow that may accumulate up to a couple inches before the warm air is advected into the region that may change it to sleet and then freezing rain before becoming all rain on Monday. At Stony Brook University, the column has been moistening all day from how dry it was from this morning's soundings by the precipitation aloft. The image below shows data from a vertically-pointing radar that provides fall speed at the top (how fast the hydrometeors are falling to the ground) and reflectivity which provides an idea of the size and concentration of hydrometeors. There's been snow falling but evaporating around 1.5 miles above the surface at 10 AM EST to only about a half-mile above the surface at 3:30 PM EST. According to this trend, we would expect snow to reach the ground by 5:30 - 6:00 PM EST.

Dr. Brian Colle provided this snapshot of the time (x-axis) vs. height (y-axis) data from the vertically-pointing radar on the roof at SoMAS at SBU.
 While temperatures are expected to remain below freezing for the start of the precipitation, the SBU-WRF simulated soundings of temperature with height show that after 1 AM EST, the temperatures at low-levels and near the surface should warm to well above freezing (~ 7C/44F) by 7 AM EST. In between 1-7 AM there is the chance for freezing rain, as air above the surface warms faster than the air at the surface leaving the surface temperature near or below freezing.

If you have any questions about precipitation type, don't hesitate to comment on this post!

For more information about winter weather from the NWS New York Office, please visit this site:
For SBU-WRF data, please visit this site:
If you see something falling, mPING it! For more info please visit this site: