It’s winter, and it finally snowed in these parts, and these two facts have combined to make my son quite happy. Every day after I pick him up from his preschool, he asks if we can go walk in the snow – something that turns into us hiking across snow-covered fields and throwing snowballs at each other, and which ends with us having to leave snow-caked shoes in the hallway and hanging snow-covered coats and gloves (and sometimes pants) in the bathroom to dry.
“Why does it snow?” he asked yesterday, after we were back inside and bundled up under some lovely fleece blankets.
Uhm… I haven’t the slightest idea. I mean, I know it must be related to the concept of rain. Both rain and snow are precipitation, after all. But I don’t really know why snowflakes form instead of, say, sleet. Which makes this a great question! And a great question to combine with another one he asked, which came up while he was drinking some water: “why is ice made out of water?”
So let’s find out.
What is “Freezing”?
To understand freezing, we’ll need to take a quick look at matter. For help with this, we’ll turn to LiveScience.com and Matter: Definition & the Five States of Matter. They define matter as “anything that has mass and takes up space). The state of matter is the form the matter takes, with the most common states being solids, liquids, gasses, and plasma. (The article lists Bose-Einstein condensate as the fifth state; Wikipedia lists twenty-one different states). Matter can also be said to be in a phase, which is “a region of space (a thermodynamic system) throughout which all physical properties of a material are essentially uniform”.
Using ice as an example, an ice cube is water in the solid state in a phase the size of the cube. Ice cubes in a glass of water then represent a system which has water in two states (solid and liquid) and two phases (the liquid volume, and the volume of the ice cubes).
With that in mind, we’ll turn to Purdue University and Freezing:
When a liquid is cooled, the average energy of the molecules decreases.
At some point, the amount of heat removed is great enough that the attractive forces between molecules draw the molecules close together, and the liquid freezes to a solid.
The temperature of a freezing liquid remains constant, even when more heat is removed.
The freezing point of a liquid or the melting point of a solid is the temperature at which the solid and liquid phases are in equilibrium.
The rate of freezing of the liquid is equal to the rate of melting of the solid and the quantities of solid and liquid remain constant.
What happens when water freezes?
When water freezes, it undergoes a phase change from liquid to solid. Energy is removed from the water molecules, slowing them down and making them become more dense. However, it hits maximum density at 4 degrees Celsius (39.2 degrees Fahrenheit) – below that temperature, water starts getting less dense.
As water begins to freeze, the molecules crystallize into open hexagonal structures.
This hexagonal structure contains more space than liquid water, making it less dense. So, by the time it has fully hardened into a solid, it floats on top of the liquid water.
How do snowflakes form?
Snowflakes start off just like rain – as water droplets forming around pollen or dust. The distinctive shapes arise because of crystalline hexagonal structure I mentioned above. A single water crystal will have six sides, and this causes the crystals to build up into a symmetrical pattern as they grow in size. The specifics of the shape are determined by the temperature and the atmospheric conditions:
Ultimately, it is the temperature at which a crystal forms — and to a lesser extent the humidity of the air — that determines the basic shape of the ice crystal. Thus, we see long needle-like crystals at 23 degrees F and very flat plate-like crystals at 5 degrees F.
The intricate shape of a single arm of the snowflake is determined by the atmospheric conditions experienced by entire ice crystal as it falls. A crystal might begin to grow arms in one manner, and then minutes or even seconds later, slight changes in the surrounding temperature or humidity causes the crystal to grow in another way. Although the six-sided shape is always maintained, the ice crystal (and its six arms) may branch off in new directions. Because each arm experiences the same atmospheric conditions, the arms look identical.
Sleet, Hail, and Frozen Rain
All of this made me wonder, though. If snow is literally created by the same process that creates rain, where to the other types of “winter weather” come from? That is, sleet, and hail, and frozen rain. Fortunately, NOAA has the answer.
Snow generally begins life in the “dendritic growth zone” (aka the “snow growth zone”), a layer in the atmosphere with temperatures between 10.4 and 0.4 degrees Fahrenheit (-12 to 18 degrees Celsius), and cannot form if the atmospheric temperature rises above 32 degrees Fahrenheit (0 degrees Celsius). Additionally, the relative humidity of the atmosphere must be at 70% or greater. Note, however, that the dendritic growth zone is not a formal layer of the atmosphere (hence the term “zone” instead of “layer”) – it can form at different altitudes, depending on the weather.
At some point – one which is determined by the size of the snowflake and the weather conditions – the snowflake falls. It may partially melt as it falls, but as long as it passes through no more than “a very shallow melting layer” (no more than 1,500 feet thick) that is no more than 33.8 degrees Fahrenheit (1 degree Celsius) and then refreezes. In fact, partially melted and refrozen snowflakes help produce the wet snow that is so beloved of people who build snowmen and pack snowballs.
Sleet starts life as snow, and goes through a similar process to the creation of wet snow. However, it hits a melting layer that is less than 2,000 feet thick and has a temperature between 33.8 and 37.4 degrees Fahrenheit (1 to 3 degrees Celsius). All of which is a fancy way of saying that sleet is partially melted snow.
Freezing rain also starts life as snow. However, it hits a melting layer with a temperature above 37.4 degrees Fahrenheit (3 degrees Celsius). This makes it melt completely. The exterior cools below freezing as it falls, but it doesn’t fall long enough to turn into sleet before it hits the surface. But, since the exterior is at the freezing point of water (or extremely close), it freezes on contact with the surface.
Hail is as unrelated to snow as anything that starts out life as “ice crystals in the upper atmosphere” can be. It israin that gets blown up into extremely cold layers of the atmosphere and freezes. Then it falls when the wind won’t hold it up, only to (possibly) get blown upwards again and have more layers of ice freze around it. This can continue until the updrafts that keep throwing it upwards are no longer able to do so.