Ask Weather Dan: Winter precipitation – all about snow

After the last few winters we’ve had, Richmonders obviously have snow on the brain. Here Weather Dan offers up a treatise of sorts on that wintry precipitation that makes us all loony and sends us to the grocery store to wage war over that last loaf of bread.

February 8, 2011; 11:59 AM • by Weather Dan

Michelle asks:

Can you explain why we get snow vs sleet vs freezing rain?

Scott requests:

I would like a treatise on the relative fluffiness of the snow.

I can’t promise a treatise, but hopefully this will suffice. In order to avoid totally overwhelming you, I’m going to split this up into two parts. First, I’m going to go over the atmospheric conditions required to see snow.

To really understand what’s going on in the formation process for both snow and ice, we’ve got to first understand what’s happening in the upper atmosphere as these systems develop. What does that mean? Microphysics, that’s what!

As I’ve mentioned in some of my forecast discussions, the temperature in the troposphere – especially in the area between 5,000 and 15,000 feet – makes all the difference. While I’ve talked about how the appearance of a warm air layer within that range can alter precipitation type, what I haven’t talked about are the processes that go on aloft to even form precipitation to begin with.

Let’s start with the basics. This entire process is predicated on the existence of water vapor in the atmosphere. We’re all taught about the physical properties of water in school; while it exists normally as a liquid, it can change to a gaseous state above 212F, and to a solid below 32F. Within the atmosphere at any given time, you’ll find water existing in all three states at various locations. Within the lowest layer of the atmosphere, the troposphere, as a general rule, temperature will decrease with height. (This isn’t always the case – there are small features that can change this cause warm layers of air to appear above colder ones in the atmosphere. These are called inversions and worthy of another column all their own.)

In school, (hopefully) you learned about two processes — freezing and deposition. Freezing is pretty straightforward: you take water, expose it to air colder than 32 degrees F, and it becomes a solid under normal atmospheric pressure. Deposition you may have heard of before; in school, it’s often taught as sublimation. However, this isn’t entirely correct – sublimation only refers to a change from the solid phase to the gas phase. For example, exposing dry ice (solid carbon dioxide) to room temperatures will cause it to transition directly to a gas. Deposition is the reverse; in air that is below freezing water vapor will, rather than condensing to a liquid, change directly to a solid. Both of these processes happen during snow and ice formation, usually at temperatures between 0 and about negative 20 degrees Celsius, and are most efficient between -12 and -15 degrees celsius.

Different types of ice crystals will form at different temperatures within this range. The six-pointed snowflakes you’re commonly used to seeing are called dendrites, which form in that prime temperature range of -12C to -15C. Other shapes, including hexagonal columns, sheets, and other six-sided figures form at other temperatures.

So if snow forms in temperatures that is quite cold, and assuming that the temperature on the ground is also below freezing, why don’t we always see snow?

There are two factors that would keep snow from reaching the surface: temperature and moisture.

To best illustrate this point, we’re going to look at a diagram called a skew-t plot. If you’ve never seen one before, it’s a graph of the vertical structure of the atmosphere. Vertical height is plotted along the left hand side, in thousands of feet. Temperature is plotted across the bottom and skewed to the right, so that lines of equal temperature are on the diagonal. Two lines denote the vertical profile, a red line (on the right) denotes temperature, while the green line on the left indicates dewpoint.

The light blue line you see on the image denotes the 0 degrees Celsius temperature line. This is an ideal profile for snowfall. When the dewpoint and temperature lines are close together, the atmosphere is considered to be saturated — that is, no additional water can exist in vapor form. If a portion of the atmosphere isn’t saturated, then falling precipitation will evaporate before it reaches the ground.

Think of the atmosphere as a giant layer cake. Except instead of each of the layers being uniform, they’re of different sizes, and they’re changing on a regular basis. Some of these layers are warm, some are cold, and some are even colder still. Some are dry, and some are moist. In order to get snow to reach the ground, every layer in the cake, from the point aloft where the snow develops, all the way to the ground, has to be moist. And not only that, it has to be cold, too. While that range of -12C to -18C is best for snow development, in order for snow to make it all the way down, that temperature has to stay below 0C in every layer of the cake. As you can see in the sounding above, the red temperature line stays to the left of the 0C isotherm.

So now that we’ve established what conditions are best for snowflakes, we are left with determining what makes snowflakes “fluffy.”

[youtube]http://www.youtube.com/watch?v=D4i7vS_UO4Q[/youtube]
(Video included not because it adds any additional scientific value; it’s just so darn cute!)

Once snowflakes form and start falling, they have to pass through the remainder of those layers in the cake in order to reach the ground. As the temperature and moisture properties of the atmosphere change, snowflakes will be altered as well.

The key to wet, heavy snow is a very shallow (not very tall) warm layer at or very close to the surface. The water vapor capacity of air is a function of temperature; by warming the air, you increase the amount of water vapor that is mixed in. Let’s take a warm layer of air, very close to or slightly above freezing (somewhere ideally around 30-34 degrees Fahrenheit), and place it very near the surface. Once the warm air (warm in a relative sense, remember) is in place, once the layer becomes saturated with moisture, flakes will begin to melt slightly and accumulate condensed water while they fall, and begin to stick together. Large clumps of these flakes fall from the sky, and because of the large structure and extra water from being partially melted, the snow appears wet and is much heavier. This wet snow is much denser, and compacts easier once it reaches the ground and isn’t as easily transported by wind, and so doesn’t drift as much.

Conversely, as the temperature of the air closest to the surface drops, the snow becomes smaller, drier, and lighter. Once on the ground, wind transports drier snow much more easily, and drifts several feet deep can result in the right conditions.

If you’re wondering, snow CAN fall when the surface temperature is above freezing. What matters is the height of the layer of warm air at the surface. If it is shallow enough, the flakes will not have had enough time to melt before they reach the ground. If the ground is still below freezing as well, then snow can accumulate as well.

As anyone who’s lived in Richmond knows, snow is far from the only thing that falls from the sky during winter. There are some other classes of precipitation that we should explore. By modifying various layers in that big “cake” that is the atmosphere, different precipitation types can be created.

We’ll explore that a bit more in part 2.

Further (slightly more technical) reading:

“Determining Winter Precipitation Type.” National Weather Service, Louisville, KY.

February 8, 2011 11:59 AM 4