Ha! A Snow Day!!!

First Snow Day of the year! Teachers can use one every now and then. I couldn't have ordered it more perfectly ... 3-4 inches on the ground and more falling, gray skies, middle of the week, Vince Guaraldi's Charlie Brown Christmas playing and two little squibbers drinkin' hot chocolate and eatin' scrambled eggs and bacon!

Here are some Frequently Asked Questions from the website of the National Snow and Ice Data Center ...

How big can snowflakes get?
Snowflakes are agglomerates of many snow crystals. Most snowflakes are less than one-half inch across. Under certain conditions, usually requiring near-freezing temperatures, light winds, and unstable, convective atmospheric conditions, much larger and irregular flakes close to two inches across in the longest dimension can form. No routine measure of snowflake dimensions are taken, so the exact answer is not known.

Why is snow white?
Visible sunlight is white. Most natural materials absorb some sunlight which gives them their color. Snow, however, reflects most of the sunlight. The complex structure of snow crystals results in countless tiny surfaces from which visible light is efficiently reflected. What little sunlight is absorbed by snow is absorbed uniformly over the wavelengths of visible light thus giving snow its white appearance.

What causes the blue color that sometimes appears in snow and ice?
Generally, snow and ice present us with a uniformly white face. This is because most all of the visible light striking the snow or ice surface is reflected back without any particular preference for a single color within the visible spectrum. The situation is different for that portion of the light which is not reflected but penetrates or is transmitted into the snow. As this light travels into the snow or ice, the ice grains scatter a large amount of light. If the light is to travel over any distance it must survive many such scattering events, that is it must keep scattering and not be absorbed. The observer sees the light coming back from the near surface layers (mm to cm) after it has been scattered or bounced off other snow grains only a few times and it still appears white. However, the absorption is preferential. More red light is absorbed compared to blue. Not much more, but enough that over a considerable distance, say a meter or more, photons emerging from the snow layer tend to be made up of more blue light than red light. Typical examples are poking a hole in the snow and looking down into the hole to see blue light or the blue color associated with the depths of crevasses in glaciers. In each case the blue light is the product of a relatively long travel path through the snow or ice. So the spectral selection is related to absorption, and not reflection as is sometimes thought. In simplest of terms, think of the ice or snow layer as a filter. If it is only a centimeter thick, all the light makes it through, but if it is a meter thick, mostly blue light makes it through.

Is it ever too cold to snow?
No, it can snow even at incredibly cold temperatures as long as there is some source of moisture and some way to lift or cool the air. It is true, however, that most heavy snowfalls occur with relatively warm air temperatures near the ground - typically 15°F or warmer since air can hold more water vapor at warmer temperatures.

When is it too warm to snow? How does snow form if the ground temperature is above freezing?
Snow forms when the atmospheric temperature is at or below freezing (0 Celsius or 32 Fahrenheit) and there is a minimum amount of moisture in the air. If the ground temperature is at or below freezing, of course the snow will reach the ground.

However, the snow can still reach the ground when the ground temperature is above freezing if the conditions are just right. In this case, snowflakes will begin to melt as they reach this warmer temperature layer; the melting creates evaporative cooling which cools the air immediately around the snow flake.This cooling retards melting. As a general rule, though, snow will not form if the groud temperature is 5 degrees Celsius (41 deg Fahrenheit).

Why can snow fall when temperatures are above freezing?
Snow forms in the atmosphere, not at the surface. So snow can fall when surface temperatures are above freezing as long as atmospheric temperatures are below freezing and the air contains a minimum moisture level (the exact level varies according to temperature).

Is snow edible?
Clean snow is certainly edible. Snow in urban areas may contain pollutants that one should not eat but they would probably be in such low concentrations that it might not matter. Still, eating snow should be restricted to "wilderness" areas. Sometimes snow contains algae which gives it a red color. This snow can be eaten and some say it actually tastes "good" but we have never tried it.

Why do weather forecasters seem to have so much trouble forecasting snow?
Snow forecasts are better than they used to be and they continue to improve, but snow forecasting remains one of the more difficult challenges for meteorologists. One reason is that for many of the more intense snows, the heaviest snow amounts fall in surprisingly narrow bands that are on a smaller scale than observing networks and forecast zones. Also, extremely small temperature differences that define the boundary line between rain and snow make night-and-day differences in snow forecasts. This is part of the fun and frustration that makes snow forecasting so interesting.

Does snow change how sound waves travel?
Yes, when the ground has a thick layer of fresh, fluffy snow, sound waves are readily absorbed at the surface of the snow. However, the snow surface can become smooth and hard as it ages or if there have been strong winds. Then the snow surface will actually help reflect sound waves. Sounds may seem clearer and travel farther under these circumstances.

Why does snow crunch when you step on it? At what temperature does it crunch?
A layer of snow is simply composed of ice grains with air in between the ice grains. Because the snow layer is mostly empty air space, when you step on a layer of snow you compress that layer—a little or a lot, depending on how old the snow is. As the snow compresses, the ice grains rub against each other. This creates friction or resistance; the colder the temperature, the greater the friction between the grains of ice. The sudden squashing of the snow at lower temperatures produces the familiar creaking or crunching sound. At warmer temperatures, closer to melting, this friction is reduced to the point where the sliding of the grains against each other produces little or no noise. It's difficult to say at what temperature the snow starts to crunch, but the colder the snow, the louder the crunch.

Enjoy the day! Let it snow, let it snow, let it snow ...