Thundersnow Electrifies Denver

Thundersnow Hits Denver

The phones lit up and the e-mails poured into the 24/7 Weather Center late on Friday as a relatively rare early February thunder-snowstorm hit between 9 PM and 11 PM.

It is not especially common to get thunder and lightning during the winter months, but it does happen periodically.  The winter air is usually too cold and heavy to be lifted high into the sky to produce thunderstorm clouds, but under certain conditions, there is enough instability in the air to form some small, but intense snow squalls that can produce thunder and lightning.  When these squalls develop, they are similar to summer thunderstorms in that they can drop very heavy snow over local areas in just an hour or two. The snow squalls typically last an hour or so before moving on to the east, much like a thunderstorm. When we get a really big storm, like the one in March of 2003, sometimes the thunder snow will last several hours and dump a foot or more of snow.

It does seem that an awful lot of our snow tends to occur at night, especially in Denver and along the Front Range foothills (elevations from 6,000 to 9,000 feet).

I have had folks ask "why 75% of snow falls at night" - that may be an overstatement, but I put the matter to Nolan Doesken our State Climatologist at CSU. Nolan has received this question often, and one of the graduate students at CSU actually has done a research paper on this matter. Some of this is a little technical, but I thought that I would throw it in anyway!

As for causes, it is likely a combination of radiational cooling, and mountain drainage wind patterns that raise relative humidity and enhance condensation. It is most apparent from the little data we have in Nov-Feb. From late Feb on, then convective processes kick in and begin tofavor the afternoon and early evening hours for the greatest snowfall.

I have compiled a variety of information about snow and snowstorms; I hope that you will find it useful and interesting...

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 and 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 15F or warmer since air can hold more water vapor at warmer temperatures.

More About Blue Snow and Ice...

It is a common misconception that the blue color exhibited by glaciers, old sea ice, or even holes poked into a snow bank is due to the same phenomenon that makes the sky blue-light scattering. But nature has more than one recipe for producing the color blue. In frozen water and in the sky the processes are almost the reverse of each other. A blue sky results when light bounces off molecules and small dust particles in the atmosphere. Because blue light scatters more than red does, the sky looks blue except in the direction of the sun (particularly when the sun is near the horizon and the blue light is scattered out of the sunlight, leaving the red color of sunrises and sunsets).

When light passes through ice, however, the red light is absorbed while the blue is transmitted. Were the operating process scattering as in the atmosphere, then the transmitted light would be red, not blue. However, because of the large size of snow grains and ice crystals, all wavelengths of visible light are scattered equally. Scattering therefore does not play an appreciable role in determining the color of the transmitted light.

It takes an appreciable thickness of pure ice to absorb enough red light so that only the blue is transmitted. You can see the effect in snow at fairly shallow depths because the light is bounced around repeatedly between ice grains, losing a little red at each bounce. You can even see a gradation of color within a hole poked in clean, deep snow. Near the opening, the transmitted light will be yellowish. As the depth increases, the color will pass through yellowish-green, greenish-blue and finally vivid blue. If the hole is deep enough, the color and light disappear completely when all the light is absorbed.

The color of ice can be used to estimate its strength and even how long it has been frozen. Arctic Ocean ice is white during its first year because it is full of bubbles. Light will travel only a short distance before it is scattered by the bubbles and reflected back out. As a result, little absorption occurs, and the light leaves with the same color it had when it went in.

Arctic explorers and mountain climbers know that old, blue ice with fewer bubbles is safer and stronger than white ice. An added bonus for explorers is knowing that floating camps built on blue ice will last longer.

Colorado Ski Country USA says all 26 resorts have received fresh snow is the past 36 hours. Combined, the ski resorts have seen 4,750 inches of snow this season, that's about 395 feet. Since Dec 1, it snowed 59 out of 62 days at the Colorado resorts. Southwest Colorado has been hit the hardest. This week, Wolf Creek received 60 inches of snow and Silverton 47 inches.