Weather 101 - Let it Snow, Let it Snow.

My son (who lives and works in Minnesota) was talking to a colleague and the subject of. "Is it too cold to snow?" surfaced. That's because it was quite cold in Minneapolis and snow was in the offing. When the storm ended, about 7 inches of snow had covered the Minneapolis area. Much of the snow fell with surface temperatures below 20 degrees; some even accumulated as temperatures fell to below 15 degrees (Fig 1). In the clouds above, temperatures were much colder.

To answer this question, I turned to Dr. Kenneth G. Libbrecht, a noted snow researcher at Caltech. Dr. Libbrecht has documented almost all that is known about snow crystal formation at his web site.

Snow Crystals

Snow crystals, the umbrella classification of the different types of snow that can form in the atmosphere, include prisms, columns, plates and various other crystalline forms. How each of these form hinges on cloud temperature and atmospheric water vapor content (linked to temperature). The mostly hexagonal shapes are tied to how the atoms of hydrogen and oxygen interlink; the branching or dendritic patterns occur because the ends of the crystals stick out into the air more and become the easiest places on which water vapor can transform directly into ice (a process known as deposition).

To better appreciate the patterns and the complexity (yet simplicity) of these crystalline forms, take a look at this table showing relatively simple graphical representations of many of these snow crystal forms. There are other, more complicated snow classification systems, as well.

Dr. Libbrecht grows snowflakes in his laboratory under controlled conditions to better understand how they form. This is important because conditions in the atmosphere can be highly variable in both time and 3-dimensional space. Remember that the snowflakes that you see on the arm of your ski jacket formed in clouds thousands of feet above you and then had to travel all the way down to terra firma. Conditions in the clouds are often much different that you experience at the ground. For example, have you ever seen it snow when the air temperature you experience is 40 degrees or more? I have. Hence cloud temperatures had to be at least below freezing.

This growth behavior is summarized in a "snow morphology diagram," that describes preferred crystal shape under different conditions of temperature and moisture. Perhaps the most striking characteristic is that more simple shapes (those without branching patterns) occur when moisture values are ever so slightly lower; the more moisture in the air (at any given temperature), the more likely that flakes will be dendritic. The most extreme shapes -- long needles around 20 degrees F and large, thin plates around 5 degrees F -- form when the humidity is especially high. And, dendritic shapes, most likely to catch onto other dendritic shaped-flakes occur when air temperatures are close to the freezing/melting point and the snowflake surfaces become "wet." This capturing process creates those "cotton ball" or "silver dollar" sized snowflake clumps that so many of love to gawk at as they literally drop from the sky.

Is It Ever Too Cold To Snow?

Which brings us to the question that prompted this article - "Is it ever too cold to snow?" And the answer, based in part on the snowflake morphology diagram, and from observations, is that it can snow at temperatures even as low as -40 degrees F. For example, it snows at the South Pole, where temperatures rarely top - 40 degrees F. And it snows in Alaska, as well (Fig. 2).

But, Libbrecht notes that, "it doesn't snow so much when the temperature is below around - 4 degrees F." This is because as moist air cools, it starts producing snow before it gets that cold. By the time the temperature drops further, the snow has already fallen and the air becomes drier. Even though clouds filled with ice crystals may remain, these don't produce much, if any snow.

Higher up in the sky, cirrus clouds, always filled with ice crystals, may start to produce snow. But falling from heights of 20,000 feet to 60,000 feet or more, these crystals will sublimate (transform from ice to water vapor) on their descent. It is rare for crystals to fall from cirrus clouds and reach the ground.

Still, individual snow events (and even different parts of particular storms) can have very different snow:liquid water content ratios. Typically, the average ratio of 10:1 (i.e., 10 inches of snow melts to one inch of liquid or rainfall) is used in discussions. But, for the Minneapolis storm of December 7-9, 2009, the ratio actually turned out to be 20:1. 7.4 inches of snow fell and this yielded a scant 0.37 inches of liquid. With a dry snowfall like this, it was not surprising that the fallen snow blew around and drifted so much.

Enjoy!

So, the only things to do now are to bundle up, get out there and shovel your sidewalk and driveway and then have fun playing in the snow. And when the next snow arrives, whatever, the air temperature, just be ready to observe those flakes as they fall on your ski jacket. Check out their shapes and compare them to what's expected from the snow morphology diagram. Finally, extend your tongue and catch some of those tasty flakes!