SNOW

Here in Minnesota, we spend a lot of time thinking about snow during this time of the year. At Rock Leaf, snow has a huge impact on our daily planning: when is the snow getting here, how much of it will fall, how will it impact our travel/work, and how good will the skiing be? If snow has also been impacting your plans lately, you may have wondered how and why this stuff forms.

Snow forms in the lower portion of Earth’s atmosphere. This happens when moisture is present and when the temperature is below freezing (32°F). Since warmer air is able to hold more moisture, heavy snows usually occur when ground temperatures are between 15°F and 32°F. During Rock Leaf’s recent Plot Based Inventory (PBI) trip to central Minnesota, we had a few very cold but snow-free days. The low temperatures (around -15°F) and low air moisture content created the crisp, sunny conditions you see in the photo below. So, if the conditions are right, frigid temperatures can make for beautiful days!

Snowflakes form as ice crystals within clouds stick together, and the structure of these ice crystals is influenced by the moisture and temperature conditions they are formed in (see Snow Crystal Diagram). You may have heard that no two snowflakes are exactly the same. While snowflakes may form in similar shapes, their shape changes in response to fluctuations in temperature and moisture as they fall through the air. The likelihood of two snowflakes experiencing identical conditions as they journey from a cloud to the ground is very low, but it isn’t impossible. If you want to take a look at the shape of falling snow, a great way to do this is to catch some snowflakes on a dark surface (e.g., a jacket sleeve, hat, or glove). A hand lens or magnifying glass can help if the crystals are very small.

Snow Crystal Morphology Diagram. Credit: Kenneth G. Libbrecht, http://www.snowcrystals.com/

The excitement doesn’t end once the snow has fallen. Changes to snow crystals can happen once they’ve reached the ground surface. Snow can melt and refreeze to form layers of ice within the snowpack. Layers of snow can be compressed by the weight of additional snowfall, and heavy winds can break down snow crystals and pack them tightly together. Between the layers of snowpack and the ground surface are small spaces known as subnivean (Latin for “under” and “snow”) zones. Small mammals like mice and voles use these spaces to avoid predators and to keep warm. Air temperatures in the subnivean zone are kept stable because the snowpack contains so much trapped air. Snow is actually an excellent insulator.

So when you’re trudging through the snow this winter, know that there are many factors that created the specific conditions of the stuff crunching under your boots. If you want to learn more about snow, the National Snow and Ice Data Center is a great resource for snow info and data: https://nsidc.org/learn/parts-cryosphere/snow.