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Processes in meteorology can be quite fascinating, down to the finest details of how precipitation is formed. Perhaps you have wondered on a cold winery day when the sky turns silvery gray and the snow begins to fall, how nature creates the millions of exquisitely crafted ice crystals that gather into mounds of snow.
Snow formation begins very high in the atmosphere, usually between 10,000 and 20, 000 feet in elevation, where the temperatures are very cold, often between -20 to -40 degrees F. A snow crystal begins as water molecules start to gather on a tiny particle of dust, known in meteorology as condensation nuclei.
The structure of the water molecule allows them to form into a six-sided lattice creating a tiny hexagonal ice plate in very cold temperatures. The formation of these tiny ice plates can occasionally be seen in your backyard when fog forms on bitterly cold mornings. The process of water molecules gathering into ice crystals is known as deposition.
High in the atmosphere, the tiny ice crystals be
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gin to descend as they grow bigger. They must continue to pass through saturated air in order to grow. As the small ice crystals fall into the lower layers of the atmosphere, they encounter super-cooled water droplets. "Super cooled" means the water droplets are below freezing, even though they are still in a liquid state. A solid surface is needed to initiate ice formation. When a falling ice crystal encounters a "super-cooled" water droplet, the droplet will quickly freeze onto the ice crystal. This process is knows as riming.
As ice crystals continue to fall through saturated air, they will continue to grow by way of both deposition and riming. Larger crystals will overcome smaller crystals forming the clusters we know as snow flakes. If the air temperature is very cold during a snowfall, then the snow crystals may be too dry to gather into clusters, and will descend as individual crystals. Likewise, if air temperatures are relatively warm during a snowfall, perhaps at or near the freezing mark, the snow crystals may be wet, causing them to conglomerate into large snowflakes reaching as much as an inch in diameter.
Snow crystals come in a variety of exquisite sizes and shapes. The structure of a snow crystal may be able to indicate the temperature or humidity of the atmosphere is passed through as it formed and
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Exquisitely Crafted Ice Crystals
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descended. Slight variations in snow crystals may affect how slippery or sticky a freshly fallen surface of snow may be, and hence, how it may effect winter travel or winter related sports.
Things to think about as you watch the first, or the last, snowfall of the season.
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On Friday, June 15th of this year, 5 members of the NWS office here in Caribou completed a one day climb of Mt Katahdin. This represented just about one quarter of our entire staff and was especially impressive considering this was one of the hottest days of the summer. Temperatures reached a high of 90 degrees in Caribou and 95 in Bangor on that day. Fortunately, it was much more pleasant on the summit with temperatures estimated to be in the mid 70s. We ascended via the Abol trail and
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came down via the Hunt trail and despite the heat, all hikers had a successful trip with no significant problems encountered.
To see a larger picture, visit our website at:
www.nws.noaa.gov/er/car
Click on "Visit our Office", and scroll down a short distance.
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