By the end of this month winter is typically beginning to loosen its grip on Minnesota, which gives us an excuse to explore some of the different forms of snow and ice, which still remain with us.
The pretty patterns we can see here are frost patterns on my bedroom window. Frost is formed when air that contains some moisture comes into contact with a surface, like a window-pane, that is very cold; below freezing. This is very similar to what happens when you breathe on a mirror or a cold window. Your warm breath contains lots of moisture which condenses as tiny droplets of water onto the glass. The patterns in the frost may depend on many things; how clean and smooth the window is, how cold it is, how cold the air is, and how much moisture it contains. Some windows show the same sort of frost patterns each time they form, others show different sorts of patterns.
If there are scratches or dirt on the window then the frost will tend to start forming on them. If there are lots of places on the window where frost crystals want to start growing, or if lots of moisture condenses at once (because of a sudden change in temperature for example), then the frost crystals that form will be small. On the other hand if there are only a few places where the frost crystals start to grow, then it is easier for these crystals to grow rather than starting new ones. The pattern will then have only a few large and beautiful crystals. The images here show two different examples where different sized crystals can be seen together. Why do you think this might happen?
It is very difficult to get good pictures of snow and ice in the microscope, because the microscope is usually warm so the snow melts. However a dedicated man called Wilson A. Bentley spent many years photographing snowflakes in his back yard, and in 1931 he published them in a book called Snow Crystals. Today there are ways to look at snow and ice at higher magnifications in the scanning electron microscope (SEM). The two black-and-white images below (which should be moving if you have a browser that can view animated GIF files) are SEM images if ice. In the first movie (on the left) the ice is growing as frost, because the moisture in this particular SEM is condensing on the ice which has already formed and is still below freezing. In the second movie (in the middle) snow crystals which are still at a temperature below freezing are shown evaporating or subliming, because there is not enough moisture in the air around them. This is one way that we can lose a lot of snow cover in the spring. Another way when the temperature gets warmer is by melting. One of the more beautiful results of this melting is that icicles may be formed like the one shown here on the right.
Next time you see a frosty window see what patterns you can find there, and try and guess how they got to look like that.
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Last Update: 2/25/96