The first official day of Winter isn't until December 22, however in cities and towns across the nation temperatures have already begun to drop. Here in North Idaho, a blanket of freshly fallen snow covers Koinonia House. Like the staff here at K-House, many of you will have the opportunity to enjoy the poetry and beauty of the ice and snow of this special season.

Have you ever noticed that ice floats? Why?

Virtually every material substance contracts when it cools. As it gets warmer, the molecules increase their vibrational energy and require more room: the substance therefore expands as it warms. And, conversely, it contracts as it cools. Materials decrease in volume as they get colder. Water is the astonishing exception. It expands when it freezes into a solid. Why does water violate this general rule? Why does water expand when freezing?

The water molecule is a (not-so-simple) combination of two atoms of hydrogen bonded to one of oxygen. Yet this particular combination possesses an amazing array of unique characteristics that distinguish it from any other material known!

The Freezing Process

Although almost all materials decrease in volume as they get colder, water has an astonishing characteristic. As it drops toward its freezing point of 0^oC (32^o F), its volume also reduces until it reaches 4^oC, after which it actually increases. In a pond or lake, for example, this "inverse convection" has the salutary effect of bringing oxygen dissolved at the surface down to the lower depths for use by fish and other organisms. This process continues until the entire area has reached 4^oC. As the water cools below this temperature, it dramatically increases in volume, making it lighter than the water below. This ultimately causes the top layer to freeze, which then actually acts as an insulation layer against the very low temperature of the air above. If water did not have this strange property, the entire pond or lake would freeze solid and fish and other living creatures would be killed.

This expansion can have disastrous effects on uninsulated water pipes in winter. However, this expansion effect has essential functions in nature. The rain or dew penetrates the soil, and when it freezes, the soil is shattered into small particles, breaking up the hard earth into suitable conditions in which seeds can germinate.

Why This Exceptional Behavior?

This strange behavior derives from the unusual bonding relationship between the two hydrogen atoms and the one oxygen atom that make up a molecule of water, H₂O. The oxygen atom strongly attracts the single electrons of the two hydrogen atoms, leaving the two positively charged hydrogen nuclei rather free to attract other negative atoms. This attracts the oxygen molecules in other water molecules to form rather large, but loosely coupled, frameworks.

These atoms are not in a straight line, however, and the hydrogen atoms are bent toward each other, forming an unsymmetrical three-dimensional structure. The angle formed between the two hydrogen and the central oxygen atom is 104.5^o, almost precisely that of a hexagonal tetrahedron shape (109.5^o), so it can take up this shape (slightly warped three-dimensionally) with little stress on the bonds. Opposite the hydrogen atoms, the clouds of resulting negative electrification attract the hydrogen nucleus of an adjacent water molecule to form what is called a hydrogen bond - the key to water's peculiar behavior.

These tenacious hydrogen bond frameworks give water many astonishing characteristics, including anomalously high values for viscosity, surface tension, and the temperature and heats of melting and boiling. This results in its ubiquitous role as a solvent, its remarkable thermal properties, its surface tension and capillary action, and virtually innumerable chemical properties that are essential for life.

One would expect that these unique properties would make it a very rare molecule, produced only with great difficulty under laboratory conditions. The reality is, of course, quite the opposite: it is the most prevalent material on the planet, covering three-fourths of its surface, diffused extensively in the atmosphere, and - to the surprise of geologists - also found at astonishing depths in the earth. That such a unique substance should be in such abundance is in contradiction to any expectation of random chance alone.

Snow

Water vapor is a clear gas, which, as it cools under normal conditions, condenses and forms into water droplets. At high altitudes, water vapor can cool to below freezing, but in the absence of an impurity such as dust, around which it can collect, it will remain in this state.

When ice crystals form, the molecules of water arrange themselves in a specific pattern that is determined by the tetrahedral shape of the molecule in the frozen state described above. As further molecules join those already frozen, they give up their high latent heat of freezing, and melt the adjacent molecules, which reform to a shape dependent upon the local conditions of air temperature, wind currents, humidity, etc. Each snowflake pattern is unique to itself, but is always based upon the hexagonal bonding pattern of the ice crystals familiar to us all.

Snow also has a constructive role in the ecological cycle. It filters dust out of the air, absorbs nitrogen which then enters the soil, and acts as an insulating blanket to the plants and roots in the ground. The difference in temperature between the air and the ground covered by two feet of snow can be as much as 40^oC.

When snow melts, it requires considerable heat to effect this, and therefore melts slowly, lowering the rate of melt water and reducing the flooding that could occur if the latent heat of freezing were lower.

In addition to all these unique properties, snow also has the added ability of reflecting all the colors of the spectrum to yield pure white. Is no wonder that we often find ourselves in awe of the sheer beauty of a layer of freshly fallen snow. May the tell-tale signs of winter throughout the coming months serve to remind you of the wonderful works of our Lord.