When winter comes, bird baths solidify, roads glaze, lakes and rivers freeze, pipes burst, and exposure causes hypothermia and frostbite. Learning some physics of water and ice will reveal how ice also furnishes protective functions in natural systems.
The Shore Effect
People who live along the shore know it is warmer in winter and cooler in summer there than inland. That is because water requires more energy to change its temperature than the air and soil around it. This is a property of water called “latent heat.” It takes one calorie of heat to change the temperature of a milliliter (ml) of water one degree Centigrade. A quart of water has a volume of about 1000 ml. Thus, for a quart of water to change temperature by one degree Centigrade, it would require about 1000 calories. Food Calories (note this word is capitalized) are each composed of 1000 calories, and one Calorie will change the temperature of a quart of water by one degree Centigrade. All bodies of water are heat sinks, and a lot of heat can be pumped into or taken out of them without changing their temperatures by very much.
Trees Moderate Temperature
A summer's cool forest glade is one result of water's latent heat. Leaves and stems are composed of living cells and each of these cells is about 80% water. A single tree may hold hundreds, sometimes thousands of quarts of water above ground, and this water absorbs a tremendous amount of the sun's energy. In addition to water's properties, photosynthesis utilizes another 10% of the sun's energy. By eliminating huge heat sinks and shade generators as they shed leaves, deciduous trees ensure that their trunks and roots will remain as warm as possible during the winter: Enough to resume photosynthetic activity in the twigs on warm winter days. Winter temperatures underneath leafless trees are much higher than under evergreens, and deciduous trees transport nutrients stored in roots earlier than do evergreens.
Snow and Ice Insulative Properties
Snow is honeycombed with air pockets and can serve as a thermal blanket to protect organisms beneath it. We see effects of this insulation when late spring snowstorms are accompanied by deep frost. Buds exposed to the air are killed by the frost while the buds covered by snow are protected. After the temperature warms, the shrubs may bloom and leaf out at the base but have temporarily naked twigs at the top. Animals sometimes utilize these snow blankets: Mice and other rodents move beneath it, grouse sometimes fly directly into a snowbank on a cold winter's evening, and many forest birds survive by roosting overnight in the quaminik, an open space under snow-covered evergreens, rather than in a naked tree.
As water freezes, the individual molecules that once moved past each other become locked in an ice lattice and release energy that warms the surroundings. Farmers in Florida make use of this attribute by spraying water on tender fruits and vegetables on nights they expect frosts. The ice forming on their surfaces both warms and insulates the crops, preventing frost damage. The opposite occurs when the ice melts: It takes energy to loosen the molecules from the ice lattice and that energy is taken from the environment, cooling the surroundings. Unless there is sunlight, ice may not melt until the temperature is one or two degrees warmer than freezing.
Ice Floats
Ice is lighter than water and floats because molecules in ice are further apart than molecules in liquid water. If ice sank, there would be no insulating barrier between the water and the air. Pond, stream, and lake surfaces would continue to freeze at the surface as the ice dropped to the bottom. These bodies of water would rapidly become solid blocks of ice with no free water for fish and other wildlife to swim in, and the animals would freeze solid. There would be no dissolved oxygen to support life, and animals and plants would suffocate. Additionally, the layer of meltwater covering the ice surface would reduce heat transfer to deeper layers and the bottoms of deep bodies of water might remain frozen all summer, inhibiting the growth of rooted plants.
Biologists often state that life would be impossible without water. Scientists may never know whether this is true or not, but latent heat, resistance to thermal change, and density are protective qualities of water for life as we know it. Water has many other qualities necessary for living cells, but there is not enough space to discuss these here.
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