From outer space our planet Earth reveals its true nature. It appears as a tiny blue sphere traversed by swirling bands of white, wispy clouds. The blue is due to liquid water that covers 72% of the earth's surface while the white clouds consist of water droplets and ice crystals suspended in its atmosphere. The polar ice caps of frozen water add a further touch of solid white.

Many of the familiar physical, chemical and biological processes on Earth could not occur without the presence of water. Water has played an important role in regulating the Earth's thermostat over the past several billion years. Water is all-important in transporting heat energy from the tropics to the poles. The layer of the atmosphere (the troposphere) where all weather phenomena occur is where most atmospheric water resides. Our bodies are mainly water. Water is a universal solvent that we use liberally in our everyday lives. Water is so ubiquitous that we tend to take it for granted; yet water is one of the strangest naturally occurring substances in the entire universe.

Water, unlike most substances, reaches its maximum density as a liquid rather than a solid. For this reason, ice floats allowing lakes and the polar seas to build an insulating layer of ice protecting the underlying waters from freezing. If water behaved like most substances, ice would sink to the bottom, and these bodies would freeze solid making it impossible for most marine life to survive.

When pressure is applied to most solids, they become denser. Ice, however, behaves quite differently. Water is most dense as a liquid at 4° C above its freezing point. When ice is compressed, it becomes denser and warms to its liquid state i.e. it melts! The pressure of a car moving over an icy road actually warms and melts the ice beneath its tires creating a thin layer of liquid water, which lubricates the surface making the road seem slippery. When the air temperature is very cold, this effect is much reduced and tires seem to stick to the road. Ice itself is not very slippery. If it were not for this same effect, it would be difficult to ice skate or ski. The pressure from our body weight melts the snow beneath our skis creating a thin, lubricating layer of liquid water. Similarly, copious melt waters issue from the base of the Sound's glaciers because pressure from the overlying ice sheet compresses and melts the ice at the glacier's base.

Water is peculiar in another way. It takes more heat energy than any other naturally occurring substance to change it from its liquid to its gaseous state. For example, heating 1 gram (a sugar cube sized sample) of water from its melting to its boiling point would require 100 calories. But to change that same gram of water to one gram of steam would require an additional 597 calories. Secondly, water is slower to absorb heat and to release stored heat than any other naturally occurring substance except liquid ammonia. For example, it takes five times as much heat to raise the temperature of water 1°C as the same amount of sand. These properties are important in determining Prince William Sound's weather and climate and indeed the weather and climate of the Earth in general.

Unlike most familiar compounds and elements, water exists as a liquid, a solid, and a gas in precisely the narrow temperature range that exists at or near the Earth's surface. Oddly enough, the very presence of water in our atmosphere and oceans may in turn be the reason for this narrow temperature range. Water is a potent greenhouse gas that has warmed and regulated the temperature of our planet over the past several billion years. Although we are familiar with solid iron bars that melt and various liquids which vaporize, it is usually the familiar but unconventional compound, water, we use to form our conception of the three phases of matter.

To understand what causes our weather and climate, we must first examine the fundamental nature of water and of the other gases which make up our atmosphere. Physicists tend to conceptualize all forms of matter as fundamentally empty space occupied by extremely small electrically charged or neutral particles in a constant, rapid motion. Depending on how substances reflect and refract light and how they resist the pressure of our sense organs, we classify them as liquids, solids, or gases. Physicists, however, attempt to understand these gross properties by visualizing the three states of matter as the interactions of tiny charged and uncharged particles known as atoms, ions and molecules. For the purposes of mathematical modeling, physicists treat these particles as tiny billiard balls exhibiting a certain degree of attraction for each other yet always in constant motion-either vibrating in place, sliding past one other or randomly bouncing off each other with perfect elasticity. Furthermore, physicists posit that a substance's temperature is nothing more than a measure of the motion of its molecules-the greater the temperature, the faster the molecules are moving; and conversely, the faster the molecular motion, the higher the temperature. In solids and liquids the attractive forces between molecules oppose this motion. In gases, it is a billiard ball, bouncing free for all.