Plant and animal life of the lands, upon which man depends for much of his food, require fresh water. Man himself uses fresh water in many ways. The only basic source of fresh water is from the atmosphere through condensation of water vapour.
Water also leaves the land and ocean surfaces by evaporation and so returns to the atmosphere. Evidently, the global pathways of movement of water form a complex network. We have already mentioned that there is a global water balance, just as there is an energy balance, and that the water balance deals with flow of matter and so compliments the energy balance.
Water states and heat - Water occurs in three states,
(1) frozen as ice, a crystalline solid,,
(2) liquid as water, and
(3) gaseous as water vapour.
From the gaseous vapour state, molecules may pass into the liquid state by condensation, or if temperatures are below the freezing point, they can pass by sublimation directly into the solid state to form ice crystals. By evaporation, molecules can leave a water surface to become gas molecules in water vapour. The analogous change from ice directly into water vapour is also designated sublimation. Then, of course, water may pass from liquid to solid state by freezing, and from solid state to liquid state by melting. All of this can be represented by a triangle in which the three states of water form the comers. Arrows show the six possible changes of state.
Of great importance in weather science are the exchanges of heat energy accompanying changes of state. For example, when water evaporates, sensible heat, which we can feel and measure by thermometer, passes into a hidden form held by the water vapour and known as the latent heat of vaporization. This results, in a drop in temperature of the remaining liquid, the cooling effect produced by evaporation of perspiration from the skin is perhaps the most obvious example. For every gram of water that is evaporated, about 600 calories change into the latent form. In the reverse process of condensation, an equal amount of energy is released to become sensible heat and the temperature rises correspondingly. Similarly, the freezing process releases heat energy in the amount of about 80 calories per gram of water, whereas melting absorbs an equal quantity of heat. This is referred to as the latent heat of fusion. When sublimation occurs, the heat absorbed by vaporization, or released by crystallization, is still greater for each gram of water, for the latent heats of vaporization and fusion are added together.
"The world is a book, and those who do not travel read only a page", St. Augustine said. Here at www.travel-university.org we believe that every page must be read and explored. Travel is an avenue of learning that no text or classroom can teach. The world is a living classroom and you the student. We invite you to the www.travel-university.org library where you can read general interest and detail oriented articles.
