Water and the Atmosphere

Water exists on the earth in all three states: (1) as a liquid when the temperature is generally above the freezing point of 0 C (32F), (2) as a solid in the form of ice, snow, or hail when the temperature is generally below the freezing point, and (3) as the invisible, molecular form of water in the gaseous state, which is called water vapor.

Over 98 percent of all the water on the earth exists in the liquid state, mostly in the ocean, and only a small, variable amount of water vapor is in the atmosphere at any given time. Since so much water seems to falls as rain or snow at times, it may be a surprise that the overall atmosphere really does not contain very much water vapor. If the average amount of water vapor in the earth’s atmosphere were condensed to liquid form, the vapor and all the droplets present in clouds would form a uniform layer around the earth only 3 cm (about 1 in) thick. Nonetheless, it is this small amount of water vapor that is eventually responsible for (1) contributing to the greenhouse effect, which helps make the earth a warmer planet, (2) serving as one of the principal agents in the weathering and erosion of the land, which creates soils and sculptures the landscape, and (3) maintains life, for almost all plants and animals cannot survive without water. If is the ongoing cycling of water vapor into and out of the atmosphere that makes all this possible. Understanding this cycling possible. Understanding this cycling process and the energy exchanges involved is also closely related to understanding the earth’s weather patterns.

Water tends to undergo a liquid–to-gas or a gas-to-liquid phase change at any temperature. The temperature of liquid water and the temperature of water vapor are associated with the average kinetic energy of the water molecules. The word average implies that some of the molecules have a greater kinetic energy and some have less. If a molecule of water that has an exceptionally high kinetic energy is near the surface, and is headed in the right direction, it may overcome the attractive forces of the other water molecules and escape the liquid to become a gas. This is the process of evaporation. A supply of energy must be present to maintain the process of evaporation, and the water robs this energy from the surroundings. This explains why water at a higher temperature evaporates more rapidly than water at a lower temperature. More energy is a available at higher temperatures to maintain the process at a faster rate.

Water molecules that evaporate move about in all directions and some will strike the liquid surface. The same forces that it escaped from earlier now capture the molecule, returning it to the liquid state. This is called the process of condensation. Condensation is the opposite of evaporation. In evaporation, more molecules are leaving the liquid state than are returning. In condensation, more molecules are returning to the liquid state than are leaving. This is a dynamic, ongoing process with molecules leaving and returning continuously. If the air were perfectly dry and still, more molecules would leave (evaporate) the liquid state than would return (condense). Eventually, however, an equilibrium would be reached with as many molecules returning to the liquid state per unit of time as are leaving. An equilibrium condition between evaporation and condensation occurs in saturated as long as (1) the temperature remains constant and (2) the processes of evaporation and condensation remain balanced. Temperature influences the equilibrium condition of saturated air because increases or deceases in the temperature mean increases or decreases in the kinetic energy of water vapor molecules. Water vapor molecules. Water vapor molecules usually undergo condensation when attractive forces between the molecules can pull them together into the liquid state. Lower temperature means lower kinetic energies, and slow-moving water vapor molecules spend more time close to one another and close to the surface of liquid water. Spending more time close together means an increased likelihood of attractive forces pulling the molecules together. On the other hand, higher temperature means higher kinetic energies, and molecules with higher kinetic energy are less likely to be pulled together. As the temperature increases, there is therefore less tendency for water molecules to return to the liquid state. If the temperature is increased in an equilibrium condition, more water vapor must be added to the air to maintain the saturated condition. Warm air can therefore hold more water vapor than cooler air. In fact, warm air on a typical summer day can hold five times as much water vapor as cold air on a cold winter day.

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