Describe the movement of water through the hydrologic cycle. List and describe water’s unique properties.

Water is found everywhere on Earth—in the oceans, glaciers, rivers, lakes, air, soil, and living tissue. The vast majority of the water on or close to Earth’s surface (over 97 percent) is saltwater found in the oceans. Much of the remaining 3 percent is stored in the ice sheets of Antarctica and Greenland. Only a meager 0.001 percent is found in the atmosphere, and most of this is in the form of water vapor.

Movement of Water Through the Atmosphere

The continuous exchange of water among the oceans, the atmosphere, and the continents is called the hydrologic cycle (Figure 4.1). Water from the oceans and, to a lesser extent, from land areas evaporates into the atmosphere. Winds transport this moisture-laden air, often over great distances, until the process of cloud formation causes the water vapor to condense into tiny liquid cloud droplets.

The process of cloud formation may result in precipitation. The precipitation that falls into the ocean has ended its cycle and is ready to begin another. The remaining precipitation falls over the land. A portion of the water that falls on land soaks into the ground (infiltration) to become groundwater. The remainder, which flows along Earth’s surface, is called runoff.

Much of the groundwater and runoff eventually returns to the atmosphere through evaporation. A smaller amount of groundwater is taken up by plants, which release it into the atmosphere through a process called transpiration (or evapotranspiration).

The total amount of water vapor in the atmosphere remains fairly constant. Therefore, the average annual precipitation over Earth must be roughly equal to the quantity of water lost through evaporation. However, over the continents, precipitation exceeds evaporation. Evidence for the roughly balanced hydrologic cycle is found in the fact that the level of the world’s oceans is not dropping.

This continuous movement of water through the hydrologic cycle holds the key to the distribution of moisture over the surface of our planet and is intricately related to all atmospheric phenomena.

Water: A Unique Substance

Water has unique properties that set it apart from most other substances. For instance, (1) water is the only liquid found at Earth’s surface in large quantities; (2) water is readily converted from one state of matter to another (solid, liquid, gas); (3) water’s solid phase, ice, is less dense than liquid water; and (4) water has a high heat capacity—meaning changing its temperature requires considerable energy. All these properties influence Earth’s weather and climate and are favorable to life as we know it.

These unique properties are largely a result of water’s ability to form hydrogen bonds. Hydrogen bonds are the attractive forces that exist between hydrogen atoms in one water molecule and oxygen atoms of any other water molecule. To better grasp the nature of hydrogen bonds, let’s examine a water molecule. Water molecules (H2O) consist of two hydrogen atoms that are strongly bonded to an oxygen atom (Figure 4.2A). Because oxygen atoms have a greater affinity for the bonding electrons (negatively charged subatomic particles) than do hydrogen atoms, the oxygen end of a water molecule acquires a partial negative charge. For the same reason, the hydrogen atoms of a water molecule acquire a partial positive charge. Because oppositely charged particles attract, a hydrogen atom on one water molecule is attracted to an oxygen atom on another water molecule (Figure 4.2B).

Hydrogen bonds hold water molecules together to form the solid we call ice. In ice, hydrogen bonds produce a rigid hexagonal network (Figure 4.2C). The resulting molecular configuration is very open (lots of empty spaces). When ice is heated sufficiently, it melts. Melting causes some, but not all, of the hydrogen bonds to break. As a result, the water molecules in liquid water display a more compact arrangement and no rigid structure. This explains why water in its liquid phase is denser than it is in the solid phase.

Because ice is less dense than the liquid water beneath it, a water body freezes from the top down. This has far-reaching effects, both for our daily weather and aquatic life. When ice forms on a water body, it insulates the underlying liquid and slows the rate of freezing at depth. If water bodies froze from the bottom, many lakes would freeze solid during the winter, killing its aquatic life. Further, deep bodies of water, such as the Arctic Ocean, would never become ice covered. Such changes in freezing patterns would alter Earth’s energy budget, which in turn would modify atmospheric and oceanic circulation patterns.

Water’s heat capacity is also related to hydrogen bonding. When water is heated, some of the energy is used to break hydrogen bonds rather than to increase molecular motion. (Recall that an increase in average molecular motion corresponds to an increase in temperature.) Thus, under similar conditions, water heats up and cools down more slowly than most other common substances. As a result, large water bodies tend to moderate temperatures by remaining warmer than adjacent landmasses in winter and cooler in summer, as discussed in Chapter 3.

• hydrogen bond: The attractive force that exists between hydrogen atoms in one water molecule and oxygen atoms in another water molecule.

• hydrologic cycle: The continuous movement of water from the oceans to the atmosphere (by evaporation), from the atmosphere to the land (by condensation and precipitation), and from the land back to the sea (via stream flow).

• The unending circulation of Earth’s water supply is called the hydrologic cycle.

• Water has unique properties: (1) It is the only liquid found at Earth’s surface in large quantities; (2) it is readily converted from one state of matter to another (solid, liquid, gas); (3) its solid phase, ice, is less dense than liquid water; and (4) it has a high heat capacity—meaning it requires considerable energy to change its temperature.