Page 218 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

182 part II The Water, Weather, and Climate Systems
Water is everywhere in the atmosphere, in vis­ ible forms (clouds, fog, and precipitation) and in microscopic forms (water vapour). Out of all the water present in Earth systems, less than 0.03%, some 12900 km3, is stored in the atmosphere. If this amount were to fall to Earth as rain, it would cover the surface to a depth of only 2.5 cm. However, the atmosphere is a key pathway for the movement of water around the globe; in fact, some 495000 km3 are cycled through the atmo­ sphere each year.
Water is an extraordinary compound. It is the only common substance that naturally occurs in all three states of matter: liquid, solid, and vapour. When water changes from one state of matter to another (as from liq­ uid to gas or solid), the heat energy absorbed or released helps power the general circulation of the atmosphere, which drives daily weather patterns.
This chapter begins our study of the hydrologic cycle, or water cycle. This cycle includes the movement of water throughout the atmosphere, hydrosphere, lithosphere, and biosphere. We discuss surface and subsurface components of the cycle in Chapter 9, and they are summarized in Figure 9.1. Although the hy­ drologic cycle forms a continuous loop, its description often begins with the movement of water through the atmosphere, which includes the formation of clouds and precipitation over land and water. These are the processes that power weather systems on Earth.
Water vapour in the air affects humans in numer­ ous ways. We examined the humidex, a summation of the effects of heat and humidity that cause heat stress, in Chapter 5. Water vapour forms clouds, which affect Earth’s energy balance. As discussed in Chapter 4, clouds have a warming effect, seen by comparing humid and dry regions over the world. In humid areas, such as the equa­ torial regions, clouds reradiate outgoing longwave energy so that nighttime cooling is decreased. In dry regions with few clouds, such as in the areas of the subtropical highs, nighttime cooling is more significant.
In this chapter: We examine the dynamics of atmo­ spheric moisture, beginning with the properties of water in all its states—frozen ice, liquid water, and vapour in the air. We discuss humidity, its daily patterns, and the instruments that measure it. We then study adiabatic processes and atmospheric conditions of stability and in­ stability, and relate them to cloud development. We end the chapter with a look at the processes that form clouds, perhaps our most beautiful indicators of atmospheric conditions, and fog.
Water’s Unique Properties
Earth’s distance from the Sun places it within a most remarkable temperate zone when compared with the positions of the other planets. This temperate loca­ tion allows all three states of water—ice, liquid, and
▲Figure 7.1 Surface tension of water. Surface tension causes water to form beads on a plant leaf. [céline kriébus/Fotolia.]
vapour—to occur naturally on Earth. Two atoms of hy­ drogen and one of oxygen, which readily bond, make up each water molecule. Once the hydrogen and oxygen atoms join in covalent bonds, they are difficult to sepa­ rate, thereby producing a water molecule that remains stable in Earth’s environment.
The nature of the hydrogen–oxygen bond gives the hydrogen side of a water molecule a positive charge and the oxygen side a negative charge. As a result of this polarity, water molecules attract each other: The positive (hydrogen) side of a water molecule attracts the negative (oxygen) side of another—an interaction called hydrogen bonding. The polarity of water molecules also explains why water is able to dissolve many substances; pure water is rare in nature because of its ability to dissolve other substances within it. Without hydrogen bonding to make the molecules in water and ice attract each other, water would be a gas at normal surface temperatures.
The effects of hydrogen bonding in water are observable in everyday life, creating the surface tension that allows a steel needle to float lengthwise on the surface of water, even though steel is much denser than water. This surface tension allows you to slightly overfill a glass with water; webs of millions of hydrogen bonds hold the water slightly above the rim (Figure 7.1).
Hydrogen bonding is also the cause of capillarity, which you observe when you “dry” something with a paper towel. The towel draws water through its fibres because hydrogen bonds make each molecule pull on its neighbor. In chemistry laboratory classes, students observe the concave meniscus, or inwardly curved sur­ face of the water, which forms in a cylinder or a test tube because hydrogen bonding allows the water to slightly “climb” the glass sides. Capillary action is an impor­ tant component of soil­moisture processes, discussed in Chapter 18.