A World of Weather: Chapter 5 Introduction

CHAPTER 5

MOISTURE IN THE ATMOSPHERE
One of the most striking features in images of the earth taken from space is the overwhelming impression of blue, a majesty of color befitting the royalty of the oceans and seas that robe 70% of the planet's surface. No other planet in our solar system still has access to such vast reservoirs of liquid water, although Mars and Venus probably once had water on their surfaces. Channels in the Martian surface photographed by Mariner 9 in the early 1970s supported the possibility that the Red Planet was once warm enough to support running water (see Figure 5.1). Then, in July 1997, the first pictures from the Mars Pathfinder revealed a geological landscape consistent with a titanic flood that occurred long, long ago. Geological evidence suggests that water has likely flowed on earth for nearly 3.8 billion years, more than 80% of geologic time. Gravity likely played a role in water's initial appearance, given that large quantities of water are believed to have been transported to earth by meteorites. And gravity's tug holds water to our planet today.

Water is special not only because it covers 70% of the earth's surface, but also because water is the only known substance in the universe that can exist in gaseous (water vapor), liquid, and solid (ice) forms within the relatively small range of temperatures and pressures found near the earth's surface. This trinity of phases is the basis for the Bergeron-Findeisen process, an in-cloud give-and-take between water, ice and water vapor that creates much of the precious precipitation that falls to earth. In addition, large exchanges of energy take place between water and its surroundings whenever water changes its form - exchanges that have direct impacts on weather.