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.