The color photograph of the full Earth taken by astronauts on their way to the
moon ushered in a new age. Looking at Earth from space, we cannot see our political
boundaries. Nor can we usually see the effects of our own activities.
What was and is striking about the image of the Earth from space is its exquisite
and delicate beauty: the patterns of white clouds against the deep blue of the
oceans. These patterns alter from moment to moment, from day to day, from season
to season. They are probably quite different now than they were a billion or
4 billion years ago. Despite the fact that cloud patterns, amounts, and thicknesses
change, the worlds climate has remained remarkably constant, and no doubt
clouds play an important role in determining and regulating that climate. Defining
the role of clouds in climate is one of the trickiest problems for scientists
developing computer models of climate. (The modelers might dearly love to have
the clouds go away!) It is also essential to determining whether and how human
activities are upsetting the Earths delicate balance. To perceive, for
example, how pollution may be affecting clouds, we must understand what a cloud
is and what forces may shape or alter it.
These are questions that this module addresses. It introduces the basic features and classifications of clouds and cloud cover, and explains how clouds form, what they are made of, what roles they play in determining climate, and how they both cause and are affected by climate changes.