Introduction

Water vapor is a key climate variable, serving to link a variety of complex and poorly understood processes. Although comprising less than 1% of the atmospheric mass, water vapor is the dominant gaseous absorber of thermal radiation. The disproportional importance of water vapor stems from the fact that it is the only atmospheric constituent to possess a permanent dipole moment. This feature, combined with the asymmetrical arrangement of mass in the water-vapor molecule, leads to a rich and complex distribution of absorption lines through the electromagnetic spectrum. Radiative absorption by water vapor not only plays a key role in determining the atmosphere's “greenhouse effect” but, because the concentration of water vapor depends strongly on the surface temperature, it also comprises the largest known feedback mechanism for amplifying global warming (ICCP, 1990). Current estimates are that radiative feedback by water vapor increases the climatic sensitivity to carbon dioxide by roughly a factor of two when considered in isolation from other feedbacks, and by as much as a factor of three or more when the interactions with other feedbacks are considered. In addition to its radiative effects, the strong dipole moment of water vapor is also responsible for the large latent heat associated with its phase transitions which, in turn, provides much of the energy for driving the atmosphere's largescale circulation.

The fundamental importance of water vapor in Earth's climate underscores the need for an accurate understanding of its distribution and variation.