The Clouds of the Middle Troposphere: Composition, Radiative Impact, and Global Distribution Kenneth Sassen • Zhien Wang Received: 29 June 2011 / Accepted: 14 November 2011 / Published online: 8 December 2011 Ó Springer Science+Business Media B.V. 2011 Abstract The clouds of the middle troposphere span the temperature range where both ice and liquid water in a supercooled state can exist. However, because one phase tends to dominate, of the two midlevel cloud types, altostratus are deep ice-dominated, while altocumulus are shallow water-dominated, mixed-phase clouds with ice crystal virga typically trailing below. Multiple remote sensor examples of these cloud types are given to illustrate their main features, and the radiative consequences of the different cloud microphysical compositions are discussed. Spaceborne radar and lidar measurements using the CloudSat and CALIPSO satellites are analyzed to determine the global distributions of cloud frequencies and heights of these clouds. It is found that together these little-studied clouds cover *25% of the Earth’s surface, which is about one-third of the total cloud cover, and thus represent a significant contribution to the planet’s energy balance. Keywords Tropospheric clouds Á CloudSat Á CALIPSO Á Lidar Á Radar 1 Introduction Clouds play a critical role in maintaining the energy balance of the Earth via their redistribution of the incoming solar radiation and outgoing infrared radiation (Stephens et al. 1990; Wielicki et al. 1995; Stephens 2005). The factors that affect the flow of radiation through clouds are controlled by a number of things, mainly their geographical locations and heights, and their microphysical properties including cloud particle phase, ISSI Workshop on Observing and Modeling Earth’s Energy Flows, January 10–14, 2011, Bern, Switzerland. K. Sassen (&) Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA e-mail: ksassen@gi.alaska.edu Z. Wang Department of Atmospheric Science, University of Wyoming, Laramie, WY, USA e-mail: ZWang@uwyo.edu 123 Surv Geophys (2012) 33:677–691 DOI 10.1007/s10712-011-9163-x