Clouds, Chemistry and Climate

Owen B. Toon
Laboratory for Atmospheric and Space Physics
Department of Atmospheric and Oceanic Science
Duane Physics Building
Campus Box 392
University of Colorado
Boulder, CO 80309-0392
Email: toon@lasp.colorado.edu

Clouds are ubiquitous in planetary atmospheres. They are important for a number of reasons. They get in the way of remote sensing observations. Their composition, and behavior, can provide clues to processes occurring at the surface or within the atmosphere. Hence they may help provide signals of life. They play a major role in planetary climates and in the chemistry of planetary atmospheres. Finally, they are often beautiful. Clouds provide much of the color, motion and drama we associate with living on Earth, and with viewing other planetary objects.

There are several classes of clouds and aerosols. Clouds are usually thought of as volatile particles suspended in air. Their volatility links them closely to atmospheric dynamics since the vapor pressure is exponential in temperature, while temperature is controlled by dynamics or radiation. An aerosol is a suspension of particles in air. Hence, in its widest definition, clouds are a type of aerosol. However, aerosols are often thought of as being relatively involatile, or composed of such small particles that their sedimentation velocities are too low for significant precipitation.

The general production mechanisms of clouds and aerosols provide important clues to the behavior and distribution of clouds and aerosols. Aerosols are often generated mechanically, or by the condensation of a photochemically produced vapor whose vapor pressure is relatively low. The evaporation and condensation of a volatile species, however, produce clouds. These differences between clouds and aerosols can be a function of temperature. For instance, the upper level sulfuric acid clouds on Venus are so cold that they cannot evaporate easily, due to their low vapor pressures. Hence they have long lifetimes of weeks or months. In contrast, in the warmer lower sulfuric acid clouds temperatures are high enough for the particles to grow and evaporate in minutes. Hence one expects the lower clouds to be tied to dynamics and be highly variable, while the upper clouds should be much more uniform.

Some aspects of cloud physics can be seen in the cloud coverage. Mars and Earth are dominated by condensational clouds and are only partly cloud covered. Titan and Venus have high altitude photochemical clouds. These have long lifetimes and therefore cover the entire planet, despite dynamical motions that are both ascending and descending. Condensational clouds once again dominate Jupiter and Saturn. However, in this case the atmospheres are so deep that several overlapping areas of clouds are present so that the planets are essentially totally clouds covered.

The microphysical processes that govern clouds and aerosols are well known (Pruppacher and Klett, 1997; Seinfeld and Pandis, 1998). Fairly sophisticated models have been applied to Venus (James et al., 1997;Krasnopolsky and Pollack, 1994; Imamura and Hashimoto, 1998). Numerous observations of the clouds have been made (Esposito et al., 1983; Carlson et al. 1993, Grinspoon et al., 1993; Knollenberg and Hunten 1980). Similarly many models of Mars cloud physics have been designed (Colaprete et al., 1999; Forget and Pierrehumbert, 1997; Michelangeli et al., 1993). Likewise, some data on Martian clouds is available and more should soon be forthcoming from Mars Global Surveyor (Curran et al., 1973; Jaquin, Gierasch, and Kahn, 1986;Kahn, 1984). There are also numerous studies of the clouds of Titan and of the outer planets.

Clouds cover a huge range of physical dimensions, and particle sizes. The particle size range is similar to that from baseballs to planets. Not surprisingly the physics of the clouds varies significantly over such a large particle size range. There are several aspects of cloud physics that we take for granted on Earth, but which are not general. For instance it is quite possible to have rain without clouds. Clouds occur on Earth because it is relatively difficult to grow particles to large enough sizes for coalescence to occur. Similarly, significant supersaturations can occur without cloud formation. It is thought that as much as 20% of the upper troposphere of Earth is supersaturated with respect to ice, but clouds are not present due to the difficulty of ice nucleation. Such supersaturated regions may be even more common on cold planets.

Spectroscopically clouds are difficult to treat. If the particle sizes are comparable to the wavelength then they are usually neutral scatterers at wavelengths where absorption is not present. Hence it is difficult to obtain information about particle size and composition. Likewise observations at wavelengths where the particles are strongly absorbing provide little information if the particles are larger than the typical absorption pathlength for the material composing the clouds. In that case the clouds appear black or gray. For these reasons water clouds on Earth are best studied in the near infrared, where the absorption pathlength is changing rapidly with wavelength, or in the far infrared where the particles may be small compared with the observing wavelength.

References:

Carlson, R. W., et al. (1993). Variations in Venus Cloud-particle properties: A new view of Venus’s cloud morphology as observed by the Galileo near infrared mapping spectrometer. Planetary and Space Science, 41 471-486.

Colaprete,A., O. B. Toon and Julio A. Magalhaes (1999). Cloud formation under Mars Pathfinder conditions J. Geophys. Res., in press.

Curran, R.J., B.J. Conrath, R.A. Hanel,V.G. Kunde, and J.C. Pearl (1973).Mars: Mariner 9 spectroscopic evidence for H2O ice clouds, Science,182,381.

Esposito, L. W., R. Knollenberg, M. Marov, O. Toon, and R. Turco (1983). The clouds and hazes of Venus. In Venus (D. M. Hunten, L Colin, T. M. Donahue, and V. I. Moroz, Eds. ) pp 484-564, University of Arizona Press, Tucson.

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Michelangeli, D., O. B. Toon, R. M. Haberle, and J. B. Pollack, (1993) Numerical simulations of the formation and evolution of water-ice clouds in the Martian atmosphere, Icarus,100, 261-285.

Pruppacher, H. R., and J. D. Klett, (1997) Microphysics of Clouds and Precipitation, Kluwer, Dordrecht.

Samuelson, R. E., and L.A. Mayo, (1997). Steady-state model for methane condensation in Titan’s troposphere, Planet. Space Sci., 45, 949-958.

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