ScienceWeek

HISTORY OF ATMOSPHERIC PHYSICS: ON CLOUDS

The following points are made by Graeme L. Stephens (American Scientist 2003 91:442):

1) On a December evening in 1802, Luke Howard (1772-1864), a London pharmacist and amateur meteorologist, aired his ideas about the classification of clouds. These ideas were presented to a small gathering of young science-minded intellectuals who called themselves The Askesian Society. Howard's lecture on that evening was titled "On the Modification of Clouds" and opened as follows:

"My talk this evening concerns itself with what may strike some as an uncharacteristically impractical subject: it is concerned with the modification of clouds. Since the increased attention which has been given to meteorology, the studies of various appearances of water suspended in the atmosphere has become an interesting and even necessary branch of that pursuit. If clouds were the mere result of condensation of vapour in the masses of the atmosphere which they occupy, if their variations were produced by the movements of the atmosphere alone, then indeed might the study be deemed a useless pursuit of shadows...."

2) This was a historic lecture for many reasons. Most importantly, it heralded the beginning of meteorology, a previously unrecognized area of natural science. This lecture was published the following year as an essay and appeared in subsequent publications over a span of almost 20 years. It is a remarkable testimony that Howard's classification, with minor changes, remains in use today by practicing meteorologists. His classification was a revelation, bringing a sense of order and understanding to a subject that had lacked coordinated thought --let alone any documented theories as to how pressure, temperature, rainfall and clouds might be related. Perhaps even more impressive than Howard's classification of clouds, or "modifications" as he referred to them, was his intuition, inspired by the earlier ideas of his close acquaintance John Dalton (1766-1844) that clouds must be considered as "subjects of grave theory and practical research governed by fixed laws." Howard's ideas about the physics of clouds were generally sound despite the poor understanding of the physics of air and water vapor in his time.

3) By contrast, for the past 50 years the modern science of meteorology has fixated on the ever-expanding capability of computer technology and the numerical prediction of the movements of invisible air. To the non-meteorologist this must appear most odd. Clouds, after all, are the most visible manifestation of weather in all its forms, and their prediction should be more than an object of curiosity. However, even the task of numerically integrating forward in time the Navier-Stokes equations governing the behavior of invisible air turned out to be substantially more complex than was originally conceived.

4) Present-day classification of clouds adopted by the World Meteorological Organization:

High clouds (bases > 6 km): cirrus, cirrocumulus, cirrostratus

Middle clouds (bases 2 to 6 km): altocumulus, altostratus, nimbostratus

Low clouds (bases < 2 km) stratocumulus, stratus, cumulus

The 10th type, cumulonimbus, extends through all ranges of altitude.

American Scientist http://www.americanscientist.org

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COSMIC RAYS, CLOUDS, AND CLIMATE

The following points are made by K. S. Carslaw et al (Science 2002 298:1732):

1) The correlation between cosmic rays and Earth's cloud cover over a solar cycle, first reported by Svensmark and Friis-Christensen in 1997, was hailed by some as the missing piece in the puzzle of understanding how the Sun could influence climate change. The intensity of cosmic rays varies globally by about 15% over a solar cycle because of changes in the strength of the solar wind, which carries a weak magnetic field into the heliosphere, partially shielding Earth from low-energy galactic charged particles. Although long suspected of having some influence on atmospheric processes, the correlation between cosmic rays and global cloudiness was to some researchers the clearest indication that such a link might exist.

2) Changes in cloud cover are important because clouds exert a strong control over Earth's radiative balance. Since the original observation, improved satellite data have become available and the cosmic ray-cloud effect seems to be present in low-altitude clouds. Because low clouds exert a large net cooling effect on the climate, this determines the sign of the possible cosmic ray-cloud effect: More cosmic rays are associated with more low clouds and lower temperatures. The observed variation of low clouds by about 1.7% absolute corresponds to a change in Earth's radiation budget of about 1 Wm^(-2) between solar maximum and minimum. This change in energy input to the lower atmosphere is highly significant when compared, for example, with the estimated radiative forcing of 1.4 Wm^(-2) from anthropogenic CO2 emissions.

3) In summary: It has been proposed that Earth's climate could be affected by changes in cloudiness caused by variations in the intensity of galactic cosmic rays in the atmosphere. This proposal stems from an observed correlation between cosmic ray intensity and Earth's average cloud cover over the course of one solar cycle. Some scientists question the reliability of the observations, whereas others, who accept them as reliable, suggest that the correlation may be caused by other physical phenomena with decadal periods or by a response to volcanic activity or El Nino. Nevertheless, the observation has raised the intriguing possibility that a cosmic ray-cloud interaction may help explain how a relatively small change in solar output can produce much larger changes in Earth's climate. Physical mechanisms have been proposed to explain how cosmic rays could affect clouds, but they need to be investigated further if the observation is to become more than just another correlation among geophysical variables.

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