Weather Almanac for February 2006
JOHN TYNDALL
SKY BLUE AND GREENHOUSE GASES
 In the annals of science, the names of those individuals who made landmark discoveries have often become well known outside the scientific community. Watson and Krick, Newton, Archimedes are but a few among those so remembered. But many others are relative unknowns outside their specialty field today. One such Nineteenth Century scientist is John Tyndall, a scientist whom the Encyclopedia Britannica linked with Darwin and Huxley as inseparably connected mid-19th Century scientists who strove to make science part of general life. Though generally forgotten today, one of Tyndall's great discoveries is the topic of many news items these days: the so-called greenhouse gases including carbon dioxide. It was Tyndall who first recognized that carbon dioxide readily absorbed heat radiation leaving the Earth's surface, inventing a spectrophotometer in order to make the critical measurements for his theory. But Tyndall was not a "one-hit wonder" as he also was the first to explain. more or less correctly. why the sky is blue.
John Tyndall was born in the village of Leighlin Bridge, Carlow, Ireland in 1820. In addition to his common school education, Tyndall's father tutored him in many other subjects, particularly theology. Leaving school at 17 with a firm background in basic mathematics, surveying and English composition, Tyndall joined the Irish ordinance survey as a practical surveyor and draftsman two years later. When friends thwarted his desire to emigrate to America, Tyndall became a railway engineer in 1844, travelling extensively in Great Britain during the construction of the railway network.
In 1847, he took a position as mathematics master at Queenwood College Hampshire. His friendship with the science master, a young chemist named Edward Frankland, resulted in both men deciding to go to Germany together for a formal science education. Arriving in Marburg, Germany in 1848, the pair was given lab space by Robert Bunsen. Tyndall enrolled in introductory lectures on chemistry, physics and calculus. Under Bunsen's inspiration, Tyndall completed all doctoral degree work in under two years. Decided to concentrate on physics, Tyndall began studies on diamagnetism and the magnetic optical properties of crystals. This was to be his major research for nearly six years, and the research published in his treatise on Crystals was read across the scientific world. That lead to studies in 1851 at the Berlin laboratory of Professor Heinrich Gustav Magnus where he won the respect of many of the finest scientific minds in Germany.
Returning to England in 1851, Tyndall spent another two years at Queenwood College, supplementing his income through translating and reviewing foreign science for the Philosophical Magazine. During this time he became friends with many influential scientists such as Thomas Huxley and Michael Faraday. In 1853, Tyndall was named Fellow of the Royal Society and appointed Professor of Natural Philosophy at the Royal Institution, working alongside Faraday. In 1862, Tyndall succeeded Faraday as Royal Institution Superintendent.
Like many of the Victorian-era scientists, John Tyndall took an interest in a wide variety of subjects including chemistry, physics, geology and climate, and he often combined his work with his recreation. After visiting the Alps on holiday in 1849, Tyndall returned there yearly to study the glacier formations. His visit to Switzerland in 1856 in the company of Huxley resulted in a joint treatise: On the Structure and Motion of Glaciers. Tyndall became the first to climb Weisshorn in 1860, and climbed Mont Blanc several times. His mountaineering adventures were chronicled in his 1860 book Glaciers of the Alps.
In addition to his deep understanding of several scientific fields, Tyndall had the great skill of making difficult scientific conceptions understandable and entertaining to the laymen, both in lecture and in writing. As a result, he rose to become one of the foremost lecturers of his day, in the grand company of Faraday and Huxley. In 1872-73, Tyndall toured throughout the United States giving popular science lectures. Although the lecture tour earned him several thousand dollars, he took none of it and asked it be directed to advance American science.
According to the Eleventh Edition of the Encyclopedia Britannica, "His strong, picturesque mode of seizing and expressing things gave him an immense living influence both in speech and writing, and disseminated a popular knowledge of physical science such as had not previously existed. But besides being a true educator, and perhaps the greatest popular teacher of natural philosophy in his generation, he was an earnest and original observer and explorer of nature."
At the Royal Institution he continued his study of diamagnetism and then branched out into other endeavours including research on radiant heat, spontaneous generation and the germ theory of disease, glacier motion, sound, the diffusion of light in the atmosphere and a host of related topics. These studies brought Tyndall a great reputation among his scientific peers.
During his lifetime, Tyndall published more than 16 books and 145 papers, including Faraday as a Discoverer. He received five honorary doctorates and was elected a respected member of thirty-five scientific societies.
Tyndall also used his scientific knowledge for practical applications, inventing an improved fog horn, the fireman's respirator, and the "light pipe." Originally constructed using just a torch and a bucket of water, this latter device is the forerunner of the gastroscope used in hospitals today to view inside the stomach. The light-pipe concept eventually led to the development of fibre optics.
By the mid-1880s the bouts of ill health and the sleeplessness that had plagued Tyndall for years began to take their toll. In 1887, he resigned from the Royal Institution and retired to Hampshire. The Summer of 1891 marked the first time in over thirty years that Tyndall did not visit the Alps. As his sleeplessness worsened, he began experimenting with drugs that might induce sleep. Tragically, in 1893 his wife Louisa accidentally administered a fatal overdose of chloral hydrate, and Tyndall fell into the sleep of eternity.
Tyndall's work included several topics of interest to weather and climate. In one study, published as an 1872 book The Forms of Water, he used the structure of the snowflake to attempt to understand how water molecules bond together in forming ice. He is also credited with the first atmospheric pollution measurements, using instruments to measure infrared and scattering in order to monitor the London atmosphere. His two most important discoveries related to the atmosphere, however, were on the thermal radiative properties of the gases within the air and the physical properties of air that made the sky blue.
Explaining the Ice Ages and Discovering the "Greenhouse Effect"
During the mid-19th Century, Tyndall joined the hot climate change debate of his day: what caused the occurrence of ice ages. Had huge ice sheets really once covered northern Europe tens of thousands of years ago? And if so, what caused the climate to change so dramatically? Might changes in the atmosphere's chemical composition hold the answer? Tyndall wondered. Scientists had already speculated the atmosphere might retain the Sun's heat but did not know how. Tyndall decided to find out if gases could indeed trap heat.
Inventing a spectrophotometer to measure the heat-absorption power of gases, he discovered, in 1859, major differences among atmospheric gases in their abilities to absorb radiant heat. He found oxygen and nitrogen, the two main components of the atmosphere, are almost transparent to radiant heat, but other trace gases: notably water vapour, "carbonic acid" — as carbon dioxide was then called — and ozone readily absorbed passing heat rays.
Tyndall thus concluded that water vapour, the strongest radiant heat absorber, is the most important gas controlling Earth's temperature. Without water vapour, he wrote, the Earth's surface would be "held fast in the iron grip of frost."
Applying his radiation studies to minimum nighttime temperatures and dew/frost formation, Tyndall correctly postulated that dew and frost are caused by a loss of heat through radiative processes. He even suggested these factors gave London a "heat island" — meaning the city was warmer than its surrounding areas.
Tyndall speculated fluctuations in water vapour and carbon dioxide could be related to climate change and the onset of ice ages. At century's end, Swedish scientist Svante Arrhenius applied Tyndall's discovery to the ice-ages riddle, mathematically showing that halving the atmosphere carbon dioxide concentration could lower European temperatures to an ice-age chill.
Why The Sky Is Blue
One of the more frequently asked questions of meteorologists is: "Why is the sky blue?" It was a question that intrigued Tyndall as well; he wrote: "these questions [the color and polarization of skylight] constitute, in the opinion of our most eminent authorities, the two great standing enigmas of meteorology." In the course of his studies of light beams, the answer became clear.
Forwarding the work of Isaac Newton on visible light a century before, Tyndall noted that when light, such as sunlight, passed through a clear fluid holding small particles in suspension, the shorter blue wavelengths were scattered more strongly than the red. Thus, large molecules and dust would diffuse light, an 1869 observation that became known as the Tyndall effect. To measure the scattering of blue light waves in the sky, he invented a spectrometer. The observations lead Tyndall to hypothesize that the blue colour of the sky resulting from the scattering of solar rays by small particles of dust and droplets of water in the atmosphere. Later, Lord Rayleigh would develop the theory behind these observations, showing that the amount of light scattered by sufficiently small particles is inversely proportional to the fourth power of wavelength. Today, we know the process as Rayleigh scattering, and we also know that it is the oxygen and nitrogen molecules rather than small dust particles and droplets of water that cause this scattering.
Those light rays scattered by the Tyndall effect became known for a time as Tyndall blue and the polarized envelope formed by the scattering of light by suspended particles as the Tyndall cone. The Tyndall effect causes some blue colouration in nature such as blue eyes, the opalescence of some gem stones, and the colour in the blue jay's wing. The hue varying according to the size of the scattering particles.
Thus, while the reflected light gives us, at noon, the deep azure of the Alpine skies, the transmitted light gives us, at sunset, the warm crimson of the Alpine snows.
SIX LECTURES ON LIGHT (1885), Lecture IV:11. The Blue of the Sky. |
Learn More About Light Scattering From This Relevant Book
Chosen by The Weather Doctor
Written by
Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
February 1, 2006
The Weather Doctor's Weather Almanac John Tyndall
©2006, Keith C. Heidorn, PhD. All Rights Reserved. Tyndall portrait courtesy NASA
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