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Weather Almanac for April 2006
KITES LIVE IN WEATHER
No one knows exactly when the first kites were flown, nor of what they were made, but most agree flying kites was well established in China over two thousand years ago. These early kites were constructed of bamboo, for the frame, and silk, for the sail and bridle.
The earliest (about 200 BC) known written account of kite flying chronicled the Chinese General Han Hsin, founder of the Han Dynasty, using a kite to overthrow a tyrannical emperor. According to the accounts, he flew a kite over the emperor's palace to determine its distance from the city walls. By knowing the distance, his troops were able to tunnel directly into the palace and surprise the defenders.
Nearly two millennia would pass before kites would rise in the service of weather science, though I can fully imagine someone watching the dance of a kite in the wind and beginning to wonder and speculate on what was happening high above the ground. Our first documented meteorological experiments with kites occurred during the mid-18th Century, and one of these was the famed, and extremely dangerous, kite and key experiment of Benjamin Franklin.
In July 1749, Dr Alexander Wilson of the University of Glasgow (Scotland) wondered whether the air high above the ground was colder or warmer than the air at the surface. With the help of colleague Thomas Melville, he launched the first upper air sounding using a kite. In the experiment, they attached several thermometers along the line connecting six paper kites used to achieve greater heights. The smallest kite headed the train, and others, which ranged from 4-7 feet (1-2 metres) in height, followed at intervals. Along this line, the several thermometers were attached.
In order to read the thermometers and determine the temperatures aloft, each thermometer had a slow-burning fuse device at its attachment point. When the fuse had burned down, the instruments were released and fell to earth. The thermometers, bundled in strips of cloth to prevent breakage, had bushy tassels of paper tied to them to slow their descent to avoid breakage. As soon as the thermometers reached the surface, Melville read the temperatures. From the readings, the first record of temperature variation with altitude was obtained. Though Wilson's experiment only probed a few hundred feet about the surface, his method showed that kites could be used as meteorological observation platforms.
Franklin's Famous Kite
American Founding Father Benjamin Franklin became interested in electricity and particularly whether lightning was electricity around 1747. During the summer of 1752, he conceived an experiment to probe a thunderstorm and its electricity using a silk kite with a thread attached to it. There has been some conjecture recently as to whether Franklin ever performed this experiment, in part because the self-promoting Franklin left no direct account of it. We do know that Joseph Priestley, the renowned British scientist, wrote in about it 1767:
"The kite being raised, a considerable time elapsed before there was any appearance of its being electrified. One very promising cloud had passed over it without any effect; when, at length, just as he [Franklin] was beginning to despair of his contrivance, he observed some lose threads of the hempen string to stand erect, and to avoid one another, just as if they had been suspended on a common conductor. Struck with this promising appearance, he immediately presented his knuckle to the key, and (let the reader judge of the exquisite pleasure he must have felt at that moment) the discovery was complete. He perceived a very evident electric spark." (History and Present State of Electricity, 1767).
If Franklin and his adult son did fly that kite, they were lucky to be alive. If lightning had truly struck the kite, whoever held the string or touched the key would likely have died. Probably, the kite string set up an electrical potential difference between the kite and key and the man at the ground, and that is what caused the small spark to jump to Franklin.
Early Meteorological Research
As the next half century passed, kites were occasionally used for serious meteorological research, carrying instruments into the free air. One interesting sounding series came from the arctic expedition during the winter of 1822-23 when Captain Sir Edward Parry and the Reverend George Fisher attached self-registering thermometers to kites and flew them into the arctic sky above Igloolik, an island off the northeast North American coast.
American meteorologist James Espy enlisted the help of his friends at the Franklin Kite Club who met once a week to fly their kites for scientific experiments for his research in 1840. Using a number of kites flown simultaneously, Espy determined the cloud base height for convective (cumulus) clouds, and for the first time, detected updrafts below these cloud bases. With the data collected during these flights, he related the cloud base heights to the surface temperature and dewpoint. This study led to the first estimates of the temperature lapse rate under convective conditions. (Espy photograph c.1850, courtesy Historic NWS Collection, NOAA)
In September 1847, Sir Francis Reynolds and William Radcliff Birt of England's Kew Gardens Observatory developed a six-sided meteorological kite that could be held relatively stationary by cords attached at the three lower corners. This gave them the ability to raise and lower weather instruments to measure temperature, humidity and wind aloft using a pulley system.
The innovations and research of E. D. (Douglas) Archibald during the 1880s initiated what would perhaps be the golden age of the scientific kite. In 1883, Archibald fastened self-recording anemometers to his tandem kite line at several intervals. To measure winds to great heights, and thus establish a wind profile, greater strength was needed in the kite line. So Archibald came up with a landmark innovation. He replaced his flax string line with one of piano wire. Not only was the wire stronger, it was thinner, lighter and cheaper. With the new line, he was able to reach altitudes of up to 500 m (1640 ft) and bring his instruments safely back to earth. With stronger lines, meteorologists could probe the atmosphere to much greater heights and thus use the information to significantly improve weather forecasts. Three years later, Archibald attached a camera to his kite and became the first to take aerial photographs.
In the early 1890s, the next revolution in weather kiting came with the introduction of two new kite shapes. As the use of kites to measure weather properties high above the surface increased, it became apparent that neither the old standard diamond-shaped kites nor the hexagonal kites developed by American Alexander McAdie of the Blue Hill Observatory were stable enough to meet the needs of researchers. Within a year, two new designs made kites practical for weather research and operations: the Hargrave cellular or box kite (1893) and the Eddy bowed kite (1894).
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