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Weather Almanac for July 2005
BALLOONS SAMPLE THE WEATHER
On occasion when I travel north of Victoria, I have seen hot-air balloons arise in the calm summer airs as balloonists take advantage of the light breezes over the Saanich Peninsula. Ballooning has increased in popularity over the last few decades after years of taking a back seat to powered flying thanks to improved technology. And I am sure many of you keep an eye on the 2002 flight of Steve Fossett, as he became the first to circle the globe nonstop and alone in a balloon three years after, Bertrand Piccard and Brian Jones had completed the first nonstop global balloon circuit.
The use of probe balloons to aid manned balloons is as old as manned balloon flight. History tells us the first human balloon ascent carried physicist Jean-Francois Piollaatre de Rozer and Major Marquis Francois d'Arlandes of the French Army aloft on November 21,1783 over the Tuileries in Paris. The hot-air balloon designed by the Montgolfier brothers, Joseph and Etienne, stayed aloft for 25 minutes and landed safely 8 km (5 miles) outside Paris. Ten days later, December 1, 1783, Jacques Alexandre Charles, made an ascension with one passenger Noel Roberts, the Duke of Chartres, in a different type of balloon, the first hydrogen-filled balloon, carrying a barometer and thermometer. Charles was an associate hired by the Montgolfiers to carry out research on balloons. According to accounts, the Montgolfiers launched a small pilot balloon about two metres (six feet) in diameter to judge the wind conditions prior to Charles' flight.
Ballooning never truly caught on as a major means of transportation, though the large dirigibles had some early success prior to the Hindenburg disaster. However, balloons have proven to be a useful tool in the probing of our atmosphere for scientific purposes. Today, balloons carrying radiosondes are routinely released around the globe, and pilot balloons or pibals are frequently used to study wind currents. The "big daddy" balloon probes are those launched by researchers to study the high atmosphere far above the ground where use of aircraft is not efficient and expensive.
Of Pibals and Radiosondes
Untethered manned ballooning, which was always at the mercy of the air currents, required knowledge of upper-level wind speed and direction to determine the balloon's flight direction and speed. Early balloonists quickly adopted the practice of releasing small paper balloons prior to an ascent to determine the wind currents aloft. It appears that the term pilot balloon arose from these pre-flight probes and the name was later shortened to pibals.
While balloonists estimated the wind character from the flight of the pilot balloons by eye, some scientists saw the pilot balloons as a way to study the wind patterns high above the ground. Likely Thomas Forster first observed and recorded the drift of small free balloons to determine high altitude wind character in 1809. In the same year, Wallis, who used a telescope to track the balloon movements, concluded from his observations that the wind high above the surface followed complicated paths and often contained multiple current streams.
Prior to the advent of radio transmission, probing the atmosphere's properties had to be made by either manned flights, free balloons that dropped their sensors during the flight, or tethered balloons. One of the first expeditions to measure atmospheric properties was undertaken by the French duo of J.-A. Bixio and J. A. Barral in July 1850. Ascending from the Paris Observatory to measure temperature, solar radiation and air composition changes with altitude, their five-hour flight reached its apex at 7000 m (23,000 ft). Many difficulties surrounded such manned probing, most importantly the observer's need for oxygen as the air thinned with altitude.
Free balloons carrying meteorographs came into use in the 1890s. After a predetermined time, the meteograph instrument package would be released and descend on a parachute. Hopefully, the instrument would be recovered undamaged so that its data could be abstracted. This method was too unreliable for routine use. However, the French meteorologist Leon Philippe Teisserenc de Bort completed nearly 600 unmanned balloon soundings between 1898 and 1904. The temperature profiles he obtained to an altitude of approximately 14 km (46,000 ft) allowed him to identify a high-level temperature inversion that confirmed the existence of the stratosphere.
An alternate solution to the problem placed recording instruments on balloons, or small blimps, that were tethered. The instrument package could be sent aloft and then reeled back in to read the observations. This method proved rather successful and could be used under many weather conditions too dangerous for a human observer to be sent aloft. In many instances, the tethered balloon was replaced with a tethered kite. The US Weather Bureau began using tethered kites at selected locations across the country in the late 1890s.
It took over half a century before pilot balloons were regularly used to determine the wind character. In 1872 Cleveland Abbe, first chief scientist of the US weather service suggested a practical method for determining a balloon's position in the air:
The balloon should carry a suspended light thread from 50 to 500 feet long, at the bottom of which hangs suspended a light object. The observer can at any time ascertain the linear distance and altitude of the balloon by observing the altitude of the upper and lower end of the vertical line thus carried by the balloon...(Treatise on Meteorological Apparatus and Methods, 1888)
Visual tracking of swiftly moving balloons by theodolite is a skill, one I never mastered as I usually quickly lost the balloon as it moved across the sky. Low clouds, fog and precipitation always hamper this method as a practical and routine observation practice. With the advent of radio, compact electronics, and radar, a giant leap was made in balloon probing.
In addition to studying the atmosphere vertically, meteorologists often want to know about the atmosphere across horizontal space, particularly to determine trajectories of air parcels. Again, they turned to a form of balloon: the tetroon. The tetroon is a tetrahedral balloon whose straight seals joining its four triangular faces are stronger than the curved seals of the more traditionally shaped balloons made from mylar. The strong construction allows the tetroon to maintain a constant volume under most atmospheric conditions. When filled with helium to a desired level of bouyancy, the tetroon will float at a desired air density level (desired altitude) and drift with the wind currents. By tracking their movements, researchers are able to further our understanding of atmospheric turbulence, low-level vertical motions, and air pollution dispersion.
The US Signal Corps directed their first tests of radio-tracked balloons in 1923. In 1927 two scientists P. Idrac and R. Bureau from France attached a radio transmitter to a free balloon setting the stage for the radiosonde (from radio and sonde which in old English means messenger). In January 1930 Russian meteorologist Pavel A. Molchanov achieved the first official flight of a radio meteorograph attaching a telemetered instrument package, recording temperature and pressure, to a sounding balloon. Less than two years later, Vilho Väisälä of Finland launched his version of the radiosonde. This package was presented in 1935 to the international meteorological community, and in 1936, it was considered suitable for routine meteorological use. Väisälä's company manufactured this useful package used worldwide for decades.
Developments in radar gave a big boost to rawinsonde (the use of a small balloon with diameter about 1 metre [39 inches] to determine only wind information radio wind sonde) and radiosonde technology during World War II. By the later years of the 20th Century, advances in radar, microelectronics and satellites led to fully automated observations with accurate spatial positioning. Today, daily radiosonde launches occur at about 1100 permanent sites around the globe as well as soundings from many ships at sea, synchronized to 0000 UTC and 1200 UTC. The data from these flights are compiled and distributed worldwide for use in plotting upper air charts and as input data to weather analysis and forecasting models.
The teardrop-shaped radiosonde balloon filled with helium gas rises at a rate of about 500 cm/sec (16.7 ft/sec) and expands as the surrounding air pressure decreases. By the time the balloon reaches it peak altitude, around 35 km (about 115,000 feet), it has stretched to nearly as big as a house. When it has reached its limit of stretch, the balloon ruptures, and the gas inside escapes. A small parachute deploys as the radiosonde and burst balloon fall to earth in order to lessen any damage on the ground.
Routine radiosonde flights can exceed two hours in length during which time the balloons may drift more than 200 km (about 125 miles) from the release point. The balloon carries aloft a very small package of instrumentation (barometer, temperature sensor, and humidity sensor) and communication equipment, hardly larger than a tube of toothpaste. The package hangs at the end of a 30-metre cord attached to the radar reflector which, in turn, is connected to the balloon. The silvery radar reflector provides a target for the radar tracking the balloon. (For more details on the radiosonde package, see the NOAA sites: http://www.srh.noaa.gov/mob/balloon.shtml and http://www.erh.noaa.gov/er/gyx/weather_balloons.htm)
Radiosonde, tetroons, and pibal sensing of the atmosphere are just a few of the ways balloons have been used to probe the atmosphere. Manned balloons and tethered balloons with instrument packages aboard have also been used for research and routine analysis of the lower atmosphere in a variety of applications. The altitude limit of such probes is constrained by the length of the tether cable, or the human limits of ascent (in the days previous to pressurized cabins and suits).
Today, the other chief use of balloons as scientific platforms involves research reaching the highest levels of the atmosphere and lowest limits of outer space. Aeronomers, those scientists whose scientific research is aimed at discovering and understanding the chemical, dynamical, and radiative processes of the Earth's upper atmosphere, use large weather balloons to lift heavy payloads of scientific instruments to altitudes generally above 30.48 km (100,000 feet). They have used balloons to study light from the aurora and the gases in the high altitudes, including the ozone layer.
By midway through the Twentieth Century, better materials allowed giant plastic balloons to carry heavy cargoes to extreme altitudes. These huge balloons are so rugged they can withstand 250 km/h (155 mph) jet stream winds and air temperatures as low as minus 86 degrees Centigrade (minus 123 degrees Fahrenheit) while being exposed to the full force of cosmic and solar radiation. In 1972, the largest balloons measured 229 m (750 ft) tall and had a 1.5-million cubic-metre (53-million cubic-ft) gas capacity that was capable of carrying 6,350 kilograms (seven tons) of instrumentation to "low altitudes" or lighter packages to 50 km (31 miles).
These balloon flights have proved so successful that they are used as often by astronomers as by aeronomers.
All photographs courtesy NWS Collection, National Oceanic & Atmospheric Adminstration (NOAA), US Department of Commerce
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