Little Radios and Big Balloons

Upper-air observations were critical to the successful application of Bergen School methods. Although tethered balloons and kites, free-floating balloons, and aircraft had been used to gather upper-air data since the early 20th century, they were all deficient in some way. Tethered balloons and kites could not fly as high in the atmosphere, and free-floating balloons carrying equipment packets had to be found before the data could be retrieved. (One set of instruments launched from Harvard's Blue Hill Meteorological Observatory landed in a pasture in Amherst, Massachusetts, only to be eaten by a hungry cow—both cow and sensors suffered fatal injuries.) Data from aircraft were used when available, but aircraft could not be launched in stormy weather, just when meteorologists most needed the data. The solution would be to create a free-floating balloon that could carry not only meteorological equipment but also a radio transmitter that would send the data back to a receiving station as they were measured.

In 1930, the Russian meteorologist Pavel A. Moltchanoff was the first to sound (gather information from) the stratosphere successfully with his 4.4-lb. (2-kg) "radio-meteograph" launched from Sloutsh in the Soviet Union (USSR). His instrument was called a "Kammgerat" because of the comblike metal strips that created the signal. On December 30, 1931, in Helsinki, the Finnish mathematician-turned-meteorological-instrument-inventor Vilho Vaisala (1889-1969) launched his new radiosonde for the first time. Unlike the Kammgerat, Vaisala's radiosonde transmitted data by using radio signals. The original prototypes only measured temperature, but by 1935 his radio sondes could also measure pressure and humidity. Used routinely at Finnish meteorological observation stations, they quickly spread throughout the Scandinavian countries. Vaisala started shipping radiosondes to the United States in 1936.

While this work was taking place in Finland, meteorologists and technicians at Harvard's Blue Hill Observatory were developing their own radiosonde. They had two main problems: making the equipment small enough and finding balloons with enough lift to get the equipment off the ground. The scientists attacked both problems simultaneously. The first successful launch of a two-pound (890-g) radio transmitter carried by three small hydrogen-filled balloons took place in October 1935. They were able to receive a signal from a height of 43,000 feet (13.1 km) and a distance of 60 miles (96 km). To launch both the radio transmitter and the meteorological instruments, the team needed larger balloons. Working with balloon manufacturers, they were able to obtain balloons with diameters of four to six feet (1.2-1.8 m) that could carry loads weighing between 1.8 and 2.6 lb. (800-1,200 g). Their first successful radio-meteograph launch took place on December 23, 1935. The four balloons carrying the instruments took 1.5 hours to reach an altitude of approximately 52,500 ft. (16 km).

By the end of the decade, improvements in radiosonde construction had made them much easier to use and they were widely utilized at upper-air stations around the world. The data obtained from these devices were critical for meteorologists who were providing aviation forecasts and would play an even larger role in flight safety during World War II.

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