An important breakthrough in atmospheric observation came in the late eighteenth century with the invention of the hot-air balloon. Balloon flights made it possible to carry instruments thousands of feet into the atmosphere to take measurements. The English physician John Jeffries is often given credit for the first balloon ascension for the purpose of making meteorological measurements. In 1785 Jeffries carried a thermometer, barometer, and hygrometer to a height of 9,000 ft (2,700 m) in his balloon.
For the next 150 years, balloons were the primary means by which instruments were lifted into the atmosphere for purposes of observation. A number of devices were invented specifically for use in weather balloons. Most commonly used of these devices were the meteorograph and the radiosonde, both of which are combinations of instruments for measuring temperature, pressure, humidity, and other atmospheric properties.
The radiosonde differs from a meteorograph in that it also includes a radio that can transmit the data collected back to the earth. When the radiosonde is used to collect data about atmospheric winds also, it is then known as a rawinsonde. In most cases, data collected with the meteorograph is recovered only when the instrument is jettisoned from the balloon or airplane carrying it. At one time, scientists paid five dollars to anyone who found and returned one of these measuring devices.
Balloons are still an important way of transporting weather instruments into the atmosphere. Today, they are often very large pieces of equipment, made of very thin plastic materials and filled with helium gas. When the balloons are first released from the ground, they look as if they are nearly empty. However, as they rise into the atmosphere and the pressure around them decreases, they fill to their full capacity. Balloons used to study the properties of the upper atmosphere are known as sounding balloons.
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