Balloons and Meteorology

Preparing to launch America''s first "ballon-sonde." Since this first launch on September 15, 1904, in St. Louis, Missouri, literally millions of weather balloons have been launched by the National Weather Service and its predecessor organization.

Balloons are ideal for gathering meteorological information and have been used for that purpose throughout their history. Meteorological measurements of wind and air pressure have gone hand in hand with the earliest balloon launches and continue today. Balloons can climb through the denser air close to the Earth to the thinner air in the upper atmosphere and collect data about wind, the different layers of the atmosphere, and weather conditions as they travel.

The first meteorological balloon sondes, or "registering balloons," were flown in France in 1892. These balloons were relatively large, several thousand cubic feet, and carried instruments to record barometric pressure (barometers), temperature (thermometers), and humidity (hygrometers) data from the upper atmosphere. They were open at the base of the balloon and were inflated with a lifting gas, which could be hydrogen, helium, ammonia, or methane. The lifting gas in the balloon exited through the opening as the balloon expanded during its ascent and the air became thinner and the pressure dropped. At the end of the day, as the lifting gas cooled and took up less space, the balloon descended very slowly. The meteorologists had to wait until the balloon descended all the way to Earth to retrieve their instruments, which often had drifted up to 700 miles (1,126 kilometres) from their launch point.

The German meteorologist Assmann solved the problem of drifting balloons and retrieval of instruments in 1892 by introducing closed rubber balloons that burst when they reached a high altitude, dropping the instruments to Earth by parachute much closer to the launch site. These balloons also had fairly constant rates of ascent and descent for more accurate temperature readings. Assmann also invented a psychrometer, a type of hygrometer used to measure humidity in the air that laboratories generally use.

In the 1930s, meteorologists were able to get continuous atmospheric data from balloons when the radiosonde was developed. A radiosonde is a small, radio transmitter that broadcasts or radios measurements from a group of instruments. Balloons, usually unmanned, carry the transmitter and instruments into the upper atmosphere. The radiosonde transmits data to Earth while measuring humidity, temperature, and pressure conditions.

Two men performing balloon tests for the U.S. Weather Bureau

Today, three types of balloons are commonly used for meteorological research.

Assmann's rubber, or neoprene, balloon is used for measuring vertical columns in the atmosphere, called vertical soundings. The balloon, inflated with a gas that causes the balloon to rise, stretches as it climbs into thin air, usually to around 90,000 feet (27,400 meters). Data is taken as the balloon rises. When the balloon has expanded from three to six times its original length (its volume will have increased 30 to 200 times its original amount), it bursts. The instruments float to Earth under a small parachute. The neoprene balloon can either carry radiosondes that transmit meteorological information or be tracked as a pilot balloon, a small balloon sent aloft to show wind speed and direction. Around the world, balloons equipped with radiosondes make thousands of soundings of the winds, temperature, pressure, and humidity in the upper atmosphere each day. But these balloons are launched and tracked from land, which limits what the radiosondes can measure to less than one-third of the Earth's surface. 

Zero-pressure plastic (usually polyethylene) balloons were first launched in 1958. They carry scientific instruments to a predetermined atmospheric density level. Zero-pressure balloons are the best for extremely high altitudes because the balloons can be lighter and stress on them can be distributed over the surface of the balloon.

A zero-pressure balloon being inflated at Alice Springs, Australia

About the same time, the Air Force Cambridge Research Laboratories (AFCRL) started working on super-pressure balloons, which were made from Mylar. The development of Mylar plastic films and advances in electronic miniaturization made constant-altitude balloons possible.

Mylar is a plastic that can withstand great internal pressure. The Mylar super-pressure balloon does not expand as it rises, and it is sealed to prevent the release of gas as the balloon rises. By the time the balloon reaches the altitude where its density equals that of the atmosphere, the gas has become pressurized because the heat of the sun increases the internal gas pressure. However, because Mylar can withstand great internal pressure, the volume of the balloon remains the same. By carefully calculating the weight of the balloon and whatever it is carrying, the altitude at which the balloon will achieve equilibrium and float can be calculated. As long as the pressure inside the balloon remains the same, it will remain at that altitude.

These balloons could be launched to remain aloft at specified altitudes for weeks or months at a time. Moreover, satellites could be used to track and request data from many balloons in the atmosphere to obtain a simultaneous picture of atmospheric conditions all over the globe. Another advantage of super-pressure balloons is that, since they transmit their data to satellites, they can gather data from over oceans as well as land, which is a limitation of balloons equipped with radiosondes.

The AFCRL program resulted in the Global Horizontal Sounding Technique (GHOST) balloon system. With GHOST, meteorologists at last achieved their goal of semi-permanent platforms floating high in the atmosphere.

Eighty-eight GHOST balloons were launched starting in March of 1966. The GHOST balloons and their French counterpart, EOLE, (the name Clement Ader used for one of his aircraft—named after the Greek god of the wind) used strong, plastic super-pressure balloons to trace air circulation patterns by drifting with the wind at constant density altitudes. Many super-pressure balloons were aloft at a time, grouped at constant density levels. Each balloon had a sensing device and transmitting system for gathering information on its position and weather data and transmitted atmospheric and weather data to weather satellites. They first transmitted their data to the NASA Nimbus-4 meteorological satellite in 1970.

In 1966, a GHOST balloon circled the Earth in 10 days at 42,000 feet (12,801 meters). By 1973, NASA had orbited scientific instrument packages aboard sealed balloons at altitudes up to 78,000 feet (23,774 meters). Other GHOST balloons remained aloft for up to a year. The program lasted for 10 years.

The ultimate of the super-pressure balloons was the balloon satellite Echo I. Launched into space in 1960, the balloon inflated to a sealed volume by residual air, benzoic acid, and a chemical called anthraquinone.

Constant-altitude, super-pressure balloons continue to fly over the oceans and land surfaces. These balloons have been relied on for decades to provide extensive knowledge of global meteorology and improve worldwide weather forecasting.