Using Balloons to Spy (On the Weather)

People have noticed a lot of big balloons floating across North America recently. While these balloons have caused concern, the truth is, there are thousands of them floating high above every day. In the United States, the National Weather Service (NWS) launches about 75,000 balloons over the course of a year. Each one takes important measurements that are fed into weather models to improve forecasts and to provide a better understanding of the atmosphere from near the surface to the stratosphere. This is particularly important on days where severe weather or wintry precipitation are likely.

The NWS in cooperation with other organizations release weather balloons twice a day, every day, at the same times (0000 UTC and 1200 UTC). Altogether, there are almost 900 launch sites around the world. The weather balloons are made of latex or synthetic rubber, and they carry an instrument package called a “radiosonde”, or simply “sonde”, seen in the image below attached to the bottom of the balloon. Modern sonde instrument packages measure air temperature, pressure, and humidity, and are contained in a white cardboard or plastic housing about the size of a shoebox. The instruments sample the air twice per second, then radio the information back to the scientists on the ground. The position of the radiosonde is tracked, usually with GPS, to provide wind speed and direction, as the balloon rises into the atmosphere.

The weather balloon is filled with either helium or hydrogen, and is usually about 2 m (6 ft) in diameter at launch. As the balloon rises, the air pressure around the balloon decreases, which allows the balloon to stretch and increase in diameter. Once the diameter grows to around 9 m (30 ft), the balloon bursts, a parachute deploys, and the radiosonde falls slowly back to Earth. From launch to bursting usually takes about 2 hours, with balloons ascending over 30,000 m (100,000 ft) and drifting hundreds of kilometers/miles.

There are return instructions on the housing, in case the sonde lands in an area where people may find it. The NWS appreciates any and all sondes returned to them, as the instrument packages can usually be reconditioned and reused, saving resources for the agency. Unfortunately, only 1 in 5 sondes are returned to the NWS. If you ever happen to find one, please return it!

While the use of weather balloons is ubiquitous and essential for our weather models, there was a time when sampling the upper atmosphere was impossible. How did we get from then to now?

Atmospheric Measurements Before Balloons

Evangelista Torricelli of Italy invented the barometer (a scientific instrument that measures atmospheric pressure) in 1643, pictured below. The vertical tube was about 1 meter (a little over 3 feet) tall, and filled with mercury. Torricelli theorized that we live in a “sea of air,” where we feel pressure similar to how water exerts pressure on submerged objects.

Frenchman Blaise Pascal was interested in Torricelli’s idea and concluded that, if the “sea of air” concept is correct, then air pressure should decrease with height. He wanted to observe air pressure higher up in the atmosphere, but this was in 1648 - it would be over 100 years before the invention of the hot air balloon. Luckily, he lived near Puy de D?me, a 1460 m (4790 ft) tall mountain in central France. He devised an experiment to carry a barometer up the mountain with another person marking the height of the mercury in the tube at regular intervals. In theory, the height of the mercury in the tube should lower as the weight of the atmosphere decreases. And, after a hike up the mountain, the barometer confirmed Pascal’s theory - the mercury level went down as they hiked up Puy de D?me.

Over one hundred years later, in 1787, more measurements were taken as Horace Benedict de Saussure climbed to the summit of Mont Blanc in the Alps. At this point in time, barometers had become much smaller and easier to carry, so Saussure was able to measure not just air pressure, but air temperature as well. He noted that both pressure and temperature decreased with height. Temperature decreased approximately 0.7°C per 100 meters (or 1.3°F per 100 yards), suggesting that at 30km (18.6 miles) up, the air temperature would be -273°C (-460°F), or absolute zero, where no heat remains. This spurred scientists to find ways to get instrumentation high enough to measure absolute zero.??

Introducing the Balloon

As scientific understanding of density and gasses expanded, the concept of balloon flight was developed in the mid- to late-1700’s. Hot-air and gas balloons were invented during this time, with both having their advantages and disadvantages. Hot-air balloons depend on a source of heat, which in the 1700’s meant open fire. Fire was held in iron pots under the balloon, which sometimes released burning embers into the envelope (the cloth part of the balloon) and ignited the fabric. Gas balloons often depended on hydrogen, which is extremely flammable. Other gasses could be used, including coal gas, which is less flammable, but also provides less lift. In the early 20th century, blimps used helium, but adoption of this gas was limited, as helium is relatively rare on Earth.

The first meteorological measurements taken while utilizing a balloon occurred on 1 December 1783 in Paris. A scientist and engineer rode the balloon to about 3000 m (almost 10,000 ft), then gradually descended and landed 3 km (1.9 miles) from the starting point. They noted that not only did the air get colder - they also had a difficult time breathing.

Almost 109 years later, Henry Coxwell and James Glaisher set the record of highest manned balloon flight (at the time) as part of an experiment to study the upper atmosphere. Coxwell was a famous, skilled balloon pilot, and Glaisher was a scientist with the British Association for the Advancement of Science. The two men made many research flights together, with their record-setting flight taking place on 5 September 1862. They lifted off from Wolverhampton in central England, with a thermometer and barometer in the basket.??

As the balloon rose ever higher, both men were affected by the low atmospheric pressure and cold. At almost 9 km (5.5 miles), Glaisher could no longer use his arms or legs, and was experiencing extreme altitude sickness. Coxwell allowed the balloon to rise higher - at least, until Glaisher lost consciousness and Coxwell was suffering from frostbite in both hands (the air temperature was -11°C, or 12°F). He knew he had to lower the balloon for both their sakes, but his hands could no longer move. Eventually he was able to use his teeth to open the gas valve, allowing the balloon to sink. Coxwell estimated they reached approximately 11 km (almost 7 miles) up, which was corroborated by their barometer. It is likely they nearly reached the stratosphere, but it is impossible to know for sure since Glaisher passed out and was unable to take temperature measurements (above is a dramatic wood-carving of the event printed in the London Times).

Coxwell and Glaisher demonstrated that there was a limit to how high up humans can safely go. As technology advanced, scientists developed automated recording thermometers and barometers by the late 1800’s. Scientists Teisserenc de Bort and Richard A?mann independently got the idea to attach these instruments to balloons to measure higher and higher altitudes. De Bort started in 1896 using cloth or paper balloons, and made more than 200 balloon launches over 6 years. He was confused by his readings at first: his thermometers always showed that after a while, temperature stopped decreasing, contrary to wisdom at the time. He was concerned it was instrument error, but after running so many experiments, he concluded that the atmosphere was divided into two layers, which he named the “troposphere” and the “stratosphere” (nowadays we include two more layers: the mesosphere and thermosphere). Meanwhile in Germany, Richard A?mann developed the rubber weather balloon, and was able to collect data from altitudes even higher than de Bort. Ultimately, A?mann’s measurements were in agreement with de Bort - the atmosphere eventually stopped cooling at increasing altitude, and even became warmer with height. It would be another decade before scientists figured out why temperature holds steady or increases with height in the stratosphere.

Summary

Thousands of weather balloons are launched across the world every day. The development of the science and use of weather balloons has spanned hundreds of years, and has proven essential to understanding the state of the atmosphere and forecasting. Lately, spotting balloons might be a concern due to geopolitics. However, it is most likely that you are seeing a balloon released from a nearby weather service office, and the balloon is fulfilling an important function that benefits us all.