By international agreement, each day at noon and midnight G.M.T., rain or shine, weather balloons are released from roughly nine hundred locations around the world. Within a couple of hours, most of them will be beyond the clouds, and still climbing. As they rise, they expand, going from the size of a car to the size of an orca. And then they pop. Usually, a complex device inside them, called a radiosonde, parachutes down to Earth. The radiosonde provides data on temperature, humidity, and air pressure, allowing forecasters around the world to predict sunshine in Montreal on Tuesday and rains in Mumbai on Wednesday. Often enough, someone finds one of the National Weather Service’s radiosondes on the ground, along with its bright-orange parachute. The radiosondes come equipped with mailing bags, so that they can be sent back to the N.O.A.A.’s National Reconditioning Center, in Missouri, and be reused.
Last month, a balloon the size of three buses was spotted over Billings, Montana. The Chinese Foreign Ministry claimed that it was a weather balloon blown off course, which was not entirely implausible. (It later became clear that the balloon was not a weather balloon; afterward, when more balloons were noticed, and shot down, it turned out that at least one of those likely was a weather balloon, and none were spy balloons.) But even balloons deployed for scientific aims have often carried political ballast. In 1783, one of the earliest manned balloons was built with finances from the King of France, launched with great pomp and bearing the fleur-de-lis. After the king’s overthrow, in 1790, another balloon was launched, with the goal “to see if the inhabitants of the moon were free” and, if they were not, to hand them the Declaration of the Rights of Man. That balloon faltered. But a year later, over a crowded Champs-Élysées, a balloon with a rooster-shaped basket (the rooster had become a symbol of the French people) successfully ascended twelve thousand feet. On the way down, the aeronaut, having toasted to freedom, dropped leaflets of the new constitution.
Ballooning was a performance of scientific knowledge—political armor in a time of purported democratic rationality. The French revolutionaries replaced the foot, based on the length of a man’s foot, with the metre. The “mysterious” arts—weaving, blacksmithing—appeared in plain terms in Diderot and D’Alembert’s encyclopedia. Even the republic was set to Year One, eschewing religious tomfoolery. And a military ballooning unit was formed, called the French Aerostatic Corps.
The Montgolfier brothers, the twelfth and fifteenth sons of a paper manufacturer, had started the balloon craze, in 1783. They used buttons to combine pieces of paper-lined burlap, forming a ten-metre-wide balloon, which rose more than six thousand feet, stayed aloft for ten minutes, and drifted about a mile. The brothers explained that they used “Montgolfier gas,” which was lighter than air—it was understood only later that Montgolfier air was simply warmer air.
Air is a substance that balloons have been well suited to study. The Victorian scientist James Glaisher wanted to create a map of the upper atmosphere, and see how the qualities of air changed with altitude. Earlier generations of scientists had climbed the Puy de Dôme, a dormant French volcano of modest height, carrying barometers (which, at the time, were awkward, fragile, and heavy). Glaisher wanted to check out the air even higher up. In 1862, he went up in a balloon with an expert, Henry Tracey Coxwell. At five thousand feet, they reached the clouds; at eight thousand feet, “the tops of the mountain like clouds became silvery and golden,” Glaisher later wrote. When they reached some five miles above the Earth, the barometer read 10.8 inches—at sea level, barometric pressure, which measures among other things the saturation of oxygen, is closer to thirty inches—and Glaisher’s sight began to fail. He reached for the brandy, but couldn’t extend his hand that far; he kept trying to read the barometer until he passed out. A pigeon they had brought up with them died. At that point, Coxwell, feeling weak himself, took hold with his teeth of a rope that would help them descend; his frozen hands had become ineffective. They started to descend, and survived. So: there is less oxygen up there, they learned, among other lessons.
Other people discovered the stratosphere. Léon Teisserenc de Bort sent up some two hundred balloons. He designed them himself, using kerosene paper, which is water-resistant. He also designed the instruments that his balloons carried. He dispensed with the human element, sending them up unmanned, then searching for the instruments once they fell back to Earth. He came across a mystery. Generally, as you ascend, the temperature drops. But Teisserenc’s instruments were telling him that, beyond seven miles or so, the temperature began to stabilize, and occasionally even rose. He thought that perhaps his thermometers were being affected by the sun; he wrapped them in cork and flew the balloons at night, to no avail. After two years, and similar work by a German scientist, Richard Assmann (who had fancier equipment, including balloons made of rubber), Teisserenc realized that the temperature recordings were accurate, and revealed a boundary between what we today call the troposphere—the first layer of our atmosphere—and the stratosphere, the second layer. Mt. Everest reaches up almost to that boundary, after which the air gets drier and less dense, and warmer again—these conditions prevent the two layers from mixing much.
Another momentous scientific-balloon story: in 1906, a young physicist named Victor Hess made arrangements to go to Berlin, to study optics under a revered professor. Before he arrived, the professor died, of suicide, and Hess ended up in Vienna, studying under an expert in radiation. At the time, most scientists assumed that radiation came only from the Earth itself. But one scientist had measured radiation at the top and the bottom of the Eiffel Tower—and there was more radiation at the top. Another scientist had sunk a measuring device to the bottom of the Bay of Livorno, where he found little radiation. So, in 1912, Hess took flight in a balloon, to measure radiation levels at greater heights. He went up ten times. One of those ascents was during a solar eclipse, to rule out the sun as the source of the radiation. He found that, though radiation initially decreased with height, eventually it began to rise again. After a certain point, the higher the altitude at which measurements were taken, the more radiation was found. The mysterious radiation, he deduced, must be coming from somewhere beyond Earth’s atmosphere. (Later the source of the radiation was found to be cosmic rays.) Hess had to flee Austria, in 1937; his wife was Jewish. He eventually became a professor at Fordham, in New York, and studied radioactive fallout.
I grew up with the sense that a balloon was a rocket for a modest scientist. My mother, who was a computer programmer for the National Severe Storms Laboratory, in Norman, Oklahoma, would sometimes announce that she had to go in to work for a balloon launch, and this made her work seem even more like a game, because the radar installations near the lab were housed in structures that looked like giant golf balls set up on pale-blue tees. I called up my mom’s old boss, the scientist and engineer Dušan Zrnić, to ask about the balloons. “These weren’t operational weather balloons but specialized balloons for coming up with a better way to track balloons,” he said. Doppler weather radar following the balloons, with no additional sophisticated equipment, could determine winds. “You know, balloons are relatively cheap,” he said. “If you want to do soundings with an airplane, that’s going to be much more costly.” He told me that the N.S.S.L. had its own balloons that were used every spring, launched every fifteen minutes into the storms that spawned tornadoes, to gather data both for forecasting and for research on questions about the influence of wind on storm behavior.
NASA also uses balloons, launched into the stratosphere, for research into cosmic questions. One recent NASA experiment set a series of balloons aloft over Albuquerque, one of which was launched with an All-Sky Heliospheric Imager, to get a better view of the winds surrounding and extending from our sun. Another NASA mission will soon send a football-field-sized balloon floating over Antarctica, carrying a telescope to look at the life cycles of stars. Some of the largest balloons can stay aloft for a hundred days—they’re referred to as Pumpkin balloons, since the seams of the material resemble a pumpkin’s ridges. Some of them, as their clear bodies flow upward during launch, look like enormous jellyfish. Sometimes they’re equipped with long, thin vents that hang down like legs. There are dreams of using them to visit other planets; if you were a Martian and saw one of these, you would likely be struck with as much wonder as terror.
That a large orb floating above the landscape is as much a potential weapon as it is a vector of inquiry, or a vision of lighter-than-air beauty, is evident in one of the earliest known balloon stories. Though the date of the “first” hot-air balloons is often tied to eighteenth-century France, there were earlier balloons, broadly defined. During the Three Kingdoms era, in China, from 220 to 280 A.D., Kongming lanterns were used for military signalling. These hot-air balloons, which are still in use today, are ingenious and simple: a candle is lit under a rice-paper bag; the warm air inside the bag carries it aloft. The lanterns were supposedly invented by the statesman Zhuge Liang (also known as Kongming) when he was trapped during a battle, as a way to call for reinforcements. Now they are part of lantern festivals, in China, Taiwan, and elsewhere, and notes, prayers, and worry lists are sometimes attached—with the hope that those wishes will be met, the prayers will be heard, and the worries will fly away. 
The Threat and the Allure of the Chinese Balloons
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