Jerry Trammell found one about five years ago while motorcycling through the forest roads of Craig County.
“I noticed something odd off to the side, hanging from a tree,” the Roanoke resident said. “It looked unusual enough that I stopped. And when I went over to it and pulled it down out of the tree, it was obvious what it was, because it said ‘radiosonde’ on this cardboardy-looking little package. And it was hanging from a parachute.”
There was a phone number on it with a request for the finder to return it. “But it was all kind of faded and everything and I really never called the number. I didn’t think they really wanted it back.”
So Trammell did what any curious electrical engineer would do. “I dissected it” — he laughed — “to see what it looked like and you know, it’s just a basic circuit inside of it.”
Trammell had found a radiosonde, a device launched by the National Weather Service to probe the conditions of the atmosphere.
In an era when meteorologists increasingly rely on satellite observations, GPS readings and data collected from commercial planes, there is a still a place for the radiosonde, a small electrical transmitter launched on a weather balloon.
Today’s radiosondes are simpler, lighter and less expensive than ever, and if you find one, the National Weather Service doesn’t mind if you keep it.
Traditionally they measure temperature, humidity and air pressure, said Reggie Roakes, a meteorologist with the National Weather Service in Blacksburg, near the Virginia Tech Montgomery Executive Airport. “If we have our surface pressure correctly measured before we launch a system … then we can actually calculate [air pressure] as it rises, and we no longer have to actually have a sensor to measure pressure. Now it’s a lot easier, we have GPS. So just as it rises, it’s actually measuring how far it’s moved and how far it’s risen off the ground. We can track it in real time.”
Every day, twice a day, weather balloons are launched simultaneously from some 900 stations around the world. The NWS station at Blacksburg launches at 7 a.m. and 7 p.m. (daylight time) or 6 a.m. and 6 p.m. (standard time). Extra balloons can be launched if severe weather threatens.
There was nothing threatening in Blacksburg on the early evening of May 17, 2022. Asked for a technical word to describe conditions, meteorologist Ken Kostura said, “Nice!” The temperature was 75, light breeze, pleasantly sunny with scattered high clouds.
NWS Blacksburg uses radiosondes made by Graw, a German company. The $78 white plastic device is about the size of a 1960s transistor radio, is powered by a small battery, and is labeled “HARMLESS WEATHER INSTRUMENT.” Sitting at a desk in the NWS office, senior meteorologist Stacie Hanes plugs the radiosonde into a computer to make sure it transmits on the correct frequency. She records the radiosonde’s serial number.
A metal vane on the radiosonde collects temperature and humidity data. A thin, flexible antenna transmits the data, including GPS position, to a receiver outside the NWS office.
A few minutes before 7 p.m., Hanes and meteorologist Roakes walk from the office to the Upper Air building, where the hydrogen is stored.
A vendor delivers hydrogen in red canisters once or twice month, Roakes said. Some NWS office make their own hydrogen on site.
Commercial helium must be extracted from natural gas, whereas there are several electrical and chemical methods of producing hydrogen.
Compared to helium, hydrogen is “way cheaper,” Roakes said. “It’s very easy just to make it. With helium there’s actually a helium shortage right now. It’s only getting worse. So one of the big pushes with the Weather Service is to not use it at all. I could probably count on my hands a number of locations that are still using helium.” A few locations use natural gas, a.k.a. methane, which is lighter than air.
Hydrogen, of course, is highly flammable, as anyone who has seen footage of the Hindenburg will recall. But there are lots of safeguards in the inflation system and Hanes, a 21-year NWS veteran, said she had never heard of a weather balloon explosion.
Hanes lays out a cream-colored latex balloon on a steel table that is electrically grounded to prevent static sparks. The balloon is attached to the hydrogen nozzle. Over a few minutes, it inflates to a diameter of around 5 feet.
Extra hydrogen is added if the meteorologists expect freezing rain to weigh down the balloon.
The radiosonde is attached with a tether about 80 to 115 feet long “to minimize contamination of the temperature measurements from heat shedding off the balloon skin,” according to weather.gov/upperair/factsheet. An orange parachute is tied to the radiosonde.
A few minutes after 7 p.m., Hanes lets go. Borne aloft on highly buoyant hydrogen, the balloon with its dangling device rapidly ascends toward the southeast, in the direction of Danville.
Back in the office, Hanes tracks the radiosonde on a computer. After 10 minutes, it has reached 3,000 meters, nearly 10,000 feet. Anyone with a receiver tuned to the correct frequency can track it, “We have one guy on social media … every now and then he’ll be like, ‘Hey, I’ve caught Blacksburg’s radiosonde going up,'” Roakes said.
A typical balloon ascent can last more than two hours, according to the weather.gov factsheet. The radiosonde may rise to 115,000 feet and drift more than 180 miles from its release point.
While Earth’s atmosphere may seem comfortably thick to humans, in relative terms it’s as thin as the peel of an apple. In the atmosphere’s upper reaches, the radiosonde is exposed to minus 130º F temperatures and air pressure only 1 percent of surface. A strong jet stream can drive it along at 250 mph.
On the way up, the device may pass through a commercial air lane. Collisions between jets and balloons are extremely rare and usually aren’t dangerous to the aircraft. “The theory is the balloon is breakable enough that it shouldn’t do damage,” Roakes said. However, a Soviet passenger aircraft, Aeroflot 1661, did collide with a radiosonde in 1970, causing a crash which killed 45 people. Radiosonde have become smaller over time. The Graw radiosondes launched in Blacksburg weigh less than 3 ounces.
As the balloon ascends, air pressure drops and the balloon expands. When it reaches a diameter of 20 to 25 feet, it pops and the device descends on its parachute.
NWS doesn’t know where most of them land because the radiosonde stops transmitting once the balloon pops, according to Phil Hysell, warning coordination meteorologist at Blacksburg NWS. One reason for this is to prevent curious collectors from tracking them onto private land.
“I have heard, anecdotally, that our radiosondes have reached the Atlantic Ocean, which is very likely, especially in the winter months when the upper level steering currents are strong,” Hysell said.
Roakes said he thinks many land between Lynchburg and Richmond.
With the older models, Roakes said, “the pressure sensor was valuable enough and recyclable. There would actually be a bag attached and postage was technically covered. All you had to do was put it in the bag and put it in the mail and it would go back to the recycling center.” About 25% were found and returned.
With the new ones, Hanes said, “a lot of people keep ’em. We’ve seen him on eBay, have people throw ’em away.”
NWS is trying to minimize the environmental impact when they land in the middle of nowhere “The balloon is pure latex, string is biodegradable,” Roakes said. “And that’s why the push has been to try and get them smaller and smaller … less pollution.”
So if you’re a weather geek and you happen to chance upon a “HARMLESS WEATHER INSTRUMENT” in a field somewhere, you can count yourself the lucky owner of an official NWS souvenir.
For more information on NWS radiosondes, see: