A glimpse into the atmosphere – Balloons still at the forefront of forecasting

By Morgan Levey
Medill Reports

Norman, Oklahoma. Just around the corner from the National Weather Center in Norman, Oklahoma, a small cylinder-block building with antennas and a sphere on the roof marks the site of crucial weather data collection. Reinforced to withstand a gas explosion, it’s the base for Norman’s daily weather balloon launch. Yes, balloons — one of the oldest forecasting tools reliably capturing weather warnings today.

Every day at 0 Z and 12 Z (6 a.m. and 6 p.m. CST) over 700 locations worldwide release a weather balloon. “It’s a picture of the atmosphere around the world at a given hour,” says Forrest Mitchell, the observations program leader for the National Weather Service in Norman. A weather balloon is a helium-filled vehicle that lifts a radiosonde, an instrument that takes atmospheric measurements every second, to 100,000 feet above Earth’s surface.

While the process of launching a balloon hasn’t changed since 1938, the instrument technology keeps progressing. Through its antenna, the radiosonde sends the data it collects back to the National Weather Service headquarters in Silver Springs, Maryland — between six and 7,000 readings. Computers crunch the data into figures on wind speed and direction, temperature, humidity and atmospheric pressure. These combined measurements provide a baseline for weather models and forecasts for severe weather over the next 12 hours.

Weather balloon operators Forrest Mitchell and Daryl Williams have been launching balloons for almost three decades. “So we’re starting to get the hang of it,” Mitchell jokes. On a windy evening at the beginning of May, Mitchell and Williams explained the process of launching a balloon.

While the weather balloon inflates, Forrest Mitchell explains that operators inflate the balloons to rise about 985 feet per minute. The balloon will expand as it rises and the atmosphere’s pressure decreases, growing to almost 40 feet in diameter — the size of a pick-up truck. (Morgan Levey/ MEDILL)

Daryl Williams points to the antenna of the radiosonde. Every second of its flight, the device captures information that gets converted into readings on wind speed and direction, temperature, humidity and atmospheric pressure. (Morgan Levey/ MEDILL)
In Norman, they’ve never used hydrogen to inflate balloons — only helium. Partly built to withstand a hydrogen explosion though, the building looks like a fortress. Williams sits in the room next to where they inflate balloons, which houses computers used to ready the radiosonde and monitor the flight data. (Morgan Levey/ MEDILL)
Prelaunch, Williams turns the radio on while he readies the instrument. Locations are given an hour-and-a-half window of time to launch their balloons in case something goes wrong, but they can’t launch early either. “My trade secret is when NPR goes off, I go,” he says. (Morgan Levey/ MEDILL)
Left: Just before 6 p.m. CST, Williams watches the software that calibrates the radiosonde with the location, temperature, humidity and pressure at the earth’s surface. Little green checks appear after each measurement is inputed. (Morgan Levey/ MEDILL) Right: Postlaunch, Williams watches the signal for a few minutes to make sure the balloon is sending solid flight data. “One minute and 14 into the flight and we’ve got a good, solid signal,” says Williams as he monitors the data.
Williams leaves the building with the weather balloon ready to go. The balloon is equipped with a parachute that will float the radiosonde back to earth after the balloon pops at about 100,000 feet, or 90 minutes of flying. Radiosondes cannot be reused, but return-to-sender envelopes used to accompany radiosondes on flight. Newer instruments do not include a way to send the instrument back to the launch site. (Morgan Levey/ MEDILL)
On a windy day like today, Williams has to ensure the radiosonde doesn’t get caught in the balloon’s string. (Morgan Levey/ MEDILL)
A successful balloon will rise to 400 millibars, roughly 20 to 25,000 feet. The trick to a launch is to make sure the balloon clears trees and obstacles on its way up. (Morgan Levey/ MEDILL)
Launchers refer to the moment when a balloon pops as a “bloom burst.” A poetic way to describe the rapid disintegration of the balloon. (Morgan Levey/ MEDILL)
Photo at top: Solid surfaces will reflect the radiosonde’s transmitted energy and cause it to burn out. Operators place the instrument on a styrofoam block, which absorbs the energy, preflight. (Morgan Levey/ MEDILL)