

A WVSS-II, which detects humidity, is installed on the fuselage of a Southwest Airlines Boeing 737. (Courtesy Southwest)

Twice a day, the National Weather Service launches 102 weather balloons into the sky above the United States and the Caribbean. The balloons rely critical information to forecasters. Now technology first developed for use on Mars is being attached to planes to give forecasters more information to improve their predictions.

The WVSS-II, which senses water vapor, is on 112 American planes. But given the accuracy and wealth of data gathered, the program is expected to grow. With 87,000 flights occurring every day in the United States, there’s tremendous potential to aid forecasters — and everyone who wants to know what Mother Nature has planned. Water vapor sensors measure humidity, which helps forecasters be more accurate as they predict thunderstorms, fog, snowfall and icy weather.

“What started out as ‘Will it be good enough to compete with the balloons?’ — it’s actually better than the balloons in a lot of cases,”said Rick Curtis, chief meteorologist at Southwest Airlines. “There’s a huge need in weather to get upper-air information between balloon soundings.”



A close look at a water vapor sensor on a UPS cargo plane. (UPS)

Relying on two daily weather balloon releases means meteorologists can wind up with information up to 12 hours old, which is staler than some would like. Aircraft data also offer an economic benefit. The observations from one plane flight cost 5 percent to 10 percent of the cost of releasing a traditional weather balloon, according to Frank Grooters, who leads the World Meteorological Organization team on aircraft-based observing systems.

Southwest first embraced the technology with 25 planes in 2009, and the program has grown from there. For an airline, knowing when to shut down and when to reopen after a storm is essential, so the wealth of new and fresh data is welcome. The 87 Southwest planes equipped with water vapor sensors record 48,000 observations of wind, temperature and humidity in a 24-hour span, offering more frequent readings than weather balloons can provide. While wind and temperature readings have been gathered on major commercial airlines since the mid-1990s, forecasters haven’t reaped the benefits of humidity data.



A look at where wind and temperature data was gathered over the course of one day on planes as part of the Aircraft Meteorological Data Relay (AMDAR) program. (NOAA/ESRL/GSD)

Curtis cites the example of an ice storm that was expected to hit Dallas late last year. With the help of the additional data, Southwest’s in-house meteorologists team realized the threat wasn’t as significant as expected.

“We didn’t skip a beat,” Curtis said. “We didn’t have any freezing rain, and life was fine.”

Another U.S. company, UPS, also carries the WVSS-II on some of its planes. The shipper is especially interested in fog data, as it cargo planes are in service generally around sunrise, when fog is at its worst.

The shipping company first experimented with a water vapor sensor on its planes in 1997. But the sensors lasted only about a month. Accelerating from zero to 500 mph in a minute and cruising at 40,000 feet in temperatures ranging from 90 degrees to minus-50 cause tremendous strain on the sensor. So, technology designed by NASA’s Jet Propulsion Lab to survive on a Mars rover mission was a perfect answer. SpectraSensors, which spun off from the lab, now sells the WVSS-II. As air flows through a tube attached to the fuselage of a plane, a laser measures the water vapor present.

While the WVSS-II has gained a foothold in the United States, it’s yet to catch on elsewhere, despite the urging of the World Meteorological Organization (WMO). European airline Lufthansa, which has three planes equipped with the WVSS-II, is the only airline outside the United States to use the technology.



t Meteorologist Carrie Suffern prepares to release a weather balloon at National Weather Service Headquarters in Sterling, Va. (Benjamin C. Tankersley for The Washington Post)

In countries where airlines leave meteorology to outside organizations, the WMO must appeal to those weather organizations. Then the WMO will try to persuade the airlines with the help of the local forecasters.

“It’s so difficult to convince airlines where there is no meteorological knowledge within airlines,” Grooters explained. Having a company employee on the inside who understands the technology and can advocate for it makes it an easier sell.

The places that could see some of the biggest information gains from water vapor data collection are parts of South America, Africa, the Asia Pacific and Russia, where weather balloons usage is sparse. Sensors that piggyback off existing aircraft would be a cheaper alternative to weather balloons.

Grooters expects that in 10 to 15 years 50 percent of commercial aircraft will detect water vapor.

“As soon as one or two major airlines are flying water vapor sensors, the other airlines will see and hopefully also want to participate,” Grooters said.

UPS says if results continue to show the water vapor sensor data is reliable, it will expand from 25 to more of the 237 planes in its fleet. Curtis said he hopes Southwest will continue to grow its program, which now includes about 12 percent of its planes.

“The whole community benefits from this. it’s not just for airlines,” Curtis said. “That’s the beauty. The data can be used for everybody.”