Chatham’s High-flying Science Informs Global Weather Models

By: Alan Pollock

Tim Morrissette prepares to remove the balloon from the helium hose.  ALAN POLLOCK PHOTO

CHATHAM Though we might still like to complain about them, modern weather forecasts are undeniably better than they were even a few years ago, thanks in large part to sophisticated computer modeling. But those models are only as good as the data that’s fed into them, and that’s where Chatham’s Upper Air Station comes in.

Twice a day, in all kinds of weather, Tim Morrissette and two colleagues launch radiosondes under weather balloons from a hangar on Morris Island. They work for a private company under contract with the National Weather Service, quietly collecting data on the upper atmosphere that’s priceless to weather forecasters and to aviators.

Morrissette begins each shift by attaching a large, biodegradable balloon to a helium tank inside the hangar, where it fills over the course of several minutes. Eventually, the lift from the balloon trips a valve that turns off the helium, ensuring that each flight has the same amount of lift. While the balloon is filling, Morrissette is preparing the payload, about 80 feet of string, an orange parachute, and the radiosonde itself.

At this and the 92 other upper air stations in the U.S., about 20 percent of radiosondes are returned to the National Weather Service, but not in Chatham.

“Most of ours end up in the ocean,” he said. The fact that Chatham is among the easternmost stations in the U.S. makes its data particularly valuable, since there’s very little upper air data available over the Atlantic, aside from the stray weather balloon launched by a ship, observations from aircraft, or satellite data.

Radiosonde information not only yields weather forecasts, but can help scientists better understand phenomena like air pollution and climate change. Pilots using Logan Airport are keenly interested in the information as well.

Weather balloons have been in use for more than a century, and they’re still the best way to collect certain information about the upper atmosphere, Morrissette said.

“This is ground truth for satellites,” he said. While robotic launchers are now in use in very remote parts of Alaska, it’s still cost-effective to have humans launch the balloons in most places. With a degree in meteorology from UMass Lowell, Morrissette makes sure the instrumentation gets aloft and is properly transmitting data, but he and his colleagues also watch for inconsistencies in the observations and make corrections as needed.

Ordinary radiosondes collect data on barometric pressure, temperature and humidity. The radiosondes in Chatham are technically rawinsondes, which collect that information but are also tracked from the ground, providing detailed information on wind speed and direction at various elevations.

When Morrissette lets the balloon go from the launching pad, a directional antenna inside of the dome on top of the hangar automatically tracks the radio signal emitted by the radiosonde. That keeps the data flowing without interruption, and also shows wind data as the balloon moves horizontally through the atmosphere. About five feet in diameter when launched, the balloon will increase to 20 or 30 feet in size before finally popping at an altitude of up to 100,000 feet. When that happens, the radiosonde falls back to earth by parachute, having already transmitted all of the data it collected.

Since around 2009, the radiosondes have had another feature: a GPS receiver. The current devices transmit their GPS location to the receiver, helping improve tracking.

Flights can take as long as two hours, but the upper air station begins transmitting data to weather forecasting supercomputers even before the flight is over. At a computer terminal inside the small, prefabricated building next to the hangar, Morrissette keeps watch over the balloon’s position and the data flow. It can be dull work, he said with a chuckle. He sometimes watches a movie, reads a book, or eats dinner or breakfast while monitoring the flight.

A flight is deemed successful if the radiosonde reaches an altitude where the pressure drops to 400 millibars, usually around 24,000 feet. Most flights go higher, collecting additional data, but if a flight doesn’t reach the required altitude, it may need to be repeated. During hurricane season when storms are approaching – as when Sandy made a run at New England in 2012 – the twice-daily flight schedule is doubled, with launches every six hours.

The work is solitary, which is one reason Morrissette likes it.

“I’m married with one kid, so I have no social life,” he quipped. He also likes the view of the open Atlantic, though working on the edge of a cliff on Morris Island poses its own challenges. In a few years, erosion may threaten part of the U.S. Fish and Wildlife property on which the station is located. And severe flooding along Morris Island Road – along with related power outages – shuttered the operation for five days in March.

“That’s actually the first time in my 11 years” that the station was out of commission, he said. And then there’s the wind that whips off the water and shoots up the cliff face, making launches tricky.

“I’m a little crazy. I’ll launch in anything,” Morrissette said. Plenty of those balloons have been caught by the surrounding trees and a fence. “I think I’ve hit every building out here,” he said with a laugh.