It's a little before 4 o'clock on a July afternoon atop the Environment and Natural Resources building at the University of Arizona. A nearby wind gauge detects a breeze blowing from the west. A thermometer reads 104 degrees Fahrenheit.
Outfitted in jeans, T-shirt and dark glasses, meteorologist Mic Sherwood moves catlike around the building's stark white rooftop, readying a weather balloon for launch. He connects a nozzle to the balloon, which slowly inflates. With twine and quick movements, he secures a weather instrument, known as a radiosonde, to its tail.
The radiosonde weighs about 600 grams (1.32 pounds), contains a GPS and is equipped with a hazard-orange parachute all its own.
At the same time, hundreds of other meteorologists throughout the world are doing the same thing, including those at the National Weather Service's forecasting offices throughout the United States. All are engaged in their twice-daily launch routine, one at 11:00 and one at 23:00 Greenwich Mean Time.
The U.S. meteorologists' aim is to stitch together every office's forecast to get a national read on the country's weather and to formulate respective local weather forecasts, as well.
Standard forecasts are issued about seven days out, and warnings for various weather phenomena, such as flash floods, dust storms and thunderstorms, are issued with as much lead time as possible.
"It all starts with looking at things such as satellite data," Lader says. "But we bring in data from the upper-air balloons we launch twice a day every day of the year at 4 p.m. and 4 a.m. So all that data gets ingested into computer models."
To be sure, there are many computer models to choose from, thanks in large part to the availability of supercomputers that process the wealth of data now available. Some models have a 10-day reach, others four to seven days, some only a few hours.
"If we're looking out only 12 hours, we look at a set of models that are really high resolution," Lader says. "One of those models is run by the atmospheric science department at the University."
No matter which model is used, the radiosonde measures temperature, atmospheric pressure and relative humidity. In turn, these data allow for calculation of wind speed and direction.
"The instrument collects data all the way up, and all the way down, but we don't use the data it collects while falling," Sherwood says.
Before launch, Sherwood attaches 80 to 100 feet of twine to the balloon to lengthen the distance from the balloon to the radiosonde, so that the balloon doesn't interfere with the readings as the pair move skyward.
The Tucson NWS office uses helium to fill the balloon, an unusual practice given the relative expense of the gas, but its safety is a factor because the gas line runs through the building to the roof. Other weather stations use hydrogen, a cheaper but flammable alternative, Sherwood says.
The balloons, made of rubber, are manufactured in Phoenix. The rubber's flexibility is key to their durability, given the extreme conditions they endure during their 90-minute ascent.
As the balloon ascends at a rate of five meters per second, atmospheric pressure decreases so that the balloon stretches and expands, Sherwood says. At roughly minus-70 degrees Celsius (or minus-94 Fahrenheit), around 34,000 meters (or 21 miles), the balloon disintegrates.
"It will go to a powder, so we don't have to worry about it coming back to us," Sherwood says.
In fact, the only way the radiosonde ends up returning to the meteorologists is if a human finds it and mails it back to the NWS by using a mailer attached to the instrument. Sherwood says less than 1 percent of the devices find their way back to the office — but when they do, they are reconditioned and reused.
Although it has been an unusually quiet July so far for Tucson's meteorologists, things haven't been dull, Lader says. The monsoon season, marked by wind, thunderstorms and torrential rain, runs from June 15 through Sept. 30.
"It's been a real interesting start to the monsoon this year," Lader says. "From June 15 to July 1, it was the wettest on record at the Tucson International Airport. But then we had a really big dry spell in early July through mid-July with virtually no rain. But now we're getting into a more active period again."
"We live for this moment," Sherwood says. "We like our monsoon season. I'm a lightning guy. So bring on the lightning."