Very little water vapor ever rises above the troposphere (where the majority of Earth's weather occurs) but the little bit of vapor that makes it to the stratosphere is kind of a big deal. One recent study suggests that just a one to two percent increase in mid-stratospheric humidity can retain up to twice as much CO2—the equivalent of a decade's worth of greenhouse gas emissions. However, these estimates are based on severely incomplete climate models. An ambitious NASA program hopes to patch these data gaps with the help of America's biggest UAV.

That UAV of course is the Northrop Grumman RQ-4 Global Hawk, of which NASA scored two from the US Air Force back in 2007. Each $108 million unmanned aircraft measure 44 feet long and 15 feet tall with a 116 foot wingspan. A single Allison Rolls-Royce AE3007H turbofan engine generates 7,050 pound-foot of thrust to speed the 22,000-pound UAV to speeds approaching 500 MPH.

NASA will employ the Global Hawk to study the tropical tropopause layer (TTL), where the troposphere and stratosphere meet, about 8 to 11 miles above Earth's surface. This area plays a major role in determining the stratosphere's chemical composition. However, in order to understand how the TTL affects the atmosphere above it (and thereby effectively predict changes to it), researchers need to first have a better understanding of the chemical composition of the TTL itself—an effort that has hitherto been hamstrung by the region's distinct shortness of breathable air and -115 degree F temperatures. But with the RQ-4 Global Hawk's pilots safely on the ground in the Dryden Flight Research Center at Edwards Air Force Base, the stratosphere's inhospitable conditions will have little effect on NASA's multi-year Tropical TRopopause EXperiment (ATTREX) mission.

Normally, the Global Hawk is outfitted with a Hughes Integrated Surveillance & Reconnaissance (HISAR) sensor system, which is great for spotting ground targets during combat operations. But for the ATTREX mission, it's being retrofitted with 11 separate scientific sensors designed to measure everything from the moisture and chemical composition of the air at 65,000 feet to radiation and trace gas levels.

Watch the Global Hawk Hunt For Water Vapor In the Name of ScienceS

The ATTREX project is a collaboration between four NASA departments, the NOAA, and NCAR, as well as multiple universities and private enterprises , academia and private industry. Jointly led by principal investigator Eric Jensen and project manager Dave Jordan of NASA's Ames Research Center, the ATTREX project should produce unprecedented amounts of data during the Global Hawk's fores into the upper reaches of the atmosphere.

"The ATTREX payload will provide unprecedented measurements of the tropical tropopause layer," said Jensen. "This is our first opportunity to sample the tropopause region during winter in the Northern Hemisphere when the region is coldest and extremely dry air enters the stratosphere."

So far, a half-dozen expeditions are scheduled between now and the middle of March, each lasting 24-30 hours and taking the UAVs along the equator and the Central American coast. For the 2014 season, the ATTREX team hopes to deploy the Global Hawk to Guam and Australia for similar sampling flights.

[UAS Vision - Wikipedia - NASA 1, 2, 3 - Images: NASA]