Climatologists often ponder clouds. Do they largely reflect sunlight away from Earth, helping it cool, or do they absorb and reradiate heat, accelerating rising temperatures? Their net effect in a changing climate remains an unknown. In September nasa deployed a team to the Arctic to gather more data on this question. Onboard a C-130 plane with solar, thermal and microwave radiometers, researchers recorded how sunlight and heat moved through the clouds; they also surveyed sea ice above 250,000 square nautical miles of Alaska. The work complements another nasa-supported team at the University of Alaska Fairbanks that is monitoring glacier size with planes such as the DHC-3 Otter (above). In both cases, planes collect sharper readings than satellites. “We'll be making the data set available to the scientific community within six months,” says nasa mission leader William Smith.