Airman 1st Class Andrew Tuttle purges the discharge tube on a liquid nitrogen tank Dec. 20 to clear any impurities that may have settled since its last use. The plume is the result of burnoff and the extremely low boiling point of liquid nitrogen. Airman Tuttle is assigned to the 354th Logistics Readiness Squadron at Eielson Air Force Base, Alaska. (U.S. Air Force photo/Staff Sgt Joshua Strang)