Susan Zerbe prepares to place a wreath on the headstone for her son, Tech. Sgt. Daniel Lee Zerbe, who was one of 30 U.S. troops killed when a CH-47 Chinook helicopter was shot down in Afghanistan in 2011. Zerbe's mother and other family members of fallen troops were at the Arlington National Cemetery, Va., on Dec. 12, 2015, for Wreaths Across America Day, an event to honor veterans during the holidays. (Sean Kimmons/U.S. Air Force)

Senior Airman Sonya Alexander, 77th Air Refueling Squadron boom operator, provides fuel to an E-3 Sentry aircraft during Exercise Razor Talon, Dec. 14, 2015, over the coast of North Carolina. (U.S. Air Force)

An F-16 Fighting Falcon receives fuel from a KC-135R Stratotanker during Exercise Razor Talon, Dec. 14, 2015, over the coast of North Carolina. The aircrew and other support units from several bases conducted training missions aimed at bolstering cohesion between forces. (U.S. Air Force)

Plows remove snow in front of a C-130H Hercules aircraft assigned to the 153rd Airlift Wing, Wyoming Air National Guard, Dec. 16, 2015, in Cheyenne, Wyo. The National Weather Service reported a daily maximum snowfall record for Dec. 15 at Cheyenne Regional Airport, with 6.7 inches of snowfall. (Charles Delano/U.S. Air Force)

An F-15SG from the 428th Fighter Squadron takes off at Luke Air Force Base, Ariz., Dec. 11, 2015. The F-15s came in support of the training exercise Forging Sabre from Mountain Home Air Force Base, Idaho. Forging Sabre is an exercise involving the 428th Fighter Squadron, Luke AFB 425th Fighter Squadron and members of Singapore's armed forces. The exercise includes night and weekend flying operations at Luke Air Force Base and the Barry M. Goldwater Range complex. (Devante Williams/U.S. Air Force)

An F-16 Falcon pilot from Shaw Air Force Base, S.C., walks down the Tyndall Air Force Base, Fla., flight line Dec. 10, 2015, during Checkered Flag 16-1. Airmen and aircraft from Shaw Air Force Base came to Tyndall to participate in the 53rd Weapons Evaluation Group?s Weapons System Evaluation Program and Checkered Flag 16-1. (Sergio Gamboa/U.S. Air Force)

A U.S Air Force Security Forces airmen from the New Jersey Air National Guard's 108th Wing respond to a simulated active shooter in a VirTra firearms training simulator at a U.S. Marshal training site in Lawrenceville, N.J., on Dec. 12, 2015. Security Forces airmen from the 108th Wing got to participate in high-risk traffic stop and active shooter scenarios. (Matt Hecht/U.S. Air Force)

Senior Airman Kevin O?Neil, a 71st Rescue Squadron loadmaster, waits for engine startup Dec. 11, 2015, at a Lockheed Martin Corp. C-130 ramp in Marietta, Ga. The HC-130J Combat King II is the 2,500th C-130 manufactured by Lockheed Martin and the seventh HC-130J the 71st RQS has received. (Ryan Callaghan/U.S. Air Force)