Целая наука

In the U.S. Air Force, U.S. Navy, Marine Corps, U.S. Coast Guard, British Army, Royal Marines and the Royal Air Force hand salutes are only given when a cover (protection for the head, usually a hat) is worn. Officers of the Royal Navy wearing civilian clothes render a hand salute when hatless; if wearing a hat a salute is rendered by lifting the hat (known as 'doffing') instead. The U.S. Army gives salutes both covered and uncovered. Saluting indoors is forbidden except when formally reporting to a superior officer or during an indoor ceremony.

http://en.wikipedia.org/wiki/Salute