If you’re in the Washington area Monday night into Tuesday morning and hear low-flying helicopters or planes overhead, don’t be alarmed. It’s an exercise.

The North American Aerospace Defense Command, also known as NORAD, will conduct a test between midnight and 2 a.m. Tuesday to practice interception and identification procedures.

If the weather conditions are not good, the exercise could be moved to Tuesday night, NORAD said.

NORAD has conducted such exercise flights throughout the United States and Canada since the Sept. 11, 2001, attacks.