If it has a closed circulation but under 39 mph (34 knots, or 17 meters per second) maximum sustained surface winds, it is called a tropical depression. If it lacks a closed circulation it is called a tropical disturbance. Tropical cyclones derive their energy from vertical temperature differences, are symmetrical, and have a warm core.
A tropical cyclone is a generic term for a low-pressure system that formed over tropical waters (25°S to 25°N) with thunderstorm activity near the center of its closed, cyclonic winds.
