Useful terms and definitions

This glossary supports consistent interpretation and communication across all departments during tropical cyclone operations.

Advisory

A formal message issued every six hours by the Meteorological Office, including warning details on location, intensity, and movement of tropical cyclones.

Tropical Wave

A trough or cyclonic curvature maximum in the trade wind easterlies.

Tropical Disturbance

A discrete system of apparently organized convection, originating in the tropics or subtropics, with non-frontal unitary character, maintaining identity for 24 hours or more.

Potential Tropical Cyclone

A term used in National Weather Service advisory products for a disturbance not yet a tropical cyclone but posing a threat of tropical storm or hurricane conditions to land areas within 48 hours.

Tropical Depression

An organized system of persistent clouds and thunderstorms with a closed low-level circulation and maximum sustained winds of 38 mph (33 knots) or less.

Tropical Cyclone

A generic meteorological term for a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters and has a closed low-level circulation.

Tropical Storm

An organized system of strong thunderstorms with a well-defined circulation and maximum sustained winds of 39 to 73 mph (34-63 knots).

Hurricane

An intense tropical weather system with well-defined circulation and sustained winds of 74 mph (64 knots) or higher.

Tropical Storm Watch

An announcement for specific areas that tropical storm conditions are possible, generally within 48 hours.

Tropical Storm Warning

A warning that tropical storm conditions, including sustained winds of 39 to 73 mph (34-63 knots), are expected in a specific area in 36 hours or less.

Hurricane Watch

Issued for areas where there is a threat of hurricane conditions, generally within 48 hours.

Hurricane Warning

Issued when hurricane conditions are expected in a designated area in 36 hours or less. Conditions include sustained winds of at least 74 mph (64 knots) and/or dangerous tides and waves.

Hurricane Centre or Eye

The relatively calm area near the center of a tropical cyclone, where winds are lighter and skies may be partly cloudy.