Roman candle SOS aid to navigation alarm amber light balefire beacon beacon fire bell bell buoy blinker blue peter buoy caution light danger sign early symptom falling barometer flare fog bell fog signal fog whistle foghorn gathering clouds glance go light gong buoy green light heliograph international alphabet flag international numeral pennant kick leer marker beacon nod nudge office omen parachute flare pilot flag poke police whistle precursor preliminary sign premonitory sign premonitory symptom prodroma prodrome quarantine flag radio beacon red flag red light rocket sailing aid semaphore semaphore flag semaphore telegraph sign signal signal beacon signal bell signal fire signal flag signal gong signal gun signal lamp signal light signal mast signal post signal rocket signal shot signal siren signal tower skull and crossbones spar buoy stop light storm petrel stormy petrel symptom the nod the wink thundercloud thunderhead tip tip-off touch traffic light traffic signal warning warning sign warning signal watch fire white flag wigwag wigwag flag wink yellow flag yellow jack