Cyclone warning signals: What they mean?

Weather news mentions of a cyclone signal hoisted at ports. Here is why we should worry more for signal 11 and less for 1.

Signal 8 hoisted at Puducherry port as Cyclone Vardah nears Tamil Nadu.

A visual storm warning signals are signage permanently hoisted at the mast of ports to alert ships of the weather condition in the ports they approach. While certain countries hoist flags, India uses distinct signs for day and night. Day signals are a combination of cylinders and cones, while night signals are red and white lamps.

Following the back to back storms that struck Kolkata and Machilipatnam in 1864, the government decided to set up a cyclone warning system for the undivided India. The next year, Kolkata became the first port to have a storm warning system.

India uses an elaborate signal system ranging from 1 to 11, which is hoisted at the ports to warn vessels of a possible cyclonic storm. The IMD sends information periodically to ports: usually four times a day, and once every three hours in case of a cyclone.

Signal 1: It implies a low pressure area is forming far at sea and surface wind could be upto 33 knots (about 60 kmph). This signal means the port is not affected but warns of slightly higher wind speed.

Signal 2: A depression has formed far at sea with surface winds upto 34-47 knots (about 60-90 kmph). This signal is a warning for ships leaving the port.

Signal 3: A depression has formed and could affect the port. Suface winds likely between 22-27 knots (40-50 kmph). The port likely to experience squalls.

Signal 4: A deep depression has formed at sea and is likely to affect the port later. The surface winds will be around 28-33 knots (approx. 50-60 kmph). A singal four indicates possible danger to ships stationed at the port. Signals 3 and 4 indicates bad weather at the port.

Signal 5: This signal means the deep depression has transformed into a cyclonic storm and is likely to cross coast keeping port to its left. Surface winds are likely 34-47 knots (about 60-80 kmph).

Signal 6: Similar to signal 5 but the cyclone is likely to cross coast keeping port to its right.

Signal 7: A signal 7 means the cyclone is likely to cross coast over or near the port. Signals 5, 6 and 7 indicate danger to the port.

Signal 8: A ‘very danger’ warning, which means the cyclone is now a severe or very severe cyclone and is expected to move keeping the port to its left. Surface winds are likely between 48 and 63 knots (approx. 90-120 kmph).

Signal 9: A ‘very danger’ warning, which means the cyclone is now a severe or very severe cyclone and is expected to move keeping the port to its right.

Signal 10: A ‘very danger’ warning, which means it is a very severe cyclone and is expected to cross over or near the port. Wind speed will be 64-119 knots (120-220 km/h) super cyclone - above 120 knots (over 220 kmph and more)

Signal 11: This means all communications have failed with cyclone warning office.