Explainer | Typhoon Kompasu: why is it so hard to predict and what can Hongkongers do to stay safe when typhoons hit?

Hong Kong Observatory uses signal numbers 1, 3, 8, 9 and 10 to indicate strength of the typhoon, based on wind speeds in measuring stations across city

Observatory’s work complicated by northeast monsoon winds and climate change, both of which make predicting course and intensity of coming typhoon difficult

Reading Time:5 minutes

Members of the public are advised to pay attention to the changing typhoon warnings and stay indoors where possible. Photo: Felix Wong

Published: 8:30pm, 12 Oct 2021Updated: 7:12am, 13 Oct 2021

This story has been made freely available as a public service to our readers. Please consider supporting SCMP’s journalism by subscribing.

Hong Kong is facing another shutdown after the Observatory raised a second No 8 typhoon warning signal in three days with the approach of Typhoon Kompasu, which is expected to bring more strong winds and heavy rains.

It comes on the heels of Tropical Storm Lionrock – named after Hong Kong’s iconic landmark – which brought record-breaking rains and caused 30 storeys of scaffolding to collapse, killing one woman. The city’s forecasters were caught off guard and had to change their predictions four times in a day.

With Kompasu – named after the Japanese word for compass – likely to be a stronger storm, the Post looks at the factors that complicate tropical cyclone forecasts in autumn, what residents should do to stay safe and how the weather authorities deal with typhoons in general.

What are the typhoon warning signals based on?

Typhoons usually hit Hong Kong from May to November, and are most prevalent in September. They may also form during the autumn months, but other weather conditions often complicate these storms, making them more erratic and unpredictable.