Number 8 Signal during the Passage of Typhoon Yutu

Many members of the public and the media made inquiries on the hoisting of Number 8 signal during the passage of typhoon Yutu. The following is a summary of the information provided.

Forecast wind condition over Hong Kong is the basis to determine the tropical cyclone signal to be hoisted. The Signal Number 8 will be hoisted when gale or storm force wind is expected or blowing in the Victoria Harbour, with a sustained wind speed of 63-117 kilometres per hour. Public safety is the prime concern of the Hong Kong Observatory.

Before hoisting the Number 8 Signal on 25 July 2001, the notable events were that Yutu intensified rapidly into a typhoon. The maximum wind speeds near the centre of Yutu rapidly increased from 120 kilometres per hour to 150 kilometres per hour within 3 hours, before the Number 8 Signal was hoisted. Gale force winds were affecting areas within 160 kilometres of Yutu. As Yutu was expected to come within 150 kilometres of Hong Kong before dawn, it was necessary to hoist the Number 8 Signal at 00:30 a.m. on 25 July.

Subsequently, Typhoon Yutu (Figure 1) took on a slightly more westerly track than expected and at its closest, the typhoon was about 180 kilometres from Hong Kong. Prolonged gales affected the southern part of Hong Kong while the urban areas were spared the full impact of the destructive winds (Figure 2).

Throughout the day on 25 July, Typhoon Yutu was within 200 kilometres of Hong Kong. The Observatory did not expect a rapid departure of Yutu from the territory. Indeed, the possibility of Yutu coming even closer by taking on a more northwesterly track could not be ruled out. As Yutu was a full fledged typhoon which was already causing gale force winds over the southern part of Hong Kong and on high grounds, a slight change in its direction of movement would bring it closer to Hong Kong and expose most of the territory to the full impact of gale force winds. As Yutu was posing a serious threat to the territory throughout the day on 25 July, the Number 8 Signal remained hoisted until the evening.

By the evening, Yutu gradually moved away from Hong Kong and local winds also started to fall below gale force. The Number 8 Signal was replaced by the Number 3 Signal at 7:40 p.m. on 25 July.

Fig. 1 - Satellite Image of Typhoon Yutu at 0900 (HKT) on 25 July 2001