Typhoon Shirley
August 17 - 22, 1968

The eye of Typhoon 'Shirley' was one of the very few which have passed directly over Hong Kong. There were only two previous occasions, November 23, 1939 and May 19, 1961, when the eye of a tropical cyclone has crossed the Royal Observatory. In Hong Kong early warnings of Typhoon 'Shirley' were provided and the damage caused by this typhoon was relatively slight despite its hurricane force winds and heavy rainfall. However, the force of Shirley's winds damaged a large number of trees at Victoria Park in Causeway Bay and uprooted many of them.

Early on August 17, a tropical depression formed over the western Pacific about 650 miles to the east of Manila and was moving on a generally westerly track. At 6.03 a.m. the next day, a reconnaissance aircraft reported a minimum sea-level pressure of 991 mb and the depression was upgraded to a tropical storm named 'Shirley'. A cloud picture received from a weather satellite during the morning (Figure 1) revealed that the circulation was well-organized and covered a diameter of about 360 miles. At this time, the storm began to turn to a west-northwest course and moved towards the northern tip of Luzon. 'Shirley' intensified to a severe tropical storm early on August 19 when a minimum sea-level pressure of 962 mb was reported near its centre by a reconnaissance aircraft. At 3.00 p.m. on the same day, a land station over Luzon about 65 miles west of the centre experienced winds of 40 knots. The storm moved in a more northerly direction after it entered the South China Sea, and Local Storm Signal No. 1 was hoisted at 6.10 a.m. on August 20 when it was centred about 350 miles southeast of the Colony. This was replaced by No. 3 signal at 6.15 p.m. as 'Shirley' continued to move closer. At 9.00 p.m. on the same day, the storm passed about 70 miles south-southeast of Pratas Island where winds of 60 knots were experienced.

During the early morning of August 21, 'Shirley' changed to a northwesterly course towards Hong Kong and Local Storm Signal No. 7 was hoisted at 6.00 a.m. when the storm was about 150 miles to the southeast. An ESSA 6 APT picture of 'Shirley' taken at about 11.21 a.m. is shown as Figure 2. The eye of the storm as revealed by the Observatory radar at Tate's Cairn was approximately circular in shape with a diameter of about 50 miles (Figure 3). Strong spiral bands were observed outside the eye and the height of the rain-clouds was estimated to extend to about 10 km. 'Shirley' became a typhoon during the afternoon and the maximum winds reported by reconnaissance aircraft at 12.20 p.m. and 5.45 p.m. were 75 and 80 knots respectively. Due to the intensification of the circulation, No. 7 signal was replaced by No. 9 at 2.49 p.m. to warn the increase of gales and then No. 10 at 4.10 p.m. when it was certain that the eye would pass over Hong Kong. At this time, the centre was located about 45 miles south-southeast of Tate's Cairn. The eye of 'Shirley' passed over the Royal Observatory between 7.00 p.m. and 9.30 p.m. During this period, the winds dropped but the weather remained overcast with almost continuous rain and the typhoon appeared to have weakened slightly and the minimum sea-level pressure recorded at the Observatory at 7.00 p.m. was 968.7 mb. This was the lowest figure ever recorded in August. The barogram of Typhoon 'Shirley' recorded during the period August 19-22, 1968 is reproduced in Figure 4, and the anemogram for the period August 21-22, 1968 is shown in Figure 5.

Typhoon 'Shirley' continued to move northwest during the evening and weakened into a severe tropical storm by midnight. No. 10 signal was replaced by No. 6 at 11.45 p.m. when the storm was about 45 miles north-northwest of Tate's Cairn. As winds moderated slowly, No. 6 signal was further replaced by No. 3 at 5.25 a.m. the next morning when the storm was about 110 miles to the northwest. 'Shirley' slowed down considerably over Kwangtung Province and finally degenerated into an area of low pressure later in the morning. All signals were lowered at 12.10 p.m. on August 22.

In Hong Kong, winds freshened from the north during the evening of August 20 and were strong in the morning of August 21. Gales set in at Tate's Cairn and Waglan Island at 6.00 a.m. and 8.00 a.m. on August 21 respectively and became general over the Colony by 3.00 p.m. Winds continued to increase during the afternoon and reached hurricane force in places by 4.00 p.m. The maximum gusts recorded were 110 knots at Tate's Cairn and 113 knots at Waglan Island.

With the arrival of the eye of the typhoon, winds decreased abruptly and became generally light and variable. The lull lasted for about 2 hours. A special radiosonde ascent was made in the eye of Typhoon 'Shirley' in order to record the thermal structure of its core and the results are shown in Figure 6. At 10 p.m. on August 21, southerly gales began to blow in many places and persisted until the early hours of August 22. Winds then began to moderate slowly but remained generally strong over the Colony until the early afternoon.

The track of Typhoon 'Shirley' is given in Figure 7, which also shows tracks of tropical cyclones previously giving rise to persistent gales at the Royal Observatory.

The following daily amounts of rainfall were recorded at the Observatory.

The times and heights of the highest tides and maximum storm surges recorded at the various locations in the Colony during typhoon 'Shirley' were as follows : -