Why do Hong Kong typhoon signals usually go out at 20 minutes past or to the hour?
A check by the Post found that about 93 per cent of all warnings for the past 10 typhoons, tropical cyclones and depressions were issued at 20 minutes past or to the hour by the Observatory – including the No 10 signal for when Typhoon Wipha hit the city on Sunday.
Among the 41 warnings for the 10 typhoons, tropical cyclones and depressions, only three were sent out at 10 minutes past the hour. Thirty-eight were issued at 20 minutes past or to the hour.
Among them, 22 were issued at 20 minutes past the hour and 16 went out at 20 minutes to the hour.
The Observatory's warnings are sent out in advance and include the expected time period that the next signal will go up.
In the case of Wipha, the No 1 warning was issued at 2.20am on Saturday, before it was raised to No 3 at 2.20pm that day. The No 8 signal went out at 12.20am on Sunday before the warning was upgraded to No 9 at 7.20am and then No 10 at 9.20am.
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