It's that time of year again when a narrow band of thunderstorms known as squall lines can strike unexpectedly. Known locally as shi hu feng, or "wind of the stony lake", one such squall line swept across Hong Kong on June 9 - toppling 20 trees at Ap Lei Chau in a matter of minutes.

The Hong Kong Observatory reported peak gusts of 77 and 101 kilometres an hour at Central and Shek Kwu Chau. This was the cause of  fierce winds at Ap Lei Chau and Mid-levels on Hong Kong Island that day. 

It's raining, it's blowing: The hash lines show the location of the squall line and the maximum gusts in kilometres per hour recorded at several automatic weather stations on the night of June 9. The radar image captured by the Doppler weather radar at Tai Mo Shan at 9.12pm shows the squall line had just passed Ap Lei Chau and rain had started falling there.

After studying all the available data, the observatory said there was no evidence to support speculation that a tornado, a waterspout or a microburst had occurred.

A squall line travels fast, wreaking havoc in its wake. Apart from heavy rain and thunder, it also brings a sudden change in wind direction and abrupt increases in wind speed. 

On June 9, during heavy rain and squally thunderstorms, a squall line developed over the Pearl River Estuary and moved southeastward across Hong Kong at about 50 km per hour between 8.30pm and 9.30pm. Behind the squall line, the colder air descended and spread forward on hitting the ground.

The pattern of fallen trees at Ap Lei Chau and eye-witness reports suggests winds blew in one direction throughout the event. 

Anyone caught outdoors as a squall line passes should seek shelter as soon as possible, and avoid standing under or near trees, lamp posts and other structures that might be blown down. The system usually passes within minutes.

Severe squall lines were previously reported in Hong Kong on May 9, 2005 and May 9, 2001.