The Hong Kong Observatory issued its first “Very Hot Weather Warning” for the year at 11.55am on Thursday, according to a statement.
The warning is historically the earliest one ever, following a similar alert on May 12, 2011.
At 11.40am, the Observatory recorded an average temperature of 32.7C, the highest temperature so far this year.
Meanwhile, the temperature reached 33C in Sham Shui Po, Tsing Yi, Tseung Kwan O, Tin Shui Wai, Tuen Mun and Admiralty on Hong Kong Island and 34.4C in Peng Chau on the northeast coast of Lantau Island.
The Observatory said the risk of heatstroke is currently high given the hot weather. It suggested that people should drink plenty of water and avoid over-exertion when engaged in outdoor work or activities. If people do not feel well, they should take a rest in the shade or a cooler place as soon as possible.
People staying indoors without air-conditioning should keep windows open as far as possible to ensure there is adequate ventilation, it said.
It also recommended that people to wear loose clothing, suitable hats and UV-blocking sunglasses, plus use a sunscreen lotion of SPF 15 or above when outdoors.
The warning was cancelled at 2:15pm.