A new "extremely hot" weather warning will be issued when the mercury soars this summer, the Hong Kong Observatory revealed on Thursday, as it warned that 2023 is likely to be one of the warmest years on record.

If temperatures of 35 degrees Celsius or higher are recorded at the forecaster's Tsim Sha Tsui base or in the New Territories generally it will send out the warning, reminding people to take extra precautions and mind the effects on their health of the sweltering heat.

The Observatory’s new director, Chan Pak-wai, said authorities decided to introduce a further weather warning after temperatures hit 35 degrees or above on 15 days in 2022, which he described as a "relatively high number".

Last year was also among the sixth warmest years since records began in 1884, with annual mean temperature of 23.9 degrees. July alone was the hottest month on record in Hong Kong, as the monthly average temperature reached 30.3 degrees.

The past three years have all been among the hottest on record, and they also had the highest number of days when the mercury hits 33 degrees or higher – enough to trigger a very hot weather warning.

Chan, who took up the new role earlier this month, cautioned that the annual temperatures are clearly on the rise despite some variability.

“We can conclude that [2023] is going to be among the top 10 years with the highest temperature. But then the actual number of hot days is still going to vary year to year,” Chan said.

“We look at the longer-term trend. Basically the temperature is rising and the number of very hot days is going to rise. But for year-to-year, there is still some variability, so that it's difficult to conclude at this stage.”

The Observatory also said the tropical cyclone season might start as early as June and continue until October at the latest. During the year, roughly four to seven tropical cyclones are predicted to come within 500 kilometres of Hong Kong.

Annual rainfall, meanwhile, is expected to be normal to below normal, but the forecaster said Hong Kong would still be struck by heavy downpours.

Citing global warming and extreme weather conditions, Chan said he would not rule out the possibility of another tropical cyclone with a similar strength to the intense Typhoon Mangkhut, which brought devastation to the SAR in 2018.

“Right now, we have used quite a number of different channels to give an earlier alert or information to the public," he said. "When the tropical cyclone is really going to affect Hong Kong very shortly, or is already affecting Hong Kong, we have the tropical cyclone warning signals."

Chan said the Observatory hoped to inform the public about tropical cyclones as soon as possible. But with the uncertain path and strength of tropical cyclones there might be false alarms if the Observatory sends out a signal too early, he added.