The Hong Kong Observatory yesterday issued a 'very hot' weather warning, the first such alert this year. The warning was issued at 4.30pm when the peak temperature recorded was 34.4 degrees Celsius in Sheung Shui. Temperatures are expected to reach 33 degrees today. The warning comes as a heatwave is sweeping northern and southwest regions of the mainland. An Observatory spokesman said the temperatures were being influenced by a sub-tropical weather system extending over the western Pacific. The warning advises people to seek shade, avoid overexertion and drink plenty of water to avoid heatstroke and dehydration. With the sun and clear skies likely to lure many people to beaches this weekend, precautions against sun damage are advised. According to Tammy Lee Chau Chui-ngor, an education officer with the Hong Kong Anti-Cancer Society, the incidence of skin cancer is increasing in Hong Kong. She said that it was the 10th most common cancer in 2002, and statistics showed steady increases in skin cancer cases over the past 10 years. Anthony Ying Chi-ho, an oncologist and spokesman for the society, said ultraviolet radiation was 'extremely dangerous'. The UV index, being published daily by the Observatory for the first time this year, peaked at a high 13 yesterday and is expected to reach 12 today. 'People need to be aware that lifelong damage [from sunburn] can be done before a person reaches the age of 18,' he said. 'A person only needs to be sunburnt five times to double the risk of developing melanoma... Every precaution should be made to avoid sunburn at all, as melanoma is one of the most lethal cancers.' He urged beach-goers to pay attention to weather reports and to apply suntan lotion with a minimum sun protection factor of 15 about 30 minutes before exposure.