Hong Kong's winters could vanish within 50 years, with the number of cold days declining virtually to zero due to global warming and urbanization, the head of the city's weather observatory warned on Friday.

"According to our projections, toward the end of this century ... there will be less than one cold day each winter, so winter practically will have disappeared," said Lam Chiu-ying, director of the Hong Kong observatory.

Cold days are defined as those with temperatures below 12 degrees Celsius (53.6 degrees Fahrenheit) at some point during the day.

Despite its sweltering summers, the former British colony enjoys a sub-tropical climate, with cool winter temperatures and hoar frost sometimes found on its highest peak of Tai Mo Shan.

Between 1961 and 1990, there was an average of 21 cold days every winter but this figure had already halved by 2000, Lam said.

Over the past century, temperatures in Hong Kong rose around 1.2 degrees, almost double the global average, said Lam, who warned that the city's winters might actually vanish half a century earlier if Hong Kong's rapid urbanization trends were taken into account.

"We would really start losing the very distinct seasonal march throughout the year ... We would really look more tropical than we (are) now," Lam told reporters.

The number of summer "hot-nights" in Hong Kong, with temperatures above 28 degrees Celsius, has already jumped to 30 a year -- an almost four-fold increase from the 1990s, said Lam, as heat trapped during the day by the concrete city and its teeming skyscrapers is unable to dissipate fully at night.

Lam attributed Hong Kong's forecast temperature rise of three to four degrees by the end of the century to urbanization and global warming in equal measure.