Typhoon Haitang's effect pushes the mercury to 37 Celsius, but a heat index factoring in humidity makes it 50 degrees If you felt hotter yesterday than the official temperature seemed to indicate, then by one measure you were probably right. The Hong Kong Observatory reported a high of 34.4 degrees Celsius at its Tsim Sha Tsui headquarters and the mercury touched 37.4 Celsius elsewhere as Typhoon Haitang - which battered Taiwan yesterday, leaving one person dead and 34 injured - sent a band of hot, still air across Hong Kong. But the effective temperature, as measured by the unofficial Heat Index, was as high as 50 Celsius - a level dangerous to health. The index - developed by the National Weather Service of the United States and measured in Hong Kong for the past five years on the website Weather Underground - takes relative humidity into account to measure how hot a person actually feels. The Observatory recorded relative humidity of 64 at midday, rising to 78 at 10pm. Scientists believe that the body's mechanisms for releasing heat, including sweating, work less well when humidity is high, as it is in Hong Kong's steamy summers, and that this makes people more vulnerable to heat-related illness. 'So, even at the same temperature, people in more humid places tend to feel hotter than those in drier places,' website founder Clarence Fong Chi-kong said. A heat index of 40 is described as dangerous, while heatstroke could be imminent at 55. 'The Heat Index can tell us more about the weather,' Mr Fong said. Scientific officer Lee Kwok-lun said the Observatory compiled a similar index for internal use, known as the 'weather stress index'. It referred to this index when deciding whether to issue a very hot weather warning. 'If the public does want this extra information, we could consider whether to disclose it.' Alexis Lau Kai-hon, acting director of the Centre for Coastal and Atmospheric Research, supported making the weather stress index public and believed it was time to educate the public about more sophisticated weather concepts.