Nighttime temperatures creeping ever higher

PUBLISHED : Thursday, 07 June, 2007, 12:00am
UPDATED : Thursday, 07 June, 2007, 12:00am

Summer evenings heating up in our concrete jungle

Summer evenings that once provided some respite from the heat of the day are now hotter than ever, according to research that found minimum temperatures have risen steadily over the past 40 years.

The reason, according to Yan Yuk-yee, professor of geography at Baptist University, is the city's transformation into a high-rise concrete jungle that traps heat during the day and releases it at night.

Analysing Observatory temperature data from 1965 to 2003, Professor Yan found the average minimum temperature had increased by 0.02 degrees Celsius a year on average.

The average minimum temperatures were calculated using the daily minimum temperatures from May 1 to October 15. Her study also showed the average maximum temperature over the same period had decreased by 0.014 degrees every year.

But Professor Yan said her findings did not contradict the Observatory's, which had said the weather was getting warmer. 'The reason for the average maximum temperature going down is that pollution is getting more serious and pollutants can block solar radiation and make the sky hazy and cloudy,' she said.

'We should note that the average minimum temperature is increasing at a much faster rate than the decreasing average maximum temperature,' she said, adding that it would add up to Hong Kong being a warmer place over the long term.

'We used to feel cooler in the evening when we came home after work. But now the first thing we do as soon as we return home is to turn on the air conditioners because it is still very hot during the night.'

She blamed urban planning and the high density of tall buildings for the phenomenon.

'Hong Kong is highly urbanised with tall concrete buildings. Concrete absorbs heat during the day and releases heat at night - that's why we have warmer nights than before.'

Professor Yan urged people to adopt a simpler lifestyle and to reduce pollution.

'Since we have warmer nights now, everybody turns on their air conditioners at night which means power stations have to burn more fuel to support electricity supply. This is a vicious circle.'

Professor Yan also said it was good the Observatory was studying the possibility of introducing a heat index that people could refer to.

The index takes relative humidity into account to measure how hot a person actually feels. Scientists believe the body's mechanisms for releasing heat, including sweating, work less well when humidity is high and that this makes people more vulnerable to heat-related illnesses.

'The index might be important for people with poor thermal regulatory systems and those who cannot stand high temperatures, such as the elderly. The index can remind them to take necessary precautions,' she said.

Natural balance

How we cope with changes in temperature is dependent on many factors. Some deal better than others, but the same basic principles apply to everyone

The human body can be viewed as an energy exchange surface, with heat constantly arriving and leaving

When there is a significant net flow of energy into or out of the body, our temperature changes and we are subject to heat stress. Too hot and we suffer heat stroke, too cold and we become hypothermic

The skin and lungs are responsible for heat loss through water evaporation

Heat is generated in the body by metabolism - the liberation of stored chemical energy (from food or fat deposits)

The body, being mostly water, also stores heat

Heat is also absorbed and radiated through the skin. Whether there is a net gain or loss depends on conditions