Observatory's heat warnings do not give accurate temperature

PUBLISHED : Monday, 19 July, 2010, 12:00am
UPDATED : Monday, 19 July, 2010, 12:00am

The government has decided on further exemptions for the proposed idling engine law.

A random figure of 33 degrees Celsius has been put forward as the level at which drivers may keep their engines running.

When the Observatory states the temperature is 33 degrees, it is actually measured in the shade. Also, the temperature is often different at various places in Hong Kong.

The week before last, when the Observatory warned that the temperature was 33 degrees in Yuen Long, it was 37 degrees indoors and 45 degrees in open sunlight (Clear the Air measured it).

How does that correlate, in real life, with 33 degrees measured in the shade?

The US Weather Service has a heat stress index calculator (the 'feels-like' effect on the body).

A wet-bulb temperature is the temperature you feel when your skin is wet and is exposed to moving, humid air. If we take an open-sun temperature of 45 degrees and 20 per cent humidity, this equates to a feels-like temperature of 47.2 degrees. However, with the 90 per cent humidity we get here the 45 degrees becomes a feels-like temperature to your body of 133.2 degrees. Hence the Observatory warning of 33 degrees compounded with high humidity in the open sun in the New Territories would be far higher and potentially life-threatening to people working outdoors. We also do not have a wind-chill index where the feels-like temperature is affected by the wind speed.

Take the government's proposed 33 degrees (in the shade) level with 90 per cent humidity. That equates to 53.5 degrees which your body feels, if you are in the shade. Under the sun that will be 5 to 10 degrees higher depending how clear the air and UV level are.

The US Weather Service states that with a heat index of 54.4 degrees or higher, heatstroke is highly likely, with 40.5 to 54.4 degrees heat cramps and heatstroke are possible with prolonged exposure and or physical activity.

Therein lies the reasoning why we do not publish a heat index and wind chill index.

What would the tycoons say about their construction site workers being outdoors in these heat stress temperatures?

Rather than taking a random 33 degrees without a heat index qualification the government must adopt a heat index system incorporating humidity levels which is readily available.

It must do away with this daft hot weather/cold weather warning.

The heat index should be the guideline for engine idling and should be broadcast on the radio, with announcements at tunnels and taxi and minibus stands and franchised bus stops.

LCD displays could be placed above major intersection traffic lights.

Otherwise the traffic wardens will have to carry cesium clocks and wet-bulb heat index sensors to be able to do their jobs.

James Middleton, chairman, energy committee, Clear the Air