Chill out on a hot day - or vice versa

Today is a glorious autumn day in Hong Kong: bright and sunny, not a cloud in the sky, and the distant hills actually visible for a change, indicating that the level of air pollution is unusually low. The temperature is in the mid-20s (C), with a gentle breeze; if this was England, people would be heading for the beach.

So I come in to the office this morning, wearing light trousers and a long sleeved shirt. I get into the lift, and notice that half the people in there are all bundled up in coats and sweaters, even scarves, as if it was the middle of winter.

What is this thing with Hong Kong people about dressing by the calendar instead of the thermometer? The territory's nicest weather is often around this time of year, when the stifling summer humidity has gone but there is still a pleasant warmth in the air; yet somehow as soon as it hits September each year, half the folks here seem to think it's officially winter and start to dress for the Arctic. Is this a fashion thing, or what? I would really like to know.