Can you forecast Smog?

Can the author of your article on SMOG in Hong Kong, or anyone else, say whether it is possible to tell if the next day or the next few days will be smoggy or clear? What has the greatest effect - wind, temperature, cloud, rain,  traffic (Sunday), etc.? Where would one go for such information?

I lived in Hong Kong between 1963 and 1966, and will be returning for a holiday in May 2010. This information would be very useful, so as to choose which days are best for long-distance photography from the hills or around the harbour.

Many thanks

James Carey, Paris, France

17 October 2009

Smog forecasting

Hello James!

That's an excellent question.

The HK Observatory doesn't really try to forecast air quality more than one day ahead (click the 'Other' tab and choose Air Pollution Index at the HKO site), although occasionally they will include the word 'haze' in their 7-day forecasts, which almost always means 'just plain bad air'. My impression is that such forecasts are broadly accurate, i.e. if they suggest it's going to be hazy, you'd better leave the camera home for the long shots.

There are a couple of other items you can look for in the HKO's extended forecasts. In the fall, winter and spring -- i.e. anytime HK can be affected by the northeast monsoon -- watch out for the words 'light winds'. This usually means a surge of the NE is dying out, and that often means bad air. But even moderate winds from the north or northeast can signal spells of bad air, for reasons MrB has explained.

'Light winds' are typically a bad sign in the summer as well, as it's a common condition when a typhoon is approaching HK from the east, which again usually means terrible air quality. Another sign of a typhoon approaching is northerly or westerly winds in the summer or early autumn. Again, this almost automatically means poor air quality.

The late spring is often a good time for air quality, so your trip in May is promising. Days with southwesterly or southerly winds are typically clear, and days after heavy rains (common at that time of year, as I'm sure you remember) are often the best, as the rain washes the air and leaves it pristine.

Good luck on your trip, and drop us a line if you have any other questions before you come. It would also be very interesting to hear your impressions of HK after so many years away.

Mr Tall



Dear Mr. Tall

Thank you. That is very helpful. Smoggy or clear, I know that I will enjoy my return.

Best Wishes

James Carey


smog prediction update

Just a note to follow up my general observations about smog in HK and its relation to weather and wind direction.

Today's weather is a case in point. It's unseasonably warm and humid, as the typcial autumn northeast monsoon wind pattern has died out (temporarily; a big surge is due later today), leaving us with southeasterly winds. The result? Very clear air with sweet blue skies.

Since the approaching surge is anticipated to be a very strong one, with attendant rain showers, let's see if it maintains the clear air.