Can we have our blue skies back? Please?

You don't need a chemistry degree or expensive monitoring equipment to tell when the pollution is bad. When the view from the window shows the harbour shrouded in a smoggy haze, you know that whatever is in the air can't be good for us.

But how do you tell if it is getting better or worse?

If you've lived here long enough to look back five years or more, you'll definitely know things have got worse since then. But it's harder to tell if pollution levels are still rising, or we are starting to see signs of improvement.

Enter the Batgung Smog Index, the BSI. It's based on the simple approach described above – looking out of the window. We've taken the 'hours of reduced visibility' records published by the Hong Kong Observatory, and done some simple maths to give an easy to understand number. You can get the full description of the calculations here, but simply put, the more hours of smoggy weather there are, the higher the index goes.

The index is set so that the level for October 1989 is 100. No great scientific reason for choosing that date, it just happens to be the month I first arrived in Hong Kong. We're nearly 20 years on from then, so what do you think the current level of the index is?

The latest figures from the observatory set the April 2008 BSI at 436. That means we all have to put up with over four times as much smoggy weather each year as we did in the late 1980's.

Here's the chart of the index from the early 1970's until April 2008.

The level rises and falls in the 1970's and 80's, but from the 1990's it starts climbing rapidly. From 100 in Oct 1989 it crosses 200 in Oct 1997, 300 in Dec 2003, 400 in Jan 2005, and peaks at 499 in Oct 2006.

Encouragingly the level has fallen since then, but it isn't clear yet if it's the start of a long-term improvement, or a temporary reversal like we saw around 1995 and 2001. We'll have to watch the index to find out - we'll update it as the observatory releases new figures.

Any guesses how long we'll have to wait before the index hits 100 again, and we get our blue skies back?

MrB