The Chinese air force claimed today that the biggest weather modification operation in the country's history cleared the skies over Tiananmen Square just in time for the National Day parade.

I write this post under gorgeously azure skies. Instead of the dull haze I have grown used to in Beijing over the past few years, the light is so sharp that it almost hurts my eyes.

The transformation is so dramatic it is eerie. When I flew into Beijing yesterday, the city was shrouded in what looked like a thick smog.

Weather forecasters said it would rain around midnight and, hey presto, the first drops of rain started to fall almost on the dot.

Even so, when I left the house at 4am this morning, it was still so gloomy that my taxi driver slowed the car because visibility was poor. "Wuran" （pollution）, he explained.

But was it? By 5.30am, when I arrived at the media centre for the National Day parade, the skies had cleared sufficiently to be able to see a star. But there were still thick clusters of cloud and some mist. It might rain again, I thought.

We were bussed to the press gallery outside the Forbidden City by about 7.20am. A band of cumulus lingered over the Great Hall of the People. They had darkened when I called a friend an hour later.

Just before I rang off, I described the skies to him. "It might still rain on the parade, but I think the odds are now on their [the organisers'] side," I said.

Once the march started, I concentrated on that, but I recall being impressed by the unusual vividness of the sky on some of the images on the giant screens.

Other colleagues told me they found it remarkable that the clouds seemed almost to be held back from the square, even though there were still some around the edges.

By the time of the fly-past around 11am, the skies were clear until air force jets left behind lines of coloured smoke-trails. Now, six hours later, Beijing is still enjoying perfect conditions.

What happened? According to Chinese Meterology News, there were four attacks on a bank of clouds that approached Tiananmen from the south-west between 7.30am and 9.05am. In total 432 rockets were fired to achieve the desired result.

Xinhua news agency reports that the authorities also had the capacity to delay rainfall.

"Only a handful countries in the world could organise such large-scale magic-like weather modification," it quoted Cui Lianqing, a senior air force meteorologist as saying.

But cloud seeding is generally considered far too imprecise a technique to guarantee the results seen today, which begs several questions. If clearing the skies is this easy, why don't the authorities do it all the time? Is it the cost? Concern about over-use of chemicals? Or were the authorities just lucky today?

中国空军今天声称，该国有史以来规模最大的人工影响天气作业及时地为国庆阅兵清理了天安门广场上方的天空。

我在灿烂的蓝天下撰写这篇文章。一扫过去几年我在北京已经习惯了的沉闷的阴霾，今天的光线如此锐利，几乎刺伤了我的眼睛。

如此戏剧性的转变令人毛骨悚然。当我昨天飞抵北京时，这座城市被一层看上去像是厚厚的烟雾的东西笼罩着。

天气预报员表示午夜将会下雨，然后，就像变戏法似的，第一滴雨几乎准时落下。

即便如此，当我今天凌晨4点离开家时，天空仍是如此阴沉，以致我的出租车司机因为能见度低而放慢了车速。"污染",他解释道。

但事实如此吗？早上5点30分，当我抵达国庆阅兵的媒体中心时，天空已经清晰得足以看见星星。不过天上仍然有一簇簇厚云和一些薄雾。我想，可能又要下雨了。

上午7点20分左右，我们乘坐大巴到紫禁城外的新闻记者席。一条积云带在人民大会堂上空徘徊。一小时后，当我打电话给朋友时，云层变暗了。

就在我挂断电话前，我把天空描述给他听。我说："阅兵时可能还是会下雨，不过我想胜算现在在它们的'组织者'那边。"

游行开始后，我全神贯注，但我记得巨型屏幕上展现的罕有的明亮天空给我留下了深刻的印象。

其他同事告诉我他们发现了一个不寻常的现象，那些云似乎都被阻挡在广场之外，尽管仍有一些在边缘周围。

到了上午11点左右，飞机列队飞过时天空仍是清晰的，直到空军飞机在空中拉出道道彩色烟带。现在，6小时过去了，北京还在享受着完美的天空。

发生了什么事？根据中国气象新闻，在上午7点30分至9点05分之间，一堆从西南方向接近天安门的云遭到4次攻击。为了达到预期效果，本次作业总共发射了432支火箭。

新华社报道称，当局也有能力延迟降雨。

"当今世上只有几个国家能够展开这么大规模、神乎其神的人工影响天气作业，"它引述空军高级气象学家崔廉清的话。

然而，播云技术（cloud seeding）通常被认为其精确度远远不足以保证获得今天看到的效果，这引出了几个问题。如果清理天空是如此容易，那为什么当局不一直做这项工作呢？是成本问题吗？担心过度使用化学品吗？还是当局今天只是运气好而已？