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?

    中國空軍今天聲稱，該國有史以來規模最大的人工影響天氣作業及時地為國慶閱兵清理了天安門廣場上方的天空。

    我在燦爛的藍天下撰寫這篇文章。一掃過去幾年我在北京已經習慣了的沉悶的陰霾，今天的光線如此銳利，幾乎刺傷了我的眼睛。

    如此戲劇性的轉變令人毛骨悚然。當我昨天飛抵北京時，這座城市被一層看上去像是厚厚的煙霧的東西籠罩著。

    天氣預報員表示午夜將會下雨，然后，就像變戲法似的，第一滴雨幾乎準時落下。

    即便如此，當我今天凌晨4點離開家時，天空仍是如此陰沉，以致我的出租車司機因為能見度低而放慢了車速。"污染",他解釋道。

    但事實如此嗎？早上5點30分，當我抵達國慶閱兵的媒體中心時，天空已經清晰得足以看見星星。不過天上仍然有一簇簇厚云和一些薄霧。我想，可能又要下雨了。

    上午7點20分左右，我們乘坐大巴到紫禁城外的新聞記者席。一條積云帶在人民大會堂上空徘徊。一小時后，當我打電話給朋友時，云層變暗了。

    就在我掛斷電話前，我把天空描述給他聽。我說："閱兵時可能還是會下雨，不過我想勝算現在在它們的'組織者'那邊。"

    游行開始后，我全神貫注，但我記得巨型屏幕上展現的罕有的明亮天空給我留下了深刻的印象。

    其他同事告訴我他們發現了一個不尋常的現象，那些云似乎都被阻擋在廣場之外，盡管仍有一些在邊緣周圍。

    到了上午11點左右，飛機列隊飛過時天空仍是清晰的，直到空軍飛機在空中拉出道道彩色煙帶。現在，6小時過去了，北京還在享受著完美的天空。

    發生了什么事？根據中國氣象新聞，在上午7點30分至9點05分之間，一堆從西南方向接近天安門的云遭到4次攻擊。為了達到預期效果，本次作業總共發射了432支火箭。

    新華社報道稱，當局也有能力延遲降雨。

    "當今世上只有幾個國家能夠展開這么大規模、神乎其神的人工影響天氣作業，"它引述空軍高級氣象學家崔廉清的話。

    然而，播云技術（cloud seeding）通常被認為其精確度遠遠不足以保證獲得今天看到的效果，這引出了幾個問題。如果清理天空是如此容易，那為什么當局不一直做這項工作呢？是成本問題嗎？擔心過度使用化學品嗎？還是當局今天只是運氣好而已？