HOW DO YOU FLY AROUND A 50,000- FOOT CLOUD?如何在飛行時繞過50,000呎的雲層?

Answer: you don’t. But Cathay Pacific pilots have their own way of dealing with the weather system that creates them – the Intertropical Convergence Zone.答案:根本不必這樣做。國泰航空的機師自有方法應付因熱帶輻合帶而產生的巨大雲層。撰文:

Discovery - - HOW WE FLY 翱翔之道 - Phil Heard By PHIL HEARD

IT’S A FAMILIAR routine. The gentle chime, the illuminated sign, and the request to return to your seat and buckle up. Asian weather can be volatile, particularly in summer with the sun heating warm, shallow seas, incubating the seasonal typhoons – which can make the inflight experience bumpy.

While these storms can cause disruption on the ground, it’s possible to fly over or around them at cruising altitude with barely a tremor. There is, though, a meteorological phenomenon that’s both global and yearround, and which always requires some thought on the flight deck: the ITCZ – the Intertropical Convergence Zone.

This is a belt of low pressure that spans the globe near the equator where the trade winds of the northern and southern hemispheres meet. It varies in width, and moves following the sun over the year. In summer months it drifts closer to Hong Kong, buckling further north over land, which heats quicker than the sea.

Glenn Devonport is a line operations manager and captain on an Airbus A350, with plenty of experience in avoiding the worst of the weather. The convective activity created by the heating of the sea and land creates lots of energy, says Glenn, ‘and that creates these bubbly clouds with large volumes of water, energy and turbulence. These are quite well separated, with one storm cell at a reasonable distance from the next, so we use radar to get around them.’

The ITCZ is slightly different, though. Glenn says: ‘Some of its cumulonimbus clouds can reach more than 50,000 feet. That’s higher than we fly and so you can spend hours dodging this weather, particularly south of Hong Kong.’

Pilots use whatever cues they can to plot the best route. ‘Primarily, that’s weather radar,’ says Glenn. ‘ The radar beam reflects off the water particles in a cloud, forming the picture we see on our screen. The more water in a cloud, the more active it is, so this will go from green to yellow to red. The reflectivity of the radar can also track the vertical movement of the water droplets in a cloud. If droplets are moving up and down significantly, that will create a purple colour, which indicates very strong turbulence.

‘Often we have to pick a path with the least amount of rain or cloud, but we always make sure that it’s safe to do so.’

And only then does the chime sound and the seat belt sign come on.

這種情況大家應該十分熟悉:機艙響起一陣柔和的提示音,然後扣上安全帶的標誌亮起來,接著聽到請乘客返回座位並扣上安全帶的廣播。亞洲各地的天氣經常反覆多變,尤其在夏季,太陽的高溫照射在海洋的淺水區域內,令這個地區醞釀季節性颱風,可令飛機內的乘客感到顛簸不穩。

這些風暴可在地面造成嚴重的破壞,飛機可以在巡航高度飛越或繞過風暴,其間機艙只會出現輕微的震動。這是一個全年均會出現的全球氣象,稱為熱帶輻合帶 (英文簡稱ITCZ),也是駕駛艙內的機師必須留意的天氣現象。

由於南、北半球的貿易風在赤道附近相遇,於是形成橫跨全球的低氣壓帶。低氣壓帶的闊度不一,終年隨著太陽的位置移動,夏季時會移近香港,再向北往內陸推進,而陸地的溫度比海洋上升得更快。

Glenn Devonport是航線營運經理及空中巴士A350客機的機長,對於如何避免惡劣天氣,有豐富的輕驗。Glenn表示,由於海洋和陸地受熱後出現的空氣對流產生大量能量,「造成厚厚的雲層,裡面藏有大量水分、能量和氣流。這些風暴胞彼此分隔,兩個風暴胞之間有頗大的距離,我們可以用雷達探測出來,然後避過它們。」

不過熱帶輻合帶則有點不同。Glenn說:「有些積雨雲可以達到50,000呎以上,比我們飛行的高度更高,因此可能要花上數小時來躲避這種天氣,尤其是在香港以南。」

機師會運用各種儀器和資訊來制定最妥善的飛行路線。Glenn說:「主要是利用天氣雷達來探測。雷達發出的訊號遇到雲層中的水分子,就會反射回來,在屏幕上形成一幅可以見到的圖象。雲層中的水分愈多就愈活躍,因此圖象會按不同程度以綠色、黃色及紅色來顯示。雷達的反射同時可以追蹤雲層內水滴的垂直運動,如果水滴不斷大量上下移動,就會以紫色來顯示,表示有非常強烈的氣流。」

「我們通常都會選擇雨量或雲量最少的路線來飛行,不過一定會確保安全。」

於是,那個時候機上就會響起提示音,並且亮起扣上安全帶的標誌。

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