Have you ever looked up into the sky on a clear day and seen an airplane flying high in the clouds? If so, you’ve probably also noticed the long white streaks it leaves behind in its path. They are called contrails, and there’s more to these white lines in the sky than meets the eye.
The jet fuel that powers the engines of a plane is intensely hot, often burning at more than 500°C. In contrast, the air temperature at high altitudes where commercial flights typically cruise can reach as low as -56°C. When the hot exhaust gases, which include water vapor produced by the burning of fuel, encounter the frigid surrounding air, they cool rapidly. This rapid cooling causes the water vapor to condense into tiny water droplets or freeze into ice crystals, forming visible trails known as contrails.
Contrails can help us make predictions on the weather based on their appearance. If a contrail is short and thin, it means low humidity at high altitudes, which is a sign of fair weather. A thicker, more persistent contrail suggests higher moisture levels in the air and can be a potential indicator of approaching storms. Also, contrails that look curvier and more stretched can indicate high winds in the upper atmosphere.
Photo: AdobeStock I 照片：AdobeStock
However, these contrails are not without impact on the environment. They trap heat that would normally escape Earth and lead to a warming effect. With such a high volume of flights occurring every day all over the world, the contrails we generate are having a measurable impact on climate change. For this reason, more companies are now looking into renewable fuels and hybrid airplane engines. With a little luck, these contrails can become a sign of better flying ahead.
你是否曾在晴朗的日子抬頭仰望天空，看到一架飛機在雲層中高高翱翔呢？如果有的話，你可能還曾注意到飛機在飛過的路徑上留下長長的白色條紋。這些條紋被稱為凝結尾，天空中的這些白色線條不像表面上所見的那麼簡單。
為飛機引擎提供動力的噴射機燃料非常熱，通常燃燒溫度超過攝氏五百度。相比之下，在商業航班通常進行平穩航行的高海拔處，氣溫可低至攝氏零下五十六度。當熱廢氣 ── 包含燃料燃燒產生的水蒸氣 ── 遇到周圍嚴寒的空氣時，會迅速冷卻。這種快速冷卻導致水蒸氣凝結成微小的水滴或凍結成冰晶，形成稱為凝結尾的可見軌跡。
Photos: AdobeStock I 照片：AdobeStock
凝結尾可以幫助我們根據其外觀來預測天氣。如果凝結尾又短又細，就表示高海拔地區的溼度較低，這是天氣晴朗的跡象。較厚、持續時間較久的凝結尾表示空氣中的溼氣較高，這可能是暴風雨即將來臨的潛在徵兆。此外，看起來較彎曲、較長的凝結尾可能表明較上層大氣中存在強風。
然而，這些凝結尾並非對環境沒有影響。凝結尾讓本來正常會散出地球的熱滯留並產生暖化效應。由於世界各地每天都有如此大量的航班，所以我們產生的凝結尾對氣候變遷有顯著的影響。因此，愈來愈多公司正在研究可再生燃料和混合動力飛機引擎。如果運氣好的話，這些凝結尾將來可以成為航空更美好的跡象。
What Did You Learn?
Photos: AdobeStock I 照片：AdobeStock
1. If you see a contrail that lasts for an extended period of time, what kind of weather does it indicate?
2. How do contrails pose a threat to the environment?
參考答案：
1. An approaching storm.
2. They (trap heat that would normally escape Earth and) lead to a warming effect.
Words in use
1. intensely adv. 極度地，強烈地
The student focused intensely on her studies to achieve good grades.
那名學生極其專注地用功讀書，以取得好成績。
2. cruise vi. 以平穩的速度航行，巡航
The ship is cruising through calm waters.
那艘船正以平穩的速度航行在風平浪靜的水域。
3. exhaust n. 廢氣
The exhaust from the factory’s chimney polluted the air in the surrounding area.
那間工廠的煙囪所排出的廢氣汙染了鄰近地區的空氣。
4. encounter vt. 遇到，遭遇
The divers encountered a sea turtle gliding gracefully through the water.
那些潛水客遇到一隻海龜在水中優雅地滑行。
5. condense vi. 凝結（從氣體變液體）
The cold surface of the mirror will cause steam to condense, obscuring the reflection.
鏡子冰冷的表面會導致水蒸氣凝結，模糊鏡中映像。
Practical Phrases
1. leave behind... / leave... behind 留下……
The master’s legacy will leave behind a lasting impact on future generations.
那位大師的遺留之物會對後世留下長久的影響。
2. There’s more to N than meets the eye.
……不如表面所見的那麼簡單。
There’s more to the city than meets the eye; it has a rich cultural history.
這座城市不如表面所見的那麼簡單；它有豐富的文化歷史。
3. in contrast 相較之下
Andy enjoys mountain climbing; in contrast, his sister prefers surfing at the beach.
安迪喜愛登山；相比之下，他妹喜歡在沙灘衝浪。
4. look into... 研究∕調查……
The detective will look into the case to find new leads.
那位警探將調查那起案件以尋找新的線索。
聽文章朗讀及講解: https://ivy.pse.is/455bfu
本文出自常春藤解析英語雜誌: www.ivy.com.tw
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