楊舟

What is fog？

Have you ever walked out of your front door in early morning and faced a thick covering of white air that prevented you from seeing more than a few feet in front of you？ This is known as a fog. It is actually a type of stratus1 cloud but is called by a different name because it forms by a different process.

什么是霧？

你曾經(jīng)有過在清晨走出門，面對一層厚厚的白茫茫的空氣，看不到前面幾英尺開外的地方的經(jīng)歷嗎？這就是所謂的霧。它實際上是一種層云，但由于不同的形成過程，它被冠以的不同的名稱。

How is fog formed？

A stratus cloud is any low-lying cloud that makes the atmosphere hazy2. Like all clouds， they are formed when water vapour3 condenses4 when it rises to a cooler part of the earths atmosphere. However， a fog is different because the water vapour reaches its dew point （the temperature at which it turns back into water droplets） right over the surface of the earth. We cannot see water vapour because it is a gas， but when it turns back into water droplets， it becomes visible5 to the eye. This rapid cooling just over the ground could be caused by a number of reasons.

霧是怎么形成的？

層云是使大氣朦朧的低云。和所有的云一樣，它們是在水蒸氣上升到地球大氣層較冷的地方時凝結而成的。然而，霧是不同的，因為水蒸氣達到露點（它變成水滴的溫度）是在接近地面的地方。我們看不到水蒸氣，因為它是一種氣體，但當它變成水滴時，它就變得肉眼可見了。地面上的這種快速降溫可能是多種原因造成的。

What causes fog？

Falling rain can cool down the water vapour and cause it to remain suspended6 close to the ground as a fog.

When there is a big difference between the temperatures of the day and night， the land continues to lose the heat it absorbed7 during the day. The cool night air sandwiches the water vapour close to the ground resulting in a fog.

Finally， when warm， moist8 air blows over a cold surface like ice or snow， it cools it down rapidly and turns to fog.

霧的成因是什么？

下雨可以使水蒸氣降溫，并使其像霧一樣懸浮在靠近地面的地方。

當晝夜溫差較大時，土地白天吸收的熱量持續(xù)流失。夜間的冷空氣把靠近地面的水蒸氣夾在中間，形成了霧。

最后，當溫暖潮濕的空氣吹過像冰或雪一樣冷的表面時，它會迅速冷卻并變成霧。

Fog and mist

You may wonder what the difference between fog and mist is. Well they are actually one and the same. A mist is simply a fog that isnt very thick.

Fog and mist are both created by water droplets， differing only in their overall locations and density9. Fog is a cloud that reaches ground level， even if that “ground” is a hill or mountaintop. Mist forms wherever water droplets are suspended in the air by temperature inversion10， volcanic11 activity， or changes in humidity12. Fog is denser13 than mist and tends to last longer. In terms of visibility14， fog reduces it to less than one kilometer， while mist can reduce visibility to between 1 and 10 kilometers.

The exact definition is if you cannot see an object that is less than 1 km in a mist， it is considered a fog.

霧與靄

你可能會想霧和靄有什么區(qū)別？其實它們是一樣的。靄只是一種不太濃的霧。

霧和靄都是由水滴形成的，只是它們的整體位置和密度不同。霧是到達地面的云，即使“地面”是山丘或山頂。由于氣溫逆增、火山活動或濕度變化，水滴懸浮在空氣中會形成靄。霧比靄更濃，而且往往持續(xù)時間更長。在能見度方面，霧使能見度降低到1千米以下，而靄則使能見度降低到1至10千米。

確切的定義是，如果你在霧靄中看不到距離小于1千米的物體，那那就是霧。

Different causes

Fog is formed when any cloud type makes contact with the ground. In low-lying areas， such as valleys and plains， the fog bank （a mass of fog） is essentially a cloud formation.

Clouds form when water droplets condense and merge15， but fail to achieve a size large enough to precipitate16 as rain. Clouds will form or drift closer to the ground when humidity rises or changes suddenly， or when wind speeds drop or change direction.

Mist is also formed by water droplets， but with less merging or coalescing17. This means mist is less dense and quicker to dissipate18 when wind， temperature， or relative humidity changes. Mists can form due to abrupt temperature changes （such as when exhaling19 in cold air）， high levels of humidity （in a sauna， for example）， or from evaporation20 or condensation21 （such as when rain hits sun-warmed rocks and street surfaces）.

不同的成因

任何云型與地面接觸都會形成霧。在低洼地區(qū)，如山谷和平原，霧堤（一團霧）本質上是一種云的構造。

當水滴凝結并合并，但無法達到足夠大的尺寸以凝結成雨時，云就形成了。當濕度突然上升或變化，或者當風速下降或改變方向時，云會形成或漂移到離地面更近的地方。

靄也是由水滴形成的，但合并或凝聚較少。這意味著靄的密度較小，當風、溫度或相對濕度變化時，消散的速度更快。靄的形成可能是由于突然的溫度變化（例如：在冷空氣中呼氣）、高濕度（例如：在桑拿浴室）、蒸發(fā)或冷凝（例如：當雨水碰到被陽光曬熱的巖石和街道路面）。

How to drive safely in fog and mist

When driving in misty conditions， it is important for drivers to use wipers22 with care. The water droplets in mist are often not dense enough to require the constant use of wipers， so intermittent23 patterns will probably do a better job of keeping the windshield24 clear. In foggy conditions， wipers might play a smaller role to that of fog lights.

When it seems that the speed is too fast to react to what appears out of the fog or mist， it is best for drivers to slow down. If the driving speed in the fog drops to half of the posted maximum speed， it is a good idea for drivers to find a safe spot to pull well off the road and wait for the fog to clear. Many accidents are caused in fog banks by cars going too slow and being hit from behind. When pulling off the road， drivers should keep their cars hazard25 lights flashing for additional safety.

霧天如何安全駕駛

在薄霧天駕駛時，駕駛員必須小心使用雨刷。薄霧中的水滴往往密度不夠，不需要一直使用雨刷，因此間歇模式可能會更好地保持擋風玻璃的清潔。在大霧天，雨刷的作用可能比霧燈小。

當速度太快，無法對大霧或薄霧中出現(xiàn)的東西做出反應時，司機最好減速。如果在大霧中的行駛速度降到了公布的最高速度的一半，司機最好找個安全的地方把車開離道路等霧散去。許多事故都是由于在霧中汽車開得太慢，被后面的車撞上而引發(fā)的。在駛離道路時，安全起見，駕駛員應打開車上的危險報警閃光燈。

【Notes】

1. stratus [■] n. 層云 2. hazy [■] adj. 朦朧的

3. vapour [■] n. 蒸氣;水蒸氣 4. condense [■] vi. 凝結

5. visible [■] adj. 看得見的 6. suspended [■] adj. 懸浮的

7. absorb [■] vt. 吸收 8. moist [■] adj. 潮濕的

9. density [■] n. 密度 10. inversion [■] n. 反向;倒轉

11. volcanic [■] adj. 火山的 12. humidity [■] n. 濕度

13. dense [■] adj. 稠密的;濃厚的 14. visibility [■] n. 能見度

15. merge [■] v. 合并;融合 16. precipitate [■] v. 冷凝成為雨或雪等

17. coalesce [■] vi. 合并;結合 18. dissipate [■] v. 驅散;使……消散

19. exhale [■] v. 呼氣;發(fā)出 20. evaporation [■] n. 蒸發(fā);消失

21. condensation [■] n. 冷凝;凝結 22. wiper [■] n. 刮水器，風擋雨刷

23. intermittent [■] adj. 間歇的 24. windshield [■] n. 擋風玻璃

25. hazard [■] n. 危險，風險