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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.
什么是雾？
你曾经有过在清晨走出门，面对一层厚厚的白茫茫的空气，看不到前面几英尺开外的地方的经历吗？这就是所谓的雾。它实际上是一种层云，但由于不同的形成过程，它被冠以的不同的名称。
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 earth’s 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？
层云是使大气朦胧的低云。和所有的云一样，它们是在水蒸气上升到地球大气层较冷的地方时凝结而成的。然而，雾是不同的，因为水蒸气达到露点（它变成水滴的温度）是在接近地面的地方。我们看不到水蒸气，因为它是一种气体，但当它变成水滴时，它就变得肉眼可见了。地面上的这种快速降温可能是多种原因造成的。
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.
雾的成因是什么？
下雨可以使水蒸气降温，并使其像雾一样悬浮在靠近地面的地方。
当昼夜温差较大时，土地白天吸收的热量持续流失。夜间的冷空气把靠近地面的水蒸气夹在中间，形成了雾。
最后，当温暖潮湿的空气吹过像冰或雪一样冷的表面时，它会迅速冷却并变成雾。
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 isn’t 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.
霧与霭
你可能会想雾和霭有什么区别？其实它们是一样的。霭只是一种不太浓的雾。
雾和霭都是由水滴形成的，只是它们的整体位置和密度不同。雾是到达地面的云，即使“地面”是山丘或山顶。由于气温逆增、火山活动或湿度变化，水滴悬浮在空气中会形成霭。雾比霭更浓，而且往往持续时间更长。在能见度方面，雾使能见度降低到1千米以下，而霭则使能见度降低到1至10千米。
确切的定义是，如果你在雾霭中看不到距离小于1千米的物体，那那就是雾。
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
任何云型与地面接触都会形成雾。在低洼地区，如山谷和平原，雾堤（一团雾）本质上是一种云的构造。
当水滴凝结并合并，但无法达到足够大的尺寸以凝结成雨时，云就形成了。当湿度突然上升或变化，或者当风速下降或改变方向时，云会形成或漂移到离地面更近的地方。
霭也是由水滴形成的，但合并或凝聚较少。这意味着霭的密度较小，当风、温度或相对湿度变化时，消散的速度更快。霭的形成可能是由于突然的温度变化（例如：在冷空气中呼气）、高湿度（例如：在桑拿浴室）、蒸发或冷凝（例如：当雨水碰到被阳光晒热的岩石和街道路面）。
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 car’s hazard25 lights flashing for additional safety.
雾天如何安全驾驶
在薄雾天驾驶时，驾驶员必须小心使用雨刷。薄雾中的水滴往往密度不够，不需要一直使用雨刷，因此间歇模式可能会更好地保持挡风玻璃的清洁。在大雾天，雨刷的作用可能比雾灯小。
当速度太快，无法对大雾或薄雾中出现的东西做出反应时，司机最好减速。如果在大雾中的行驶速度降到了公布的最高速度的一半，司机最好找个安全的地方把车开离道路等雾散去。许多事故都是由于在雾中汽车开得太慢，被后面的车撞上而引发的。在驶离道路时，安全起见，驾驶员应打开车上的危险报警闪光灯。
【Notes】
1. stratus [■] n. 层云 2. hazy [■] adj. 朦胧的
3. vapour [■] n. 蒸气;水蒸气 4. condense [■] vi. 凝结
5. visible [■] adj. 看得见的 6. suspended [■] adj. 悬浮的
7. absorb [■] vt. 吸收 8. moist [■] adj. 潮湿的
9. density [■] n. 密度 10. inversion [■] n. 反向;倒转
11. volcanic [■] adj. 火山的 12. humidity [■] n. 湿度
13. dense [■] adj. 稠密的;浓厚的 14. visibility [■] n. 能见度
15. merge [■] v. 合并;融合 16. precipitate [■] v. 冷凝成为雨或雪等
17. coalesce [■] vi. 合并;结合 18. dissipate [■] v. 驱散;使……消散
19. exhale [■] v. 呼气;发出 20. evaporation [■] n. 蒸发;消失
21. condensation [■] n. 冷凝;凝结 22. wiper [■] n. 刮水器，风挡雨刷
23. intermittent [■] adj. 间歇的 24. windshield [■] n. 挡风玻璃
25. hazard [■] n. 危险，风险
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.
什么是雾？
你曾经有过在清晨走出门，面对一层厚厚的白茫茫的空气，看不到前面几英尺开外的地方的经历吗？这就是所谓的雾。它实际上是一种层云，但由于不同的形成过程，它被冠以的不同的名称。
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 earth’s 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？
层云是使大气朦胧的低云。和所有的云一样，它们是在水蒸气上升到地球大气层较冷的地方时凝结而成的。然而，雾是不同的，因为水蒸气达到露点（它变成水滴的温度）是在接近地面的地方。我们看不到水蒸气，因为它是一种气体，但当它变成水滴时，它就变得肉眼可见了。地面上的这种快速降温可能是多种原因造成的。
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.
雾的成因是什么？
下雨可以使水蒸气降温，并使其像雾一样悬浮在靠近地面的地方。
当昼夜温差较大时，土地白天吸收的热量持续流失。夜间的冷空气把靠近地面的水蒸气夹在中间，形成了雾。
最后，当温暖潮湿的空气吹过像冰或雪一样冷的表面时，它会迅速冷却并变成雾。
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 isn’t 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.
霧与霭
你可能会想雾和霭有什么区别？其实它们是一样的。霭只是一种不太浓的雾。
雾和霭都是由水滴形成的，只是它们的整体位置和密度不同。雾是到达地面的云，即使“地面”是山丘或山顶。由于气温逆增、火山活动或湿度变化，水滴悬浮在空气中会形成霭。雾比霭更浓，而且往往持续时间更长。在能见度方面，雾使能见度降低到1千米以下，而霭则使能见度降低到1至10千米。
确切的定义是，如果你在雾霭中看不到距离小于1千米的物体，那那就是雾。
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
任何云型与地面接触都会形成雾。在低洼地区，如山谷和平原，雾堤（一团雾）本质上是一种云的构造。
当水滴凝结并合并，但无法达到足够大的尺寸以凝结成雨时，云就形成了。当湿度突然上升或变化，或者当风速下降或改变方向时，云会形成或漂移到离地面更近的地方。
霭也是由水滴形成的，但合并或凝聚较少。这意味着霭的密度较小，当风、温度或相对湿度变化时，消散的速度更快。霭的形成可能是由于突然的温度变化（例如：在冷空气中呼气）、高湿度（例如：在桑拿浴室）、蒸发或冷凝（例如：当雨水碰到被阳光晒热的岩石和街道路面）。
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 car’s hazard25 lights flashing for additional safety.
雾天如何安全驾驶
在薄雾天驾驶时，驾驶员必须小心使用雨刷。薄雾中的水滴往往密度不够，不需要一直使用雨刷，因此间歇模式可能会更好地保持挡风玻璃的清洁。在大雾天，雨刷的作用可能比雾灯小。
当速度太快，无法对大雾或薄雾中出现的东西做出反应时，司机最好减速。如果在大雾中的行驶速度降到了公布的最高速度的一半，司机最好找个安全的地方把车开离道路等雾散去。许多事故都是由于在雾中汽车开得太慢，被后面的车撞上而引发的。在驶离道路时，安全起见，驾驶员应打开车上的危险报警闪光灯。
【Notes】
1. stratus [■] n. 层云 2. hazy [■] adj. 朦胧的
3. vapour [■] n. 蒸气;水蒸气 4. condense [■] vi. 凝结
5. visible [■] adj. 看得见的 6. suspended [■] adj. 悬浮的
7. absorb [■] vt. 吸收 8. moist [■] adj. 潮湿的
9. density [■] n. 密度 10. inversion [■] n. 反向;倒转
11. volcanic [■] adj. 火山的 12. humidity [■] n. 湿度
13. dense [■] adj. 稠密的;浓厚的 14. visibility [■] n. 能见度
15. merge [■] v. 合并;融合 16. precipitate [■] v. 冷凝成为雨或雪等
17. coalesce [■] vi. 合并;结合 18. dissipate [■] v. 驱散;使……消散
19. exhale [■] v. 呼气;发出 20. evaporation [■] n. 蒸发;消失
21. condensation [■] n. 冷凝;凝结 22. wiper [■] n. 刮水器，风挡雨刷
23. intermittent [■] adj. 间歇的 24. windshield [■] n. 挡风玻璃
25. hazard [■] n. 危险，风险