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 stratus cloud but is called by a different name because it forms by a different process.
A stratus cloud is any low-lying cloud that makes the atmosphere hazy. Like all clouds, they are formed when water vapour condenses 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 visible to the eye.
This rapid cooling just over the ground could be caused by a number of reasons:
Falling rain can cool down the water vapour and cause it to remain suspended 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 absorbed during the day. The cool night air sandwiches the water vapour close to the ground resulting in a fog.
Finally, when warm, moist air blows over a cold surface like ice or snow, it cools it down rapidly and turns to fog.
You may wonder then- what is the difference between fog and mist? Well they are actually one and the same. A mist is simply a fog that isn’t very thick. The exact definition is if you cannot see an object that is less than 1 km in a mist, it is considered a fog.
You can create fake fog. You will have to do this with an adult. Buy some dry ice and put it into a steel container. Go into a relatively small room and pour some water into the container. You will see that this begins to produce a thick wispy layer of smoke. Can you explain why this is happening?
(Note this task involves chemicals and the by-product should not be inhaled)
Low-lying water droplets
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