You feel your glasses fog up during the night. You go out and feel you have been under a shower. You feel the air heavy. The day was not as humid, you think. What’s up with the weather?
The answer is simple. During the day, the sun is shining brightly, giving enough heat to cause water in the air to evaporate. The heat from the sun turns the water present in the air into vapour. You read how clouds are formed in school. I know most of us do not remember any lessons from school, so let us go through it once again.
The sun makes the molecules of water escape and form water vapour, which rises up and forms clouds. Not enough? Read on.
We were coming to that. At night, the sun goes down beneath the horizon and the water that is in the air stays as it is. You see, no sun to turn it into vapour. It sounds simple and it is. The relative humidity in the air depends on the amount of water in the air and air temperature. Humidity in the air indicates how much water vapour the air is saturated with.
Due to the absence of heat from the sun, water vapour stays closer to the ground at night. When the moisture in the air condenses at a rate greater than it can evaporate, you see water droplets being formed, which we call dew. It looks lovely, doesn’t it?
Image source: http://www.flickr.com/photos/pierre-lachaine/6522576905/
When temperatures are very low, dew turns into ice. You might have observed that dew is mostly settled on grass, car roofs and railings. That is because these surfaces are not warmed by the heat from the ground.
We have discussed it in the last three paragraphs, but there is more information (only because you asked). Water in the air that was present during the day and evaporated by the sun gets a chance to regroup/hug at night. It sounds sexy, but this is water vapour.
The molecules that were being sent up high in the sky by the sun combine. Together they sink because of their combined weight. The lighter ones hang up in the air and the heavy ones, too enthusiastic to stick to each other, go down and meet mother earth.
In a sentence: no sun, no heat, more water vapour.
What Happens in summers?
In summers, the temperature is very high during the day and does not drop down sufficiently to allow molecules of water to combine after sunset. Even after the sun has gone down beneath the horizon, the air temperature keeps evaporating the water. That’s why you feel the dry air in summers, day or night.
What about Fog?
Fog is formed when there is a high percentage of humidity in the air. Think of it as a cloud. It actually is similar to clouds except that it is present locally and is much close to the ground. You will mostly see fog formed close to water bodies such as lakes, rivers, oceans, marshes etc. because there are more water molecules there.
If you find yourself travelling in a foggy day, know that visibility can drop to less than 1 kilometre. In cases of mist, visibility is much better at more than 1 kilometre.
Definition of fog for school children: Fog is a collection of droplets of water or ice crystals in the air or close to the ground, which makes you turn on your fog lights.
Image source: http://www.flickr.com/photos/spirosk/4508000340/
There is more moisture in the air at night to make it hard for us to sleep comfortably.
Article by JS Umidificatoare