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Why humid air is lighter than dry air

Why humid air is lighter than dry air

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Previously we have already written about the fact that warm air is lighter than cold. But air weight is affected by its humidity. It would seem like water is heavier than air and should make the air heavier.  But it is not how it looks. Contrary to popular belief, humid air is lighter than dry air. In this lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn more about what are the weights of humid and dry air and how they are connected.

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So why exactly the humid air is lighter than dry air? The air consists of oxygen (molecular weight — 32), carbon dioxide (44) and nitrogen (28).

Now let's replace some molecules with water molecules. Water has a molecular weight of only 18. So, we replaced some molecules with lighter ones.

Thus, a wet air mass is lighter than a dry one. Therefore, the air that has accumulated moisture near water becomes lighter and rises to the top. We will talk about relative humidity in the following letters.

Humid and dry air weight. Valerya Milovanova / Windy.app

 

Text: Windy.app

Cover photo: Kamil Pietrzak / Unsplash

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