فعلاً كان السؤال خطير :)
الهواء الرطب اخف ... وعلى الاخوان الاطلاع على المكتوب لفهم السبب " عندما تدخل الرطوبة في حيز معين فانها تطرد جزء من العناصر التي تشكل الهواء " فالهواء عبارة عن 78% نتروجين +21اكسجين+ 1%عناصر اخرى " وبما ان بخار الماء اخف وزناً من محتويات الهواء من نتروجين او اكسجين، فاننا نستنتج ان الهواء الرطب اخف.
الفهم الخاطي بسبب اننا نعتقد ان بخار الماء اثقل من عناصر الهواء الجاف وهذا فهم خاطيئ، و الصحيح ان الماء هو الاثقل، و ليس بخاره.
لكن ما هو التطبيق الذي تقصد
تقبل احترامي اخ تكناوي
Humidity and air density
Most people who haven't studied physics or chemistry find it hard to believe that humid air is lighter, or less dense, than dry air.
How can the air become lighter if we add water vapor to it?
Scientists have known this for a long time. The first was Isaac Newton, who stated that humid air is less dense than dry air in 1717 in his book, Optics. But, other scientists didn't generally understand this until later in that century.
To see why humid air is less dense than dry air, we need to turn to one of the laws of nature the Italian physicist Amadeo Avogadro discovered in the early 1800s. In simple terms, he found that a fixed volume of gas, say one cubic meter, at the same temperature and pressure, would always have the same number of molecules no matter what gas is in the container. Most beginning chemistry books explain how this works.
Imagine a cubic foot of perfectly dry air. It contains about 78% nitrogen molecules, which each have an atomic weight of 28. Another 21% of the air is oxygen, with each molecule having an atomic weight of 32. The final one percent is a mixture of other gases, which we won't worry about. Molecules are free to move in and out of our cubic foot of air. What Avogadro discovered leads us to conclude that if we added water vapor molecules to our cubic foot of air, some of the nitrogen and oxygen molecules would leave -- remember, the total number of molecules in our cubic foot of air stays the same. The water molecules that replace nitrogen or oxygen have an atomic weight of 18. This is lighter than both nitrogen and oxygen. In other words, replacing nitrogen and oxygen with water vapor decreases the weight of the air in the cubic foot; that is, it's density decreases.
Wait a minute, you might say, "I know water's heavier than air." True, liquid water is heavier, or more dense, than air. But, the water that makes the air humid isn't liquid. It's water vapor, which is a gas that is lighter than nitrogen or oxygen.
Compared to the differences made by temperature and air pressure, humidity has a small effect on the air's density. But, humid air is lighter than dry air at the same temperature and pressure.