Here's why:

* Air Composition: Air is a mixture of gases, primarily nitrogen (78%) and oxygen (21%).

* Humidity: Water vapor is lighter than dry air. So, moist air is slightly lighter than dry air.

* Altitude: As you go higher in altitude, air pressure decreases. This means there are fewer air molecules in a given space, making the air less dense and therefore lighter.

However, when we talk about the "lightest air", we usually mean the air with the lowest density. And for a given altitude and pressure, dry air has the lowest density and is therefore the lightest.