>>12073925I hope this is not a troll. If air is heated up, it expands, therefore it's density becomes lower. If you have some hot air next to some cold air, the cold air will be relatively dense to the hot air. This means that per unit of volume, the cold air is "heavier" than the hot air. This means that the cold air will be pulled more strongly to the earth relative to the hot air. The means that the cold air will feel a stronger pull of gravity than the hot air. The difference between these gravitational pulls creates a small effective force on the hot air that pushes it up. You can also think of it as though the cold air is being pulled underneath the hot air by the force of gravity. This doesn't just happen for gravity though, but may occur in any direction given a force of two types of gas (or liquids) with different densities like is shown in this video:
https://www.youtube.com/watch?v=y8mzDvpKzfY 
