We used to say that hot air is lighter and so it rises up. But around the hot air, there is cold air. So, how did the cold air allow the hot air to rise up.
Asked by Rashmi and Srushti of class VIII B, KV AFS Bidar.
You might have seen that helium of hydrogen filled balloon rising up. Also remember the case of hot air balloons. It is the same process here too.
First of all, the air is a fluid and the molecules are free to move as there is plenty of space between the molecules. So, when the air below becomes hot and lighter, they can easily go up through the empty space in between the cold and heavier molecules. the space vacated by the hot molecules are immediately occupied by the comparatively heavier cold molecules.