Have you ever wondered why hot air rises? Wonder no more. This video explains the simple process of why hot air rises.
If a parcel of air becomes warmer then it's environment, it will rise. This is due to the fact that temperature is a measure of molecular activity. The hotter a parcel air gets, the faster the air molecules within that parcel are moving. Therefore the harder they bump into each other and the further they deflect from each other. This causes the parcel of air to want to expand.
If 100 air molecules were contained in 1 cubic centimeter of volume and it began to occupy 2 cubic centimeters of volume, it will become less dense then the surrounding air which would have also been around 1 cubic centimeter of volume. Therefore, being less dense it will want to float, like ice does on water.
This will cause that parcel of air to rise and in doing so it will begin to cool.
