Ask any balloonist.
Warm air rises because it weighs less than colder, heavier air.
Put a few drops of food coloring in a plastic bottle. Fill with cold water, shake to mix the color, and replace the lid. Put in a refrigerator for several hours or overnight. Fill the tub with warm water, and place the plastic bottle at the bottom. Carefully unscrew the lid so the cold water in the bottle seeps out.