"Convection currents in the atmosphere are responsible for the formation of thunderstorms as the warm and cold air masses collide. They are also responsible for warm water currents that occur in oceans." With this experiment, you are going to show what happens when warm and cold water collide!" - Steve Spangler
explanation: Warm water is lighter and less dense than cold water.
Below is another take on the convection current experiment.
Red water = warm air mass.
Blue water = cold air mass.
"A thunderstorm is caused by unstable air. A body of warm air is forced to rise by an approaching cold front. A strong, persistent updraft of warm moist air is formed. The approaching cold front helps build the updraft into a cumulus cloud. When the warm air rises and meets the cold air, it condenses. The heat helps fuel the thunderstorm. The next stage is when the cumulus cloud has grown into a cumulonimbus cloud rising above 30,000 feet. Then a downdraft forms, bringing cold air and precipitation down to the Earth's surface." - Web Weather for Kids (Click here for written instructions from Web Weather for Kids!)