What do these three things have in common?
All of these amazing forces of nature are powered by convection – changes in temperature and density of fluids that create movement. Sounds pretty complicated. But here’s a simple experiment you can do to see convection in action!
What you need: A medium-to-large clear storage tub (or any sort of large clear container), blue and red food dye, and lots of water.
- A couple days ahead of time, make some blue ice cubes by just freezing some blue colored water.
2. When the blue water has frozen into cubes, fill the tub about halfway with room temperature water. Let the water settle so it’s smooth and calm.
3. Carefully add some blue ice cubes to one end of the tub. You’re practically guaranteed to stain your hands blue. Be careful not to bump the tub or the surface it’s sitting on!
4. Carefully pour some red food dye into the other side of the bin. Again, be careful to not bump your experiment!
5. Watch how the different colors move around within the water. Where does the blue dye go as the ice melts? Where does the red dye go? Do they mix right away? How do the colors interact?
Does the red float on top of the blue? Do the red and the blue swirl around each other and mix? You have just created your own convection cell!
What’s going on here?
Convection is basically just a fancy word for heat transfer. Heat is a type of energy, which cannot be created or destroyed – only moved. When the ice is added to the room temperature water, it starts to melt, but the water that melts out is still very cold. The cold water is more dense than the room temperature water and red food dye. Remember, density refers to how many molecules are packed into a certain amount of space. Because the cold water is more dense, it’s heavier, so it sinks to the bottom. But as the cold water from the ice gets warmer in the tub, the water molecules spread apart again. It becomes less dense and lighter, so it rises back up. This circular motion is called a convection cell.
You find convection cells in all kinds of fluids, or substances that take the shape of their container. A fluid can be a liquid or a gas. When you have huge amounts of a fluid, for example, an ocean or the atmosphere, you have lots of convection cells. The denser, colder, heavier air or water sinks down, and rises as it gets warmer. Large convection cells working together create massive convection currents, like major ocean currents or air currents. These are sometimes called conveyor belts.
The air in our atmosphere is also a fluid, and different air temperatures lead to convection cells in the air. When hot air rises off the ground quickly, it leaves behind an area of low air pressure. Colder air, which dense and heavy, rushes in to fill the low pressure area. We feel that all the time – we call it wind! But when those convection cells in the air move really fast, that can create storms. Those can be small summer thundershowers or massive hurricanes.
So what about volcanoes? What do they have to do with convection? Well, the earth’s crust is divided into sections called tectonic plates, which float on top of a layer of hot magma, or molten rock, called the mantle. The mantle is also a fluid, with warmer, less dense areas and cooler, denser areas. These circulate in massive convection currents. The tectonic plates float and move (very slowly) on top of these currents. Volcanoes form mostly at the edges of tectonic plates, where hot magma is pushed out in the form of lava. The lava is pushed out of the earth by the force of convection!
Fun Fact: Some air currents, or convection cells, are so stable and long-lasting that they’ve been named. These massive convection cells are caused by the spinning of the earth! The northern trade winds blow northeast to southwest above the equator, so many European explorers used them to sail to the Americas. The strong winds that whip around the edges of Antarctica are known as Westerlies because of their direction. I was out flying my kite in the southern Westerlies just the other day!
Special thanks to Kalyani, who helped with photos and cleanup for this experiment.