The upward and downward movement of molecules of water or air is called a convection current
What Are Convection Currents?
Have you ever wondered why it's so much hotter above a campfire than next to it? Or, why when you boil a pot of water, the liquid moves around so rapidly? Both of these things are due to convection currents.
Convection is one of the three main types of heat transfer, the other two being conduction and radiation. Unlike the other two, convection can only happen in fluids. This includes liquids and gases and is because the molecules have to be free to move.
Heat energy can transfer by convection when there is a significant difference in temperature between two parts of a fluid. When this temperature difference exists, hot fluids rise and cold fluids sink, and then currents, or movements, are created in the fluid. But why does this happen? Let's look at an example of convection in action to get a better understanding.
Example: Soup for Lunch
Imagine it's lunchtime, and you're hungry. You decide to heat up some soup on a stovetop. As the soup increases in temperature, you are giving the soup's molecules more movement, or kinetic, energy. This extra movement energy will cause the molecules to spread further apart. Because of this, hotter fluids are less dense than colder fluids. The soup near the bottom of the saucepan is hottest, because it is closer to the burner. This means that the soup is less dense at the bottom.
Now, let's think about what happens when you put a cork into some water. The cork is less dense than the water, so it pops to the surface. The same thing happens with the soup. The hotter, less dense soup at the bottom rises above the colder, denser soup. As the soup rises and gets further away from the burner, it starts to cool and become denser. Before long, the soup is dense enough that it sinks again--passing the rising hot soup on its way--and the whole process repeats
These movements in the soup are called convection currents, and they are the reason that a pot of boiling water moves around so rapidly. The water is heated, becomes less dense and rises, cools, becomes denser, and sinks, over and over again. All because the burner creates a temperature difference between the top and bottom of the pot.