A fluid is any substance that flows.
Fluids include liquids, gases, and some solids like glacial ice. Fluids take the shape of a container because they do not have a specific shape and include common substances like water, shampoo, sunscreen, honey, and air. Air is a mixture of gases that flows and takes the form of its container, so it is a fluid.
Fluids flow due to different densities.
The chemical composition of fluids influences density. For example, fresh water is less dense than salt water; therefore, salt water is heavier and sinks below the freshwater. Another example (as seen in Part 1) is that carbon dioxide is heavier (more dense) than air, allowing carbon dioxide to force air out of the way and extinguish the fire. The fire cannot continue without a supply of oxygen, such as the oxygen found in air.
Temperature changes can also affect density of a fluid. Adding heat to a fluid increases the motion of the molecules, which then spread further apart. Warm fluids are less dense and rise while cold fluids are more dense and sink. The circulation of rising and sinking air is called convection.
Convection is the transfer of heat by the movement of a heated material.
In the atmosphere, on a hot summer day, the surface of the Earth is heated by energy from the Sun. Then, the warmed Earth surface warms air near the ground. The warm air rises, while cooler air sinks towards the ground. That cooler air is then warmed by the Earth surface and rises, continuing the convection pattern. Convection plays a key role in the formation of clouds and even thunderstorms. There is a limit of how far the warm air can rise in the atmosphere because air temperatures decrease with increasing altitude.
The continual cycling due to temperature-driven density, convection currents, are found in many places and on many scales in the atmosphere, ocean, and even in the Earth's interior. Smaller convection currents can be found in a cup of hot cocoa or a fish tank. Convective motions in the atmosphere are responsible for the redistribution of heat from the warm equatorial regions to higher latitudes and from the surface upward.
The main three processes of heat transfer include radiation, conduction, and convection.
Consider what happens to the water in a pot as it is heated over an open camp stove. In this example, radiation transfers heat from the burner to the pot. (Additonal background information on radiation can be found here.) Convection moves heat and water in the pot. The water at the bottom of the pot heats up first, and expands. Since the warmed water has a lower density than the water around it, it rises up through the cooler, dense water. At the top of the pot, the water cools, increasing its density, which causes it to sink back down to the bottom. This movement distributes heat within the pot.