<P> Fluids are generalized as materials that exhibit the property of flow; however, this behavior is not unique to liquids . Fluid properties can also be observed in gases and even in particulate solids (such as sand, gravel, and larger objects during rock slides). </P> <P> A convection cell is most notable in the formation of clouds with its release and transportation of energy . As air moves along the ground it absorbs heat, loses density and moves up into the atmosphere . when it is forced into the atmosphere, which has a lower air pressure, it cannot contain as much fluid as at a lower altitude, so it releases its moist air, producing rain . In this process the warm air is cooled; it gains density and falls towards the earth and the cell repeats the cycle . </P> <P> Convection cells can form in any fluid, including the Earth's atmosphere (where they are called Hadley cells), boiling water, soup (where the cells can be identified by the particles they transport, such as grains of rice), the ocean, or the surface of the sun . The size of convection cells is largely determined by the fluid's properties . Convection cells can even occur when the heating of a fluid is uniform . </P> <P> A rising body of fluid typically loses heat when it encounters a cold surface, when it exchanges heat with colder liquid through direct exchange, or in the example of the Earth's atmosphere, when it radiates heat . At some point the fluid becomes denser than the fluid beneath it, which is still rising . Since it cannot descend through the rising fluid, it moves to one side . At some distance, its downward force overcomes the rising force beneath it, and the fluid begins to descend . As it descends, it warms again through surface contact or conductivity and the cycle repeats . Air warms, becomes less dense, rises . Then air cools, becomes more dense, sinks . Then cool air at the bottom . </P>

Where is the heat that activates the convection cells generated