Convection

Another form of energy transfer is convection. Imagine an enormous kettle of cold water. In the center of the kettle is a heat source (about 10 percent of the size of the kettle base). The water is heated partially through conduction, but you would notice that the water has hot and cold areas, not a uniform lukewarm body as one might expect (if only conduction were involved). Plumes of warm water form around the heat source and move upward because they are more buoyant (less dense because they are warmer) . These rising thermal plumes, or thermals, are examples of convection. The cooler water around the thermal then replaces the buoyant fluid and begins to warm itself more quickly (it is now closer to the heat source).

Now, imagine the kettle as the atmosphere, and the heat source as the sun. Convection occurs in all fluids where there is uneven heating of the fluid.

In less idealized situations, such as the atmosphere, there may not be a stable heat source for a long enough period of time to establish the standard circular pattern generaly associated with convection. A common case in atmospheric science is that of a rising thermal, in which sunlight warms the Earth unevenly, and thermals rise from the hottest surface regions, and are lifted into cooler, lower pressure air where water droplets can form. These thermals rise into the atmosphere carrying not just warmer air, but moisture, pollutants, and anything else the air can hold. One way we describe air movement is by using air parcels. An air parcel is a theoretical bounded volume of air. An ideal air parcel can expand and contract but cannot mix with the surrounding air. In other words, the volume of an ideal parcel is variable while its mass is constant. In convection, air parcels that are warmer than their surroundings rise, and water vapor in them may condense to form water droplets. This contributes to the development of clouds. Click on the image below to view an animation of this cycle.