Air expands when it is warmed becoming less dense than the surrounding air and therefore buoyant. Similarly, air falls when it is cooled. Convection occurs when thermal energy is transported from one place to another by the motion of a liquid or a gas. During a hot summer afternoon the warm ground heats the air, the air then rises forming large convective plumes called thermals. Soaring birds and glider pilots seek out thermals and enter them to be lifted to higher altitudes by the rising air.
Convection also occurs within the tall towers of cumulus clouds.
Forced convection is used in convection ovens where fans circulate the hot air around the food. To Do and Notice Move the card from shadow into sunlight. Notice how it changes color. Repeat the experiment but cast the shadow of a finger onto the card. Notice that the card takes longer to change colors than a card held perpendicular to the sunlight.