The sensors are very responsive when they detect temperature changes. During processor intensive activities—such as processing HD video or playing a game that makes extensive use of the graphics processor—the fans speed up to provide additional airflow.
Ambient temperature plays a role in the responsiveness of the fans. The fans will turn on sooner and the fans will run faster if the ambient temperature outside the computer is high.
Where you use the system may affect fan behavior and heat as well. Using the computer on a soft surface like a couch, pillow, bed, or your lap can trap heat requiring higher fan activity. Using the computer on a hard flat surface like a table or desk allows for optimal radiation of heat.
Mac computers have air vents that allow heated air to exit. The vents are in the back of the computer on the MacBook, MacBook Air, MacBook Pro, and Mac Pro computers. iMac (Mid 2012 and later) models vent along the bottom edge of the computer. Make sure the vents remain unblocked to allow your computer to perform at its best.
In a quiet environment, increased airflow from the fans may produce an audible noise that sounds like rushing air.
The System Management Controller firmware on every Mac manages the thermal conditions of the computer regardless of the operating system. Learn more about the operating temperature of Apple portable computers.