Lesson 5 - Understanding Temperature Measurements - Page 2

Essentially, kinetic energy is an indication of how fast a molecule is moving around. Molecules are constantly in motion, but the rate of the motion can change, just like the speed of a car changes depending on whether the driver brakes or accelerates. When a molecule's velocity fluctuates, its kinetic energy will fluctuate correspondingly. To put it more scientifically, kinetic energy is always directly proportional to a molecule's velocity.

Since air molecules in warm air move more quickly than the molecules in cool air, hot air molecules generate more kinetic energy. Once these warm particles come in contact with a thermometer, the kinetic energy from the air molecules transfers to the glass molecules, which move apart as they speed up, and in a thermometer, these mercury molecules have nowhere to go but up.

When a thermometer registers a level of cold, it's really indicating an absence of heat. In the case of colder air, the mercury expands less because cold air molecules are slow, generating less kinetic energy to pass along.