

Temperature is a measure of molecular motion By Chad Palmer, USATODAY.com Air temperature is one of those things that everyone is familiar with, which turns out to be more complicated than it might seem at first. Source: USA TODAY research by Chad Palmer A thermometer actually measures the average kinetic energy of the various gs molecules that make up the air around it - let's call them "air molecules." As you can see in the graphic above, air molecules in colder air move slowly compared to those in warmer air. The kinetic energy of an air molecule is directly proportional to the velocity of the molecule. This means that colder air has less kinetic energy than warmer air. When air molecules collide with a thermometer, kinetic energy is transferred from the air molecules to the glass and then to the mercury molecules inside the thermometer. As the mercury molecules begin moving faster they move farther apart, pushing the mercury up in the thermometer. In colder air, the energy from the air molecules colliding with the thermometer transferring to the mercury molecules is less than the energy from warmer air. As a result, the mercury molecules move slower in the colder air and the mercury inside the thermometer does not expand as far up the tube as it does in the warmer air. (Related resources: Weather measurements)