Temperature

Temperature is a measure indicating the level of heat in a substance or place. In physics, it relates to the kinetic energy of the constant motion of the small particles that make up the substance—atoms and molecules. As their motion increases, temperature rises.

Temperature determines the direction in which heat flows: heat passes from a substance at higher temperature to one at lower temperature, until both reach the same level. This condition is known as thermal equilibrium.

Measurement scales and units

The Celsius scale sets 0 °C as the point at which water freezes and 100 °C as the point at which it boils under standard atmospheric pressure. The Fahrenheit scale is commonly used in the United States. The international system of units (SI) uses the kelvin (K), whose zero point is absolute zero, the lowest possible temperature. One degree Celsius and one kelvin represent the same interval.

How temperature is measured

A thermometer is the instrument used to measure temperature. Earlier types relied on the expansion of substances such as mercury or alcohol when heat was applied. Modern types use electrical methods—for example, thermocouples and instruments that measure changes in electrical resistance. There are also remote methods that detect infrared radiation emitted by a substance, used for locations that cannot be reached.

The distinction between heat and temperature

Heat is energy transferred, while temperature is a condition indicating the level of that energy. Two objects at the same temperature can contain different amounts of heat, because their size and the material they are made of differ. The quantity of energy needed to raise the temperature of something is called heat capacity.

Applications

Temperature is part of weather forecasting, health care (body temperature), food and chemical industries, and scientific research. It is also used to track climate change, by observing fluctuations in the surface temperature of the earth over long periods.