Absolute zero The lowest temperature possible. At absolute zero, an object’s atoms would be absolutely still and not vibrate at all. Absolute zero is zero on the Kelvin scale. It is -273.15°C in the Celsius scale.

Celsius scale One of the scales used to measure temperature. Zero degrees Celsius (0°C) is the temperature of freezing water and 100°C is the temperature of boiling water at normal atmospheric pressure.

Cold The absence of heat. Objects that are cold have very low temperatures.

Conduction One of the ways in which heat energy moves. In conduction, heat passes between two objects that are in physical contact. When you touch an object to see how warm it is, you receive some of its heat by conduction.

Convection One of the ways in which heat moves. Convection only occurs in fluids (liquids and gases). As some of the molecules in a fluid receive heat, they gain kinetic energy and move further away from each other. The hot part of the fluid becomes less dense than its cooler surroundings and rises upwards. But as it rises, it moves away from the source of the heat, so cools down and sinks.

Convection current The circulation of heat produced when a liquid or gas rises and sinks as it repeatedly heats up and cools down.

Fahrenheit scale One of the scales used to measure temperature. In the Fahrenheit scale, 32 degrees Fahrenheit (32°F) is the temperature of freezing water and 212°F is the temperature of boiling water at normal atmospheric pressure.​​​​​​​

Heat A type of energy that objects possess when their molecules are moving or vibrating. It is sometimes called thermal energy. The more a molecule moves, the more heat it has. Heat moves around and between objects in three main ways: conduction, convection and radiation.

Kelvin scale Sometimes known as the absolute scale, the Kelvin scale is one of the scales used to measure temperature. It starts at absolute zero, or 0 K. The temperature of freezing water is 273 K and the temperature of boiling water at normal atmospheric pressure is 373 K.

Latent heat Heat that is absorbed or released while a substance is changing state (e.g., from a liquid to a gas) and remaining at the same temperature.

Radiation One of the ways in which heat moves. Radiant heat travels in the form of electromagnetic waves. Unlike conduction and convection, radiation can travel through empty space (a vacuum). This is how the Sun’s heat reaches the Earth.

Temperature A measure of how fast the molecules of a substance are moving. Hotter objects have higher temperatures. 

Thermals Pockets of warm, rising air in the Earth’s atmosphere. They rise from areas of sea and land that have been recently warmed by the Sun. (When used as a verb, "thermal" means "to do with heat".)

Thermal conductor A substance that heat can travel through easily. Most metals are good thermal conductors.

Thermal insulator A substance that slows down the movement of heat. Air, wood and plastic are all good thermal insulators.

Thermodynamics The study of heat and the laws it obeys.

Thermogram A type of photograph that shows areas of different temperatures in different colours. Typically the hottest areas appear white and the coolest areas appear black.

Thermometer An instrument used to measure temperature. Glass thermometers are made of a long glass tube containing alcohol or mercury, which expands as it heats, and rises up the tube. Digital thermometers have small electric sensors that detect changes in temperature.

Thermostat A device that switches a heating or cooling device (such as a refrigerator, electric iron or radiator) on or off when a pre-set temperature is reached. A thermostat often contains a bi-metallic strip: a length of two different metals bonded together. Because one metal expands by a different amount to another, when the strip heats up, it bends. As it does so, the the strip completes an electrical circuit and the appliance is switched on. Other types of thermostats contain a plug, often made of wax, which expands or contracts with the surrounding temperature.

Consultant: Mike Goldsmith