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Temperature facts for kids

Kids Encyclopedia Facts

Temperature is how hot or cold something is. Our bodies can feel the difference between something which is hot and something which is cold. To measure temperature more accurately, a thermometer can be used. Thermometers use a temperature scale to record how hot or cold something is. The scale used in most of the world is in degrees Celsius, sometimes called "centigrade". In the USA and some other countries and locations, degrees Fahrenheit are more often used while scientists mostly use kelvins to measure temperature because it never goes below zero.

Scientifically, temperature is a physical quantity which describes how quickly molecules are moving inside a material. In solids and liquids the molecules are vibrating around a fixed point in the substance, but in gases they are in free flight and bouncing off each other as they travel. In a gas the temperature, pressure and volume of the gas are closely related by a law of physics.

Temperature conversions

  • To make a temperature in degrees Celsius into degrees Fahrenheit a person must multiply it by 9/5 and add 32: F = (9/5)C + 32.
  • To make a temperature in degrees Fahrenheit into degrees Celsius a person must subtract 32 and multiply the result by 5/9: C = (F - 32) * 5/9.

Useful temperatures

When they invented temperature scales scientists found there were certain things which were always around the same temperature:

  • Water freezes at a temperature of 0°C, 32°F, or 273.15 K.
  • The temperature inside the human body is close to 37°C or 98°F.
  • Water boils at 100°C, 212°F, or 373.15 K.
  • The coldest possible temperature is absolute zero. Absolute zero is 0 K,-459°F, or -273.15°C. At absolute zero molecules and atoms come to rest and so have no heat energy.

Temperature and heat

Temperature is not the same as heat. Heat is energy which moves from one thing, cooling it, to another, heating it. Temperature is a measure of the movements (vibration) of the molecules inside a thing. If the thing has a high temperature, it means the average speed of its molecules is fast. A thing may have a high temperature but because it contains very few or light atoms it has very little heat.

Heat capacity

The amount of heat that is needed to make a substance one degree higher is called its heat capacity. Different substances have different heat capacities. For example, a kilogram of water has more heat capacity than a kilogram of steel. This means that more energy is needed to make the temperature of water 1°C hotter than is needed to make the temperature of steel 1°C hotter.


Temperature is also important in weather and climate. It is related to the amount of heat energy in the air. Isotherm maps are used to show how temperature is different across an area. Temperature will be different during different times of day, different seasons and in different places. It is affected by how much heat reaches the place from the suns rays (insolation), how high the place is above the level of the sea, and how much heat is brought to the place by the movement of winds and ocean currents.


Thermometer CF
Two thermometers showing temperature in Celsius and Fahrenheit

Temperature scales need two values for definition: the point chosen as zero degrees and the magnitudes of the incremental unit of temperature.

The Celsius scale (°C) is used for common temperature measurements in most of the world. It is an empirical scale that developed historically, which led to its zero point 0 °C being defined as the freezing point of water, and 100 °C as the boiling point of water, both at atmospheric pressure at sea level. It was called a centigrade scale because of the 100-degree interval. Since the standardization of the kelvin in the International System of Units, it has subsequently been redefined in terms of the equivalent fixing points on the Kelvin scale, so that a temperature increment of one degree Celsius is the same as an increment of one kelvin, though numerically the scales differ by an exact offset of 273.15.

The Fahrenheit scale is in common use in the United States. Water freezes at 32 °F and boils at 212 °F at sea-level atmospheric pressure.

Absolute zero

At the absolute zero of temperature, no energy can be removed from matter as heat, a fact expressed in the third law of thermodynamics. At this temperature, matter contains no macroscopic thermal energy, but still has quantum-mechanical zero-point energy as predicted by the uncertainty principle, although this does not enter into the definition of absolute temperature. Experimentally, absolute zero can be approached only very closely; it can never be reached (the lowest temperature attained by experiment is 38 pK). Theoretically, in a body at a temperature of absolute zero, all classical motion of its particles has ceased and they are at complete rest in this classical sense. The absolute zero, defined as 0 K, is exactly equal to −273.15 °C, or −459.67 °F.

Absolute scales

Referring to the Boltzmann constant, to the Maxwell–Boltzmann distribution, and to the Boltzmann statistical mechanical definition of entropy, as distinct from the Gibbs definition, for independently moving microscopic particles, disregarding interparticle potential energy, by international agreement, a temperature scale is defined and said to be absolute because it is independent of the characteristics of particular thermometric substances and thermometer mechanisms. Apart from the absolute zero, it does not have a reference temperature. It is known as the Kelvin scale, widely used in science and technology. The kelvin (the unit name is spelled with a lower-case 'k') is the unit of temperature in the International System of Units (SI). The temperature of a body in a state of thermodynamic equilibrium is always positive relative to the absolute zero.

Besides the internationally agreed Kelvin scale, there is also a thermodynamic temperature scale, invented by Lord Kelvin, also with its numerical zero at the absolute zero of temperature, but directly relating to purely macroscopic thermodynamic concepts, including the macroscopic entropy, though microscopically referable to the Gibbs statistical mechanical definition of entropy for the canonical ensemble, that takes interparticle potential energy into account, as well as independent particle motion so that it can account for measurements of temperatures near absolute zero. This scale has a reference temperature at the triple point of water, the numerical value of which is defined by measurements using the aforementioned internationally agreed Kelvin scale.

Kelvin scale

Many scientific measurements use the Kelvin temperature scale (unit symbol: K), named in honor of the physicist who first defined it. It is an absolute scale. Its numerical zero point, 0 K, is at the absolute zero of temperature. Since May, 2019, the kelvin has been defined through particle kinetic theory, and statistical mechanics. In the International System of Units (SI), the magnitude of the kelvin is defined in terms of the Boltzmann constant, the value of which is defined as fixed by international convention.

Statistical mechanical versus thermodynamic temperature scales

Since May 2019, the magnitude of the kelvin is defined in relation to microscopic phenomena, characterized in terms of statistical mechanics. Previously, but since 1954, the International System of Units defined a scale and unit for the kelvin as a thermodynamic temperature, by using the reliably reproducible temperature of the triple point of water as a second reference point, the first reference point being 0 K at absolute zero.

Historically, the temperature of the triple point of water was defined as exactly 273.16 K. Today it is an empirically measured quantity. The freezing point of water at sea-level atmospheric pressure occurs at very close to 273.15 K (0 °C).

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Kids robot.svg In Spanish: Temperatura para niños

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