Temperature

Temperature is the physical quantity that determines the direction of heat flow between systems and measures the average kinetic energy of the particles in a substance. When two objects are placed in contact, energy flows spontaneously from the one at higher temperature to the one at lower temperature until both reach the same temperature, a state called thermal equilibrium.

Temperature is one of the central variables of thermodynamics, and it governs processes at every scale studied on this wiki: the rate of enzyme reactions, the phase of water, the speed of molecules in a gas, and the energy balance of the planet.

The zeroth law of thermodynamics states that if two systems are each in thermal equilibrium with a third, they are in equilibrium with each other. This is what makes temperature measurable: a thermometer reaches equilibrium with whatever it touches and reports a shared value.

More formally, thermodynamics defines temperature through entropy: temperature is the rate at which a system's internal energy changes as its entropy changes. This definition connects temperature to the second law of thermodynamics: heat flowing from hot to cold increases total entropy, which is why it happens spontaneously.

At the molecular scale, temperature reflects random thermal motion. For an ideal monatomic gas, the average translational kinetic energy of each atom is 3/2 kT, where T is the absolute temperature and k is the Boltzmann constant. Hotter gases have faster molecules; the speeds are spread across the Maxwell–Boltzmann distribution rather than all being equal.

Absolute zero (0 K, or −273.15 °C) is the temperature at which a system is in its lowest possible energy state. It can be approached but never reached. Even at absolute zero, quantum mechanics leaves particles with a residual zero-point energy.

• Kelvin (K): The SI unit of temperature. Since the 2019 redefinition of the SI base units, the kelvin is defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ joules per kelvin, rather than by the triple point of water.
• Celsius (°C): Uses the same degree size as the kelvin, offset so that 0 °C = 273.15 K. At standard atmospheric pressure water freezes near 0 °C and boils near 100 °C.
• Fahrenheit (°F): Used mainly in the United States. A change of 1 °C equals a change of 1.8 °F, and °F = °C × 1.8 + 32.

Temperature differences, such as the warming of Earth's climate, are the same number whether expressed in kelvins or degrees Celsius.

Temperature is not the same as heat. Heat is energy in transit because of a temperature difference. How much an object's temperature rises when it absorbs heat depends on its specific heat capacity: water's high heat capacity is why oceans warm and cool slowly. During a phase change, heat can be absorbed or released with no temperature change at all; this hidden energy is latent heat.

Biochemical reaction rates rise steeply with temperature: for many reactions the rate roughly doubles or triples for every 10 °C rise (a temperature coefficient, Q₁₀, of about 2–3). Above an organism's tolerance, proteins unfold and enzymes lose activity, so every species has an optimal temperature range.

Animals handle this in two broad ways. Ectotherms, such as insects and reptiles, take their body temperature largely from the environment. Endotherms, such as birds and mammals, generate metabolic heat and hold body temperature nearly constant through homeostasis. The temperature dependence of insect development is why warming changes the range and season length of disease vectors, a theme of mosquito-borne disease research.

Earth's global mean surface temperature is about 15 °C. Without the natural greenhouse effect, a planet absorbing the same sunlight would settle near −18 °C. Rising concentrations of greenhouse gases have increased the surface temperature: the IPCC Sixth Assessment Report found that 2011–2020 was about 1.1 °C warmer than 1850–1900, and later years have been warmer still.

Because the atmosphere's capacity to hold water vapor rises by about 7% per degree Celsius (the Clausius-Clapeyron relation), even small changes in temperature have large consequences for rainfall extremes, drought, and storm intensity. How much warming follows a given rise in carbon dioxide is the subject of climate sensitivity.