Cold



Cold (having less heat) refers to the condition or subjective perception of having low temperature.

The coldest theoretically possible temperature is absolute zero, also called absolute temperature, which is 0 K on the Kelvin scale, a thermodynamic temperature scale, and −273.15 °C on the Celsius scale. Absolute zero is also 0 °R on the Rankine scale, another thermodynamic temperature scale, and −459.67 degrees on the Fahrenheit scale.

Since temperature relates to the thermal energy held by an object or a sample of matter, which is the kinetic energy of the random motion of the particle constituents of matter, an object will contain less thermal energy when it is cold than when it is hot. If it were possible to cool a system to absolute zero, all motion of the particles comprising matter would cease and they would be at complete rest in this classical sense. The object would be described as having zero thermal energy. Microscopically in the description of quantum mechanics, however, matter still has zero-point energy even at absolute zero, because of the uncertainty principle.

Mammalian perception
Mammals have an endothermic ("warm-blooded") physiology. Therefore they have a perception of thermal comfort that is centered around their body temperature. Their thermal comfort is dependent upon their metabolism and environmental surroundings, and is affected by heat conduction, convection, radiation, and evaporative heat loss. Thermal comfort is maintained when the heat generated by their metabolism is allowed to dissipate, thus maintaining thermal equilibrium with the surroundings. When this equilibrium is not maintained due to excessive heat loss, then they will perceive the feeling of being too cold, and in varying degrees of discomfort depending upon how far this equilibrium is skewed.

Cooling
Cooling refers to the process of becoming cold, or lowering in temperature. This could be accomplished by removing heat from a system, or exposing the system to an environment with a lower temperature.

Fluids used to cool objects are commonly called coolants.

Air cooling is the process of cooling an object by exposing it to air. This will only work if the air is at a lower temperature than the object, and the process can be enhanced by increasing the surface area or decreasing the mass of the object.

Another common method of cooling is exposing an object to ice, dry ice, or liquid nitrogen. This works by convection; the heat is transferred from the relatively warm object to the relatively cold coolant.

Laser cooling and Magnetic evaporative cooling are techniques used to reach very low temperatures.

Notable cold locations and objects

 * The Boomerang Nebula is the coldest known natural location in the universe, with a temperature that is estimated at 1 K (kelvin) (−272.15 °C/−457.87 °F).
 * Herschel Space Observatory instruments and detectors are kept at temperatures below 2 K, using large helium tank for cooling.
 * The Universe is bathed in electromagnetic radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.725 kelvin.
 * Neptune's moon Triton has a surface temperature of −235 °C (−390 °F).
 * Uranus with an atmospheric temperature of −215 °C (−355 °F).
 * Saturn with a temperature of −175 °C (−285 °F) at cloud tops.
 * Mercury, despite being close to the Sun, is actually cold during its night, with a temperature of about −170 °C (−275 °F). Mercury is cold during its night because it has no atmosphere to trap in heat from the Sun.
 * Jupiter with a temperature of −145 °C (−230 °F) at the cloud tops.
 * Mars has a temperature of about −125 °C (−195 °F).
 * The coldest continent on Earth is Antarctica. The coldest place on Earth is the Antarctic Plateau, an area of Antarctica around the South Pole that has an altitude of around 3000 m. The lowest reliably measured temperature on Earth of -89.2 C was recorded there at Vostok Station on 21 July 1983 (See List of weather records).
 * The first "pure" Bose–Einstein condensate was created by Eric Cornell, Carl Wieman, and co-workers at JILA on June 5, 1995. They did this by cooling a dilute vapor consisting of approximately two thousand rubidium-87 atoms to below 170 nK (one nK or nano K is a billionith (10^-9) of a degree Kelvin) using a combination of laser cooling (a technique that won its inventors Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips the 1997 Nobel Prize in Physics) and magnetic evaporative cooling.