On the scale of temperatures

Although it is convenient for the purposes of every-day life to measure temperatures relative to a freezing point of water (pure for Celsius or Centigrade, maximally salty for Fahrenheit), there is a lowest-possible temperature, −273.16°C or about −460°F. If we measure temperatures relative to this, they're necessarily never negative – and it happens that this temperature is unattainable, so nothing physical is actually at zero, so everything physical is at positive temperature. Temperatures measured relative to this minimum are described as absolute temperatures; the Rankine scale uses the Fahrenheit as unit and the Kelvin scale uses the Centigrade or Celsius as unit. Following the Systèlne Internationale (as usual), I'll use the Kelvin scale.

nK, nano Kelvin

Appparently a team at MIT have managed to get a Bose-Einstein condensate down to about half a nano Kelvin; and Fins have taken the nuclei of Rhodium atoms down to 0.1 nK.

μK, micro Kelvin

mK, milli Kelvin

The cosmic microwave background radiation shows an almost uniform temperature across the whole sky; the largest variation in it is of order 0.3 mK.

K, Kelvin

Getting anything within our solar system down to even one tenth of a Kelvin is rather difficult; even 100 K is extremely cold by the standards of things natural to Earth's surface.

0.1 K to 1 K

The Boomerang nebula's temperature has been measured to be 1 K; as gas flows outwards from a star, it is expanding rapidly (at c. 164 km/s) and consequently (adiabatically, I suppose) cooling. Nothing else has been found in nature colder than the cosmic microwave background.

1 K to 10 K

The cleanest example yet observed of black-body radiation is the cosmic microwave background, at 2.7248 K. Helium (condenses or) boils at 4 K but has only ever been observed in the solid state at pressures significantly higher than atmospheric – at 103 atmospheres, it freezes (or melts) at 4 K. Several elements behave as superconductors below about 10 K.

10 K to 100 K

This is the approximate range of temperatures of (astronomical) giant molecular clouds. There are materials which exhibit superconductivity at temperatures as high as about 90 K.

Hydrogen melts (or freezes) at 14 K. Oxygen melts (or freezes) at 54 K and boils (or condenses) at 90 K; in between, Nitrogen melts at 63 and boils at 77; consequently, Earth's atmosphere (roughly one fifth Oxyten and four fifths Nigrogen, aside from some minor impurities) is almost all vapour at 100 K and almost all solid at 50 K.

kK, kilo Kelvin

Most of what's interesting to humans happens between 0.1 kK and 1 kK, although there are some things only one or two factors of ten higher than this that we can at least comprehend.

0.1 kK to 1 kK

This range can also be described as −173.16°C to 726.84°C or as −279.688°F to 1,340.312°F; it spans the range of conditions familiar to inhabitants of Earth's surface, atmosphere and most of its oceans.

100 K to 250 K

In 1983, at Russia's Vostok base in Antarctica, a temperature of 194 K was recorded. Carbon dioxide sublimes (that is, goes directly from solid to vapour, without any intervening liquid phase; or the reverse if being cooled instead of warmed) at 216.6 K.

250 K to 400 K

The zero-point of Fahrenheit's scale, just over 255 K, is the temperature at which very salty water freezes, despite its salt content (and, in this case, it's not symmetric with melting: the ice and salt crystals separate, so that it won't melt until some of the ice ments, at 273 K, to dissolve the salt and thus be capable of being liquid at lower temperatures, dissolving more ice and salt as it does so). Pure water melts (or freezes) at 273.15 K and boils (or condenses) at 373.15 K. The hottest recorded natural air temperature at Earth's solid surface was about 330 K (Libya, 1922).

400 K to 600 K

600 K to 1000 K

1 kK to 10 kK

The sun's surface temperature is about 5.8 kK, as is the centre of the Earth. Glasses typically melt (or, at least, become runny enough that the illusion of solidity gives way to obvious fluidity) between 1 and 2 kK. Aside from Carbon, the hottest any element can endure being solid is 3650 K, when Tungsten melts; at 6 kK, all pure elements are vapours (Tungsten, at about 5.8 kK, and Rhenium, at about 5.9 kK, are the last to go). Although graphite doesn't melt or boil – it sublimes, straight from solid to vapour, at about 4 kK – diamond is said to melt at 3823 K and boil at about 5.1 kK. Lithium phosphate, Li3PO4, allegedly doesn't melt until 8843 K. I don't have a figure for its boiling temperature, and there may be a few things that remain solid at higher temperatures than it, but pretty much everything (short of neutronium) is a vapour at 10 kK.

10 kK to 100 kK

Lightning in the Earth's atmosphere can be as hot as 30 kK (while carrying a current of .3 MA).

MK, mega Kelvin
plasma loops in the Sun's corona can be as hot as several million Kelvin. The heart of the Sun, or a nuclear explosion, has a temperature of around 10 MK.
… and behond
Allegedly, Fermilab have messed with temperatures of up to about a tenth of a peta Kelvin. The moderately bogus definition of the effective temperature of a molecular cloud emitting maser light (the temperature at which a black body would emit the maser frequency with the same brightness, ignoring the vast amounts of radiation the black body would be producing at other frequencies) attributes GK, TK and higher temperatures to some molecular clouds. The (seldom physically realized) Planck temperature is 355.22e30 K – 61,246 million million million million times as hot as the surface of the Sun !

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