"Sub-zero" means two different things
Worth clearing up first, because it is the source of a great deal of cross-Atlantic confusion.
In everyday American usage, sub-zero means below 0 °F, which is −18 °C. That is a genuinely cold day — the sort where exposed skin is at risk and cars struggle to start.
In Celsius-speaking countries, sub-zero means below 0 °C, which is 32 °F. That is a frost. Cold, but a Tuesday.
The two readings are 18 degrees apart. So "sub-zero temperatures expected" is either a serious warning or a mild inconvenience depending on which country wrote it, and there is no way to tell from the phrase alone.
The scale, top to bottom
| Temperature | Fahrenheit | Kelvin | What happens here |
|---|---|---|---|
| 0 °C | 32 °F | 273 K | Water freezes |
| −6 °C | 21 °F | 267 K | Pipes start to be at risk |
| −18 °C | 0 °F | 255 K | Domestic freezer; "sub-zero" in the US |
| −40 °C | −40 °F | 233 K | The two everyday scales agree |
| −62 °C | −80 °F | 211 K | Coldest recorded in the United States |
| −78.5 °C | −109 °F | 195 K | Dry ice turns straight to gas |
| −80 °C | −112 °F | 193 K | Laboratory ultra-low freezer |
| −89.2 °C | −128.6 °F | 184 K | Coldest air ever measured, Antarctica |
| −183 °C | −297 °F | 90 K | Oxygen becomes a liquid |
| −196 °C | −321 °F | 77 K | Liquid nitrogen boils |
| −253 °C | −423 °F | 20 K | Liquid hydrogen |
| −269 °C | −452 °F | 4.2 K | Liquid helium; superconductors work |
| −270.5 °C | −455 °F | 2.7 K | Background temperature of deep space |
| −273.15 °C | −459.67 °F | 0 K | Absolute zero |
Dry ice, and why it does not melt
Dry ice is solid carbon dioxide at −78.5 °C (−109 °F). At normal atmospheric pressure carbon dioxide has no liquid phase, so it goes directly from solid to gas — sublimation, which is why dry ice produces fog rather than a puddle.
Two consequences worth knowing. It cannot be handled with bare skin, because contact at that temperature damages tissue almost immediately. And in an enclosed space the gas it produces displaces air, which is why dry ice needs ventilation and should never travel in a sealed car.
The gap between dry ice and a domestic freezer is larger than most people assume: a freezer at −18 °C is 18 degrees below freezing, while dry ice is 78 degrees below it — more than four times as far.
Liquid nitrogen, and where the drama comes from
Nitrogen makes up most of the air you are breathing. Cool it to −196 °C (−321 °F) and it condenses into a clear liquid that looks like water and behaves nothing like it.
The visual effects come from the temperature difference rather than anything exotic. Poured onto a warm floor it skitters about on a cushion of its own vapour. A flower dipped into it shatters, because the water inside has frozen solid in a second. The fog is not the nitrogen — it is water vapour from the room condensing in the cold air above the liquid.
It is used for the same reason: any object dropped into it reaches −196 °C almost immediately, which is exactly what biological sample storage and metal shrink-fitting need.
Where the scale stops
Absolute zero — 0 K, −273.15 °C, −459.67 °F — is not an arbitrary point. It is where a system has the minimum energy physically available to it, so there is nothing left to take away.
Two things follow. It cannot be reached, only approached, because removing the last of the energy would take infinite steps. And there is nothing below it, which makes the kelvin scale genuinely different in kind from Celsius and Fahrenheit: kelvin starts at a real floor, so it has no negative values and needs no degree sign.
The coldest place in the universe, incidentally, is on Earth. Deep space sits at about 2.7 K, warmed by radiation left over from the early universe, while laboratories using laser and magnetic cooling have reached billionths of a kelvin.
Celsius and Fahrenheit are scales measured in degrees between chosen reference points, so a value is written 20 °C. Kelvin measures absolute thermal energy from a real zero, so a value is written 293 K — no degree sign, and never °K. The 1967 redefinition made this official.
The part that matters outdoors
Everything below about −90 °C on that table is artificial. The natural range at the Earth's surface runs from roughly −89 °C to +57 °C, and the part any of us live in is a narrow band in the middle of that.
Which is the useful perspective on a cold morning: a −10 °C night is unpleasant and consequential — pipes, plants, cars — and it is barely a tenth of the way from freezing to the coldest air ever recorded, let alone to absolute zero. If you want to convert any value on this page into a scale you think in, the converter handles all three.