# What "subzero temperature" mean? Is it below zero F or below zero C?

What "subzero temperature" mean? Is it below zero F or below zero C? Is it different from "freezing"?

• It means below zero, but on which scale would depend on your context (in reading) or your choice (in writing). Jan 7, 2014 at 23:32
• Usually it would be taken to mean freezing (of water) and therefore implicity related to zero on the Celsius scale. Nothing special happens at zero Fahrenheit (see en.wikipedia.org/wiki/Fahrenheit). The USA is one of only a few countries that still use the Fahrenheit scale, and I imagine the phrase 'sub-zero' is accordingly not so common there (happy to be corrected by US residents!) because of its potential ambiguity. Jan 7, 2014 at 23:45
• @toandfro In US the weather guys on TV using 'subzero' many times.
– alex
Jan 7, 2014 at 23:54
• Re: "Is it different from freezing?" - Yes. Water freezes at +32 °F. Jan 8, 2014 at 7:05
• @toandfro - subzero means "below zero." When used by weather forecasters in the U.S., it means "below 0°F". (It essentially means, "It's gonna be cold; bring your mittens and a warm hat.")
– J.R.
Jun 29, 2015 at 16:24

When dealing with a weather forecast in the U.S., subzero means below 0°F. It's a convenient way for the weatherman to say, "It's going to be dangerously cold!"

In a similar way, on a warmer day, you might hear a forecaster saying things like, "with lows in the teens" or "with highs in the upper 20s."

However, at a scientific conference, if I heard a researcher say, "The ion-doped material performed better in subzero temperatures," I'd probably assume she was talking about temperatures below 0°C, since scientists often use Celsius temperature scales, and because that's such a key benchmark temperature on the Celsius scale.

• Shome mishtake, shurely? If your forecaster is talking in °F (as per first paragraph), even "highs in the upper 20s" would still be below freezing to those of us who've standardised on °C. Jan 8, 2014 at 0:42
• @FumbleFingers, "in the upper 20's" is indeed below freezing. But it's still a "warmer day" than one with a high temperature below 0 F. Jan 8, 2014 at 1:42
• @Fumble - What The Photon said. This answer was made to be read in wintertime. Incidentally, where I live, we set a new record yesterday, getting colder than a previous low from 1924. (We were below 0°F, and below -20°C.) We can't wait until it climbs into the 20's (F) again.
– J.R.
Jan 8, 2014 at 3:02
• @The Photon, J.R.: oic. Yes, even in the UK our weather forecasts keep mentioning US temperatures. In the current circumstances, I imagine an awful lot of Americans would think of "freezing" (my 0°C) as "relatively warm"). Jan 8, 2014 at 12:45
• @FumbleFingers: definitely, compared to yesterday, freezing would be positively balmy. If you think "highs in the upper 20s" sounds like a mistake, try "a high of 10 degrees". Jan 8, 2014 at 23:34

It means below zero on whichever scale is in use or generally understood— centigrade, Delisle, or what have you.

It would mean the same as freezing in scales where the freezing point of water is set at zero, such as Celsius or Réaumur, but not in scales where zero is set to something else, as in Leiden or Rømer.

In Fahrenheit, zero has no particular significance, but it is not uncommon to say sub-zero temperatures to emphasize the coldness (0°F is about -17.78°C).

• Although I agree with your answer and upvoted it, I'd say that in Fahrenheit zero has psychological significance. It may just be in my head, but it really feels like it means something when the number crosses that threshold.
– user230
Jan 9, 2014 at 2:05
• @snailplane That may be because 0°F is as cold as you can get an ice/salt/water mixture. Jan 14, 2014 at 17:30

In some fields of physics, a "subzero temperature" may even mean a temperature below 0 Kelvin, see e.g. the Wikipedia article on negative temperatures. However, to emphasize the concept, other terms can be used, for example "sub-absolute-zero temperature" is used here.

• The Wikipedia article on negative temps clearly states it needs some work. Negative Kelvin is suppose to be impossible. However, as of late a study has demonstrated -1K. It is prolly an error.
– user4939
Mar 7, 2014 at 11:13

"Subzero" means literally below 0. It's impossible to say in which scale without context. If the scale is not clear from context, the term is vague.

Since, according to Wikipedia, Fahrenheit scale is used only in 5 countries (Bahamas, Belize, Cayman Islands, Palau and United States), I would suspect, if said by someone from that countries, it's Fahrenheit scale.

In any case I would assume it's Celsius scale.

If the text comes from continental Europe, I'd be sure it's about Celsius scale. Most people there aren't aware of Fahrenheit scale at all.