You would say
Es sind null Grad.
which is Plural.
Why can't you say
"Es ist null Grad."
German Language Stack Exchange is a bilingual question and answer site for speakers of all levels who want to share and increase their knowledge of the German language. It's 100% free, no registration required.Sign up to join this community
It's a language property. Logic doesn't work there. Both English and German use the plural form for any number other than 1, thus also for zero.
Other languages treat that sometimes very differently. Generally, languages are quite diverse in how they treat plurals with numbers and what type of endings they require - and considering one logical and the other not just because it doesn't match your native tongue doesn't cut it.
A few examples to the best of my knowledge:
French does it different and treats 0 and 1 as singular values, and all other numbers as plural. Chinese doesn't consider the number in the plural or singular form at all, etc.
For a software / game project (OpenTTD) which I was translation manager for some time, we have a list of different plural forms for different language localizations which knows 15 different ways for a language to form the plural, depending on the actual number. According to that list (you have to reference back the plural form list), you'd have 5 different plural forms in Irish... separate forms for 1,2,3-6,7-10 and all other numbers including 0. Or in Latvian you have 3 separate forms, one for ending in 1 except 11, one for 0, and one for all other numbers.
This is especially visible in sentences which give a quantity like "Ich sehe 5 rote Autos" (I see 5 red cars) which require the choice of singular or plural of the noun and a declension of the noun and/or adjective they refer to.
When the quantity has a unit, the unit itself usually is not subject to declension (e.g. see for instance this question on singular/plural usage with units of measurement), but the verb still has to obey the singular / plural usage ("Das sind 5 Kilogramm Kartoffeln / Es sind 0 Grad draußen" vs "Das ist ein Kilogramm Kartoffeln / "Es ist 1 Grad draußen").
Time is not a quantity and discussed here (duration of time is a quantity, like hours or minutes).
The German word for singular is Einzahl (verbatim: One-number), so it's for exactly one thing. And plural is Mehrzahl (many-number), so it's for all other numbers.
The point is, that plural is the default number, and singular is the exception used when there is only one thing.
In some languages there were also other grammatical numbers:
No matter which subset of grammatical numbers a language uses. Plural is always the default grammatical number, because it fits with all possible amounts, except those for which other grammatical numbers exist.
The number 0 is a relatively new invention. In everyday life of "normal" people it only appeared after the 17th century in Europe. So, it exists only since about 400 years in our languages, and this is too short to add a new grammatical number (a Nullzahl = zero-number) to the existing grammatical system of numbers. So, as a logical consequence, the default numner will be used which is plural.
1 These Bavarian greetings all mean "(I) greet you" in English:
Singular (greeting one person): »Griaß di«
Dual (greeting a pair): »Griaß enk«
Plural (greeting 2 people that are not a pair or 3 or more people): »Griaß aich«
I suspect, that you are expecting the singular form ist, because you consider the temperature to be the subject mandating it. I don't agree actually. Some reasons:
As Hubert Schölnast wrote, it is actually perfectly sensible to assign the number zero a plural rather than a singular. Singular refers to a thing, which does neither permit this being multiple things nor the thing being absent.
That said, all this is actually moot for the phrase "es ist/sind null Grad", because you're not actually counting degrees there. Rather you're referring to one temperature, which applies to one weather-situation, which is why contrary to the premise you may actually hear "es ist null Grad", just as much as "es ist ein Grad" or even "es ist vierzehn Grad". This should then not be understood as "there are fourteen degree-thingies in my garden" but rather as
The weather(sing.) on this day(sing.) has the property(sing.) that the temperature(sing.) equals '14.0°C'(sing.)
Which of these singulars the word "ist" refers to is a bit ambiguous.
Nowhere in the statement do any actual discrete fourteen items appear, as witnessed by the fact that it is equivalent to
The weather on this day has the property that the temperature equals '57.2°F'.
That doesn't mean you couldn't also say "es sind vierzehn Grad", and indeed this is probably the more common phrasing. But it's not inherently more logical to say it this way.