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This tweet from the National Weather Service in LaCrosse, Wisconsin, USA says:

We have officially hit -30 here at the NWS La Crosse office as of 520 a.m. The wind chill is -54. Be sure to dress appropriately if you're heading out for the day.

The photo shown below shows a digital readout of an electronic thermometer, showing -30.1 but has no units. The -30's are getting close to where the Celsius and Fahrenheit scales tend to have similar numerical values, so there's no way to use context in this case to choose the most likely answer. No units are shown on the front panel, and these instructions offer no help either.

enter image description here

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NWS Temperature forecast or new forecast

From these URLs, it appears that the unit is Fahrenheit.

and if you click on this URL - Max/MinT

Is the maximum daytime or minimum overnight temperature in degrees Fahrenheit.

In the USA NWS always used to issue forecasts and bulletins in Fahrenheit and some background can be obtained from here US Customary Units and here - Why Americans still use Fahrenheit as well here - Fahrenheit Versus Celsius: Why the US Hasn't Converted

Degrees Fahrenheit are used in the U.S. to measure temperatures in most non-scientific contexts. The Rankine scale of absolute temperature also saw some use in thermodynamics. Scientists worldwide use the kelvin and degree Celsius. Several U.S. technical standards are expressed in Fahrenheit temperatures and American medical practitioners often use degrees Fahrenheit for body temperature.

In reality, the NWS and NCDC weather forecast model outputs for the different systems (GFS, NAM, NARR...) use temperatures in Kelvin. This satisfies the international standards (e.g., CF1.6 convention compliant) and also avoids issues with advecting fields (in this case) that are equal to zero in some locations. You can see this if you look at the model output directly (for instance, here or here). Afterward, in post-processing the temperature is transformed to either Celsius or more commonly Fahrenheit for public consumption.

Scientists and meteorological researchers are more used to Kelvin and/or degrees Celsius.

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    $\begingroup$ I haven't been in the US in quite a while, I'd actually forgotten that the government still uses F. My goodness. $\endgroup$ – uhoh Jan 31 at 8:01
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    $\begingroup$ @uhoh It's the US scientists whom I feel sorry for - they have to work in Kelvin or Centigrade, but have to go through daily life in F. $\endgroup$ – UKMonkey Jan 31 at 13:15
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    $\begingroup$ @UKMonkey but Fahrenheit is so much better for daily life, the range of temperatures is just about 0 to 100, at least where I live, instead of -18 to 38 $\endgroup$ – Stephen S Jan 31 at 13:29
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    $\begingroup$ @StephenS As a general rule, the only part of the number (in Celsius) I care about in my "daily life" is is there a minus sign in the forecast - ie is there a reasonable chance for ice. It's literally impossible for someone to tell me the temperature here without highlighting if the roads and pavements will be dangerous. I would say for daily life, C is still better. $\endgroup$ – UKMonkey Jan 31 at 13:46
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    $\begingroup$ @UKMonkey agreeing with you that Celsius is better. In the place where I live (Russia) the range of temperatures is about -35° to 35°C (-31° to 97°F), even symmetrical around the zero. And the temperature of 35°C, either plus or minus (it's interesting that in Russia the plus sign usually is explicitly written when the temperature is positive, e.g. +18°C instead of 18°C), does indicate near-extreme weather conditions. And of course, negative number means frost and ice. $\endgroup$ – trolley813 Feb 1 at 6:54
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That device only shows Fahrenheit

I don't know about the full Weather Service, but that device only shows Fahrenheit measurements. You can see this in the user manual, in the appendix, page 18 enter image description here

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    $\begingroup$ Excellent, here I'd thought that the Nimbus was a broomstick. $\endgroup$ – uhoh Jan 31 at 15:44
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    $\begingroup$ I love the alternative way of getting to the answer, but the third line does say that °C is a factory option, so it is at least conceivable it was showing Celsius. $\endgroup$ – TripeHound Jan 31 at 22:21
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    $\begingroup$ The image actually shows the opposite of what you claim. It shows that it can be set of oC at the factory. $\endgroup$ – ikegami Jan 31 at 22:22
  • $\begingroup$ @ikegami good catch! $\endgroup$ – uhoh Jan 31 at 23:18

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