operating environment

I would like to know the operating temperatures of an Arduino Uno?

from experience most things are rated to like -20 to +80 C. But you can go colder if you were to build a little heated case to keep the duino warm YMMV though

IMO:
Since this is a consumer product, for that matter it is an experimenters product, the operating temperature would probably be commercial grade.
Also the experimenter may push the current limits of the hardware therefore even the commercial range of 0 °C to 70 °C may have to be reduced.

Since there is a socket and there are well used headers (mechanical issues perhaps) on the UNO, I would not want to operate it outside 0-40°C

"Commercial grade: 0 °C to 70 °C (sometimes ?10 °C to 70 °C)"

The '328P itself is rated -40C to +85C, see section 33 of the datasheet.
You'd have to look up the ratings on all the other parts - only a few of the larger parts are identifiable from their physical markings.

Folks have stuck them in coolers with cell phones and floated balloons to pretty high altitudes, where the temperature and pressure can get pretty low.

Thanks all. The Atmel site says the environment ranges from -40C to 85C, but I don’t know if that is the operating environment or the storage environment. I’m looking to operate to -20C. I’ll try it and see what happens.

I would like to know the operating temperatures

It will likely work, may be even down to -40.
The crystal(s)/resonator A/D and moisture may have an effect on your measurements.

If you have the skills I would recommend using a dedicated standalone PCB with components soldered.
Also if you have the power consider heating the enclosure.

The chart I referenced is titled "Operational Range", Industrial (-40?C to 85?C).

But as I said, that's only the '328P, no info available on most of the rest of the parts on the board.