Most extreme AirShow readings

[SeenItAll]

Two weeks ago on a CO 757 from IAD to CDG, I saw two readings on the FlightInfo screen that were beyond what I had ever seen before. About an hour or so after takeoff, ground speed was showing 704 mph (1133 kph), and about an hour before landing, outside temperature was showing -88 F (-66.7 C).

While a really big tailwind can generate the ground speed, I am more perplexed about the temperature. This reading was over Britain at 38,000 feet, and in October. Not over the North Pole in December. Is such a reading really possible? Or perhaps due to some instrumentation error?

What have been the most extreme readings that you have seen?

[Reimers]

While -56°C is the 'normal' temperature that the standard atmosphere model predicts, there are variations, just as on ground. -66°C is quite low, but not unheard of. The Airbus A320 is certified to fly at teperatures as low as -70°C, although even in warmer cruizing levels, fuel temperature might become an issue.
Over the north pole, the atmosphere at 30000 ft might be surprisingly warm, the reason being that the tropopause is lower (that's where the air doesn't get colder anymore as you go up).