I remember It was the last few days of December and many of January, and it was definitely Fahrenheit scale as O C is just freezing. The area around Oelwein can have some cold weather in NE Iowa. This was one winter back in the late 70s or early 80s long before the hothouse we have to live in now. It was also an uncharacteristically long cold spell, which is why I mentioned it.
Probably December 1983. Before and a little after Christmas it was incredibly cold.
Des Moines had 115 straight hours (Dec 21st to Dec 26th) of below 0 temps. The high Christmas Eve day was -12.
I imagine up in NE Iowa it was worse.
What is interesting is just a few months earlier in the summer of '83.... August was brutally hot. August of '83 was and still may be the hottest August on record in central Iowa. Had 8 days in the month above the century mark. And many days that weren't were very close.
Heck of a year for weather.