It’s common (as you’d expect) to see overnight lows of 3°C to 4°C producing ground and frost. However, last night the minimum I recorded was 5.0°C, and there was frost on roofs for a time this morning. That’s the highest overnight low I’ve seen produce frost. Can anyone better it?

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Not here. I assume it depends on where the thermometer is placed. Mine's on a north facing wall. If I then place the device at ground level I can then get a colder reading of course. 5C would be similar here for a car roof frost. 

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I've seen total 'white out' hard frosts at 2c to 4c.. Very strange phenomenon it must be said. 

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