On a night when the temperature is minus 20C or colder with a clear
sky and calm wind, the coldest reading measured on a weather station
thermometer is AFTER the sun has begun to rise.

Here is the reason: Prior to sunrise, the coldest air pools
immediately above the ground during the night. Thus, it is a wee bit
warmer (one or two degrees C) a metre or two further up.

The effect of the sun rising stimulates air movement, albeit slight,
which pushes the coldest air upward toward where most thermometers are
stationed. Hence, the coldest official readings are recorded
approximately a half-hour after sunrise.

This dynamic played out this morning throughout Ontario between 6:00
and 8:00 am.

Bob