On 21 Jan, 14:44, mittens wrote:
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

It's probably much more likely that losses due to outgoing long-wave
radiation continue to exceed the sensible heat inputs from incoming
short-wave radiation for some while after the visible solar disk has
risen above the horizon. The intensity of the sunshine is not normally
sufficient to trigger a sunshine recorder, even sensitive electronic
ones, until 20 or even 30 minutes after sunrise, even on a clear
winter morning, by which time the solar disk is still only at 3 or 4
degrees elevation at 50-55°N.

--
Stephen Burt
Stratfield Mortimer, Berkshire