Interesting that Tianjin - near the NE coast of China and SE of Beijing -
reported a similar phenomenon
Dry bulb 29°C and
DP -33°C ! Abrupt fall in DP to an extremely low level at the hottest part
of the day - and subsequent rise to a "normal" level in the evening and
night.

I am a little suspicious of these figures, with their diurnal variation
being the same, roughly, as at Urumqi. Air that dry at his time of year can
only have arrived by adiabatic descent from some way up, despite:

A set of 4 hourly observations with DP below -25°C so perhaps we can assume
they are correct

Tudor Hughes.