The weather conditions for the 19th and 20th of January, 1931, around 6 PM, was provided by the Met Office.
Click the image to enlarge:
Weather is notated using Beaufort Letters:
i [prefix] = Intermittent…
b = Blue sky
o = Overcast
d = Drizzle
dd = Continuous drizzle
do = Thin drizzle
ro = Light rain
rr = Continuous rain
Q = Heavy squalls
f = Fog
c = Cloudy
Around the time Wallace left for chess, the dry bulb is measured at 46.4 farenheit, or 8 celsius. It was overcast, with light rain. The day in general was foggy and overcast with intermittent rain.
On the 20th about 45 minutes before Wallace left for his fake appointment, the dry bulb is measured at 42.6 faenheit, or 5.8 celsius.