The December of 1934 is the mildest December on record along with the December of 1974 for the CET with a value of 8.1

December 1934 trackometer
1.  7.6
2.  8.4
3.  9.2
4.  9.8
5.  9.7
6.  9.5
7.  9.6
8.  9.7
9.  9.9
10. 9.7
11. 9.5
12. 9.4
13. 9.3
14. 9.1
15. 9.1
16. 9.0
17. 8.9
18. 8.8
19. 8.8
20. 8.8
21. 8.6
22. 8.4
23. 8.3
24. 8.2
25. 8.1
26. 8.0
27. 8.0
28. 8.0
29. 8.1
30. 8.1
31. 8.1

At Spitsbergen, December 1934 was +9.2C above the normal mean, Svaland it was +6.9C and at Zurich it was +5.2C

Guernsey  recorded a mean of 10.2C

It was a wet month. During the first week, Maesteg recorded 259mm of rain, Holne 262mm and Tynywaun 274mm.

Other info from this month.

By mid month, the snow line had risen to 5500ft in Switzerland due the mildness.
On the 16th, the Tiber burst it's banks near Rome.
Floods occurred in Cyprus on the 20th
Torrential rain caused flooding at Coimbra in Portugal on Christmas night causing severe flooding.
Five drowned in Crete because of flooding on the 26th
The first snowfalls for Vienna occurred on the 26th

A letter by L. Bonacina

"The heavy rainfall of December 1934 in the hilly counties south of the Thames was highly typical both as regards intensity and distribution of very wet months and marked feature of our climate which deserves comment. Apparently every county bordering on the Channel from Cornwall to Kent had between 5 and 10 inches of rain in December as a minimum, large tracts of country in the Downs of Sussex, Hampshire, Wiltshire and Dorset had between 10 and 20 inches and small areas on the flanks of Dartmoor the colossal quantity of 20 and 30 inches. To the lee of these southern ranges, on the contrary, London and Oxford had round about 5 inches and Cambridge with numerous other east Anglian and Midland places only about 3 inches.
We may recall that this very wet area which barely had a single slight frost in December 1934, lay for two or three weeks under snow in December 1933 -especially the south-western districts. Indeed the two December had little in common but dark dank days."

L.C.W.Bonacina 22nd January 1935