Reference Temperature

[Another St Ivian]

Hello all,
I was looking through the Aerodrome AIP's the other morning and came across a section on the front page of each airfields entry in the AIP called the Reference Temperature. The entry was along the lines of;

Elevation/Reference Temperature 150ft/20 C

Could anyone say for sure what exactly the Reference Temperature means? Many ideas were put forward but no one had a definitive answer.

Thanks,
A.S.I.