compass swinging
 

Hi folks,
I am trying to find the whereabouts of official written information about when a compass swing can be carried out.
More specifically regarding the time of day and conditiions. I have seen it somewhere about not being able to do swings at night and in the rain.
Many of my colleagues have all seen the info but can't remember where.
Can anyone give me a link to official documentation?
TIA
DF