On page 70 of the aforementioned they discuss a series of tests conducted on a section of Warrior's hull removed when she was made into a jetty, but stored aboard. On testing something unexpected was observed, the armours resistance to impact is highly dependent on the temperature:
Temp (degrees C): Absorbed Energy (j)
The observation that iron is less ductile and more brittle at lower temperatures is not a new one, but the degree to which protection changes is astounding. The armour offers twice the protection in the tropics than in the arctic. Thus Lambert concludes that the temperature, weather &c. all play a role, and this can explain the erratic results of armour tests.
My own observation is that it implies armour is probably more effective in a steaming warship (which is kicking out a lot of heat) than on the firing range.
In the ACW it implies that the ambient temperature of the armour is a major factor. The operating temperature in a Monitor turret was pretty high, so I guess the armour was near full strength. This bears further investigation.