I borrowed this from Wiki's page about Naga's - I think it would relate to all peppers.
"The effect of climate on the Scoville rating of Naga Jolokia peppers is dramatic. A 2005 Indian study that compared the percentage availability of capsaicin and dihydrocapsaicin in Naga Jolokia peppers grown in both Tezpur (Assam) and Gwalior (Madhya Pradesh) showed that the heat of the pepper is decreased by over 50% in Gwalior's climate (similar temperatures but less humid, much lower rainfall)."
So upping the humidity and watering should increase the scovilles.
"The effect of climate on the Scoville rating of Naga Jolokia peppers is dramatic. A 2005 Indian study that compared the percentage availability of capsaicin and dihydrocapsaicin in Naga Jolokia peppers grown in both Tezpur (Assam) and Gwalior (Madhya Pradesh) showed that the heat of the pepper is decreased by over 50% in Gwalior's climate (similar temperatures but less humid, much lower rainfall)."
So upping the humidity and watering should increase the scovilles.