alzo

I am relatively new to chronographing rounds and to reloading 9mm. In early spring, I clocked a load of 5.9gr of 3N37 with a 124gr Berry's plated RN (OAL=1.15) at 1130fps. It was about 50 deg F and dry. A week or so ago it was about 80 deg F and very humid. The same load averaged only 1086fps. Can anybody help me understand why I am seeing this difference ? I am shooting a CZ-75B-SA. Thanks.