A DHC8 Captain canceled IFR on a night approach to a CTAF airport and subsequently the aircraft momentarily entered IMC conditions. Situational awareness; fatigue and CRM were components in this error.

Narrative

Event flight was a scheduled Part 121 airline segment. Approaching our filed destination; ASOS reported 2;000 BKN ceiling. Airport was not in sight within 10 NM at 11;000 MSL; so we asked Center for the ILS/DME 21; for which we were cleared. Upon turning inbound from the procedure turn; Runway 21 was clearly visible from approach lights to end bars. Since it is often impossible to reach Center from the airline ramp at this airport; our airline's flights will often cancel IFR while airborne; as permitted by our Ops Specs. It appeared to me that the glideslope would easily take us below the clouds over the airport area; so I canceled IFR at approximately 10 NM from the Runway 21 threshold. Center switched us to a 1200 squawk and cleared us off their frequency. At approximately 5 NM it became apparent that the wind was carrying clouds across our approach path at an altitude that would intersect our approach path. The First Officer was able to gently maneuver around some of the clouds; but we did pass through a couple of them. We could always see the glow of the approach lights and were only IMC for a few seconds. The approach and landing were completed without further incident. In hindsight; I should not have canceled IFR. We were running about 40 minutes late and I did not want to deal with the hassle of contacting the AFSS (which involves transmitting on their receiver frequency and listening on the airport VOR). I clearly misinterpreted the cloud base height and wind-induced cloud movement. Furthermore; I should have consulted my First Officer for his opinion before canceling IFR; which he diplomatically pointed out to me after shutdown. I placed our flight in the position of being VFR (with no active flight plan) in IMC conditions; in an area and at an altitude with spotty radar coverage; with significant rising terrain to our immediate north. DUMB; DUMB; DUMB! My only defense is that it was well past midnight and we had flown five legs and endured an hour-long return to gate at our departure airport. Even so; it is my responsibility to assess my mental condition and make a go/no-go decision accordingly. Valuable lesson learned.