MEDEVAC Helicopter pilot reported a VFR into IMC conditions event after the weather deteriorated faster than expected requiring an immediate climb to return to VFR conditions.

Narrative

Skies Clear; 7+ miles visibility; cruise flight at 2;500 feet MSL; approached ZZZ [Airport] Class C airspace (from the east) for VFR transition to ZZZ1 hospital about 12 miles west of ZZZ. ATC confirmed transponder squawk; altitude; and heading for ZZZ1. I noticed some lower cloud cover over ZZZ; confirmed ATIS and iPAD (ForeFlight) the ZZZ was 800 scattered and 7 miles. ZZZ2 (N/W of ZZZ) was reporting the same. ZZZ ATC frequency changed me to ZZZ Tower. ZZZ Tower asked if I wanted to descend below the 800 feet scattered layer and continue to ZZZ1. I accepted. However; after descending below 800 feet and going under the scattered layer; I observed weather much less than was expected. Flight visibility reduced to 2-3 miles with mist; and the ceiling seemed more overcast than scattered. With ZZZ landing traffic behind the helicopter now; and not able to find a suitable area to land in the deteriorating conditions; I initiated IMC climb to VFR on top (about 30 seconds). During this time; the med crew asked if we were IMC; before I could inform them; and I responded yes; IMC climb to VFR conditions. The med crew was very helpful and verbally asked level attitude indicator; climb power; climb airspeed; no turns. I responded yes. The helicopter became VFR on top at 1;000 to 1;500 feet; and before I could contact ZZZ Tower; ZZZ Tower informed all participating traffic in the local area that ZZZ was now IFR. ZZZ Tower asked if we needed assistance or an IFR clearance. I responded negative; that I was VFR on top and diverting to the northwest towards ZZZ2; where the weather was clear. Helicopter diverted to ZZZ2; offloaded patient; and the med crew to a ground ambulance to ZZZ1. I checked weather at ZZZ2 and ZZZ3 (home base); contacted ZZZ1 flightwatch and departed pilot only VFR back to ZZZ3 without incident.Suggestions - Going forward; my personal pre-plan is to divert earlier; or turn around (RTB) before going below any cloud layer 1;000 feet or lower. The speed at which the flight visibility reduced was dramatic. The difference between what ATIS and Tower were reporting and actual conditions were unacceptable. Also; going forward; listening to the temperature dew point spread will be mandatory for all locations; with caution when spread is 4 degrees or less.