Question

I recently renovated an unfinished second floor attic space. It has a low sloped ceiling with just insulated rafters and an asphalt shingles above. I would say it has questionable ventilation. The finished ceiling is painted tongue and groove pine. In the summer and particularly on hot humid days the ceiling surface that runs along the ridgeline becomes saturated with water to the point where it will drip onto the floor.

I sense that I need to improve the venting. My question to you is how does this moisture form? Why does this happen in the summer, but not during the winter? We live in a cold winter climate and I am familiar with condensation/dewpoints as they relate to warm moist air contacting cold surfaces. Obviously in this case, we have the moisture, but instead of extreme cold the surface of the ceiling, particularly along the ridgeline, is extremely hot. Can you explain what is happening?