An odd cloud inversion

The cloud inversion on Wednesday was an odd one because when we set off, there was no mist in the valleys. What usually happens is that on a clear night, radiation fog forms in the valleys as the air cools and falls below the temperature at which the air becomes saturated with water vapour. When the sun rises, it heats the upper layers of the atmosphere but the sun's rays can't reach the air in the valley to warm it. Hence warm air is above the cold air. Since it is warm air that rises, the thermal energy can't reach the lower layers and the cloud stays trapped in the valley. The cloud is "inverted". The odd thing on Wednesday was that the cloud began to fill in after I had got so warm in the sun that I had to take my layers off on the ascent. At that point, sunlight wasn't prevented from reaching the valley floor. So did the humidity rise? Perhaps there was more evaporation which increased the amount of water vapour in the air above the amount that the air could hold at that temperature.