Cold air from the vents mixes with warm moist air in the cabin, as the air cools its can’t contain as much moisture which condenses forming cloud/fog. If it was raining heavily the air outside would likely be saturated and close to dew point as heavy rain normally lowers the temperature as well.
On the ground in tropical conditions it’s quite difficult to get the cabin cold enough to be uncomfortable in a short period of time. Once airborne and in cooler air with a higher rate of flow through the cabin it’s much easier, and the heating may need to be turned up especially if the number of passengers is low as people radiate heat.