The aircon was off and the windows were down.
Ok, understand this- what is fog? Its water condensing onto a cold surface. This happens because the glass temperature is lower than the dew point of the air around it.
What happens is that in the monsoon, the air is the car is already humid. When we sit in the car, we breathe out even more moisture. This moisture is usually cleared with air movement, so you dont see any fog on other times. But in the monsoon, we tend to raise the windows to keep water out. When it rains, the glass cools from the rain water. But the air inside of the car is close to saturation, so water starts to condense on the inside surface. The only way to clear this is to run the aircon. The aircon passes the car interior air over the cooling coil which cools & dehumidifies (meaning removes the moisture through condensation on the coil). Thus the aircon removes fogging on the inside. But you must use the windscreen mode to have this effect quickly.
Now you will have fogging on the outside. Whats happening is, the cabin is now cold & this cools the glass. But the outsind is now saturated (meaning nearly 100% humid). So the condensation occurs on the outside of the glass. Here you use the defogger. It basically heats the glass (there is a thin wire mesh in the glass) and that heats the glass above the dew point of the outside air & removes the fogging. Now remember, as long as it rains, you wont have fogging on the outside. Thats because the rain washes off the fog. But once it stops raining, fog will start to form on the outside. Then you got to use defogger or the wiper
We have a car with ACC, so we can avoid both issues. During rains, set the temperature to a higher setting. I personally even find 26~27 Deg C comfortable. Direct air on to your face and fore arms as they cool you the fastest. If you dont cool the canin too much, you wont have too much outside condensation & will save fuel also
Hope this helps
Ameyam