Why does windshield fog up but side window glass doesn't?

If you leave your climate control system in Auto AC, your windshield will not fog up. I leave mine in auto year round and just use the temperature control on the steering wheel to adjust my comfort level.

 

My first guess would be that far more rain hit the windshield compared to the side window, cooling it more.

 

The side windows do not fog up as rapidly as the windshield because there is a constant flow of air across side windows in all circulation settings. There is no direct flow of air across the windshield until the defog choice is made. That flow is what vaporizes the droplets of water which are the 'fog' you see.

All else being equal use defog, the icon that shows the windshield and floor blower arrows, and use the high blower level. Your 'fog' should clear within a quarter or half a minute no matter what other heat or cool functions are chosen.

If 'fog' persists select outside air rather than recirculation. That combination of outside air with defog on high will clear most recalcitrant windshield 'fog.' Recirculating excessively moist inside air is a sure prescription for window 'fog.'

If all else fails, stop your car and wait until outside conditions change; you did mention 'raining.' Some nasty weather no car is prepared to eliminate.

 

I find when it's really wet out, as in raining for days on end, it's better to use the air conditioning with some heat. Then the air dries and warms at the same time, reducing the relative humidity significantly. Drawing in cold 100% humidified air from outside doesn't really help.