As you already know, fog is common in Nathia Gali. It can lead to Zero Visibility. But thats not your only problem. Rain and Hail is probably even more dangerous than fog. They make visibility even worse. You don't get the small showers you get in London. Its a river flowing from the sky!!! and along with hail. I would advise you never to drive at night as it is almost always foggy at that time. During the day drive with indicators and lights during rain and fog, see the road signs and always use the horn before turning around corners. During rain and hail, always pause near the roadside as it generally subsides in 15-20 min but watch out for landslides and never park your car too near the cliff as the mud is soft there and you may fall down the cliff with the car.
Weather in Nathia Gali mostly remains foggy during summer so you can expect cloud bursts any time. And cloud burst usually comes with severe hail n thunderstorm making visibility even worst.
So avoid driving during such situation especially at nights. Nathai Gali road is very well known for accidents coz of visibility problems and every year many deaths are recorded. So take care!