Tractor and trailer jackknifes are a scary situation, but can be avoided. Jackknifes can happen in any kind of weather or road condition.
A trailer jackknife is more likely to happen when the trailer is empty and the road conditions are slick. If the driver brakes and the trailer tires lock up the back end of the trailer will start to slide either to the right or left; with the tractor usually still pointing straight ahead and the tractor tires having traction. If the driver can let off of the brakes and lightly use the throttle the trailer tires usually find traction again and a jackknife can be avoided.
A tractor jackknife is also more likely on slick roads; when the load is heavy, the tractor and trailer are not lined up and the brakes are applied, causing the drive tires to lock up. Always make sure that the tractor and trailer are aligned, if they are not aligned it could be a contributing factor to a jackknife accident. A tractor jackknife can also occur if the roads are slick and the throttle is applied too hard, it may cause the drive tires to lose traction and the tractor to jackknife.
In less than ideal road conditions, ensure that you are driving a proper speed for the conditions and are not forcefully applying the brakes or the throttle. As always, if you don't feel safe driving in the conditions communicate with dispatch. Stay safe!