What was unexpected about the way the children's father greeted them?

Usually, when Mr. Thornton returns from St. Anne's John and Emily rush out to greet him. Then they ride back with their father, one perched on each of his stirrups. But this time they get more than they bargained for. As a huge, violent thunderstorm starts to gather over their heads, the children rush out to greet their father as normal, only for him to yell at them to go back inside the house.
It's only at this precise moment that the children stop and realize that this is no ordinary thunderstorm; this one's unusually violent. The lightning is flashing like mad, keeping up an almost continuous blaze; rain is coming down in droves, soaking the children to the very skin. No wonder Mr. Thornton wants them to run back inside the house immediately. The children also suddenly realize that their father is afraid of the thunderstorm, and this leaves a lasting impression on them.