The mystery of frost on the window


A guy was found dead in his study by his friend. The victim’s friend called the police.
He told them that when he was walking by the victim’s house,  he noticed his study light was on and I decided to peek in from the outside to see if he was in there.

There was frost on the window and as soon as he wiped the frost to peep inside, he saw his body lying on the floor and therefore called the police.

The officer immediately arrested him for the murder of his friend.

How did the officer know that he was lying?