I’m sorry to say that this is a near impossible question to answer!
You could write a book on the subject and still not get to the bottom of the mystery.
Ground and air temperature, wind conditions and direction, moisture levels and the type of cover being hunted all have an effect – and they can (and often will) change during the course of the day.
Even the way in which the quarry has been hit can have a deciding affect.
And this is especially true of what the bird or animal – if pricked – does after being hit.
One that runs hard will give off more scent than one that tucks down with a lower pulse rate.
It’s commonly accepted that scent is caused by molecules attaching themselves to drops of moisture which rise and fall according to air and ground temperature.
This is why some days a dog will hunt quickly on an air scent or more slowly on a ground-based smell.
Days when scent conditions are poor can be very trying but any Gun who complains about the dog work at such time has no right to call himself a sportsman.
Next time you hear such comments you should ask him why – if he’s such an expert – doesn’t he have a dog of his own?