Hounds are distinguished from other breeds by their pendulous ears and by their extraordinary olfactory powers, which enable them to hunt entirely by scent. Bloodhounds are the oldest breed of sporting dogs which hunt by scent.
As the human quarry (line walker) moves across the ground, he leaves a track of particles on the soil, grass or plants over which he has passed. Where he has gone through a hedge or climbed over a wall he will have left many more scent particles as a larger proportion of his body has come into contact with the hedge or the wall.
However it is not these particles which a hunting Bloodhound smells, but the air which has come into contact with them. As most hounds attending Bloodhound Trials work free, they have the ideal opportunity to work in a natural manner without the hindrance of a long lead and a handler attached to the end. Bloodhounds which work in this style are can cover the ground at considerable speed.
Some hounds, and many other breeds in Working Trials, will track rather than hunt, using ground scent rather than air scent. Tracking is an exercise which when perfected by correct training enables the dog to follow a human scent trail accurately. However tracking dogs work in a harness with a long line held by the handler. They follow a track of ground scent made up of human scent, disturbed earth and crushed vegetation which may be reinforced by visual clues. A tracking dog will, typically, work at a slower speed than a hunting hound, and this may be very significant in real life where an escaped convict or a lost child is being sought and the line is getting colder all the time.
Weather conditions play an important part in the way a hound will work; Bloodhounds are well able to work a line up wind (the wind coming towards you bringing the scent towards you) or down wind (the wind is following you taking the scent away from you), the latter condition is more difficult. In addition, there are crosswinds, the hound could be working quite wide either side of where the runner has walked, and the scent has drifted.
If conditions are such that the scent is high, the hound could be working with his head held high, should there be a tight loop in the line a bloodhound could well cut the loop and scent across the loop.
Scent spreads with great rapidity in water; scent is dispersed as a gas and carried by air currents.