Before you can train a dog to find people, you must understand what scent we speculate dogs are responding to, and the effect wind and terrain have on scent transport.
Humans constantly shed small cornflake-shaped dead skin cells known as rafts, which are discarded at the rate of about 40,000 per minute. Each raft carries bacteria and vapor representing the unique, individual scent of the person. This is the scent sought by the trained dog. These rafts are picked in and carried by air and wind currents. They are dispersed downwind in a cone like shape that is narrow and concentrated at its source (the person), but widening as the distance grows. Trained dogs can be observed literally working the cone in open fields as they zigzag back and forth, in and out of the scent.
Differing terrain and wind currents have a major impact on the dispersion of scent and each search will present its own problems. The first day of searching may be in open woods with a nice breeze; the second day may be through dense woods in hot, windless conditions. Handlers must be well versed in the effect all these factors have on scent so that they can adapt their search plan accordingly.
Terrain features will play a role in determining how the dog is worked. Handlers in even the early stages of training must understand what effects terrain may have on scent behavior.
Open Fields. On days with a steady breeze, a trained dog should have no trouble picking up and following an airborne scent in an open field. An eager dog should alert and move in from a distance of 200 to 300 feet or more. This will be true even if the victim has been in place only a short time (15 to 30 minutes). A lighter breeze or very still day will also produce a good alert from a distance of 100 feet or more. Strong, gusty winds create the most problems because they can disperse the scent rapidly. Shifting winds make a search pattern difficult because you may start in what was originally a downwind direction, only to find that the wind has shifted and you are now working upwind. In such cases, the best approach is to remain with your original pattern since the wind will continue to shift and any attempt to keep up with the wind will leave you wandering aimlessly around the field.
Light brush is defined as a space that includes open or wooded areas with some brush or small woodpiles. Light brush problems should not pose any real difficulty for the dog as the the brush should not be so thick as to block or drastically change scent flow.
Heavy Brush. Heavy brush may be found in thick woods or unmowed fields. A heavy brush area may include brier patches and large woodpiles. Still, hot days combined with heavy brush can produce extremely difficult searching conditions because the scent remains near the victim. Detection and ranging distance will be greatly reduced, thereby making relatively close sweeps of perhaps 100 feet or less necessary for accurate coverage.
Woods. Woods can vary from an open pine forest to a swamp with large trees and very dense brush. Open woods are frequently a joy to search; dense woods will try both your and the dog's patience, particularly on hot days or in the dark. The wind velocity and terrain will dictate the handler's approach to the problem. Open woods in flat terrain should not be much more difficult than an open field search. dense woods will be similar to heavy brush problems.
Drainages. Drainages or ravines can have a definite effect on scent behavior. Since hot air rises, a drainage search during the day should be conducted along the top of the hill on the downwind side. Since cool air falls, an evening search should be conducted in the drainage itself. These search patterns will cover not only the drainage, but the slopes as well.
Drainages can also funnel scent so that it flows somewhat like a stream. If alerts are recorded in a drainage but the dog is unable to work the scent out, a careful study of terrain and airflow may reveal a funneling effect. A more thorough search can then be made of upwind areas from which the scent may be emanating.