Scent Theory: How Scent Cones Move

Scent is a moving target, carried and distorted by the wind. As a handler, one of the most challenging parts of your job is figuring out how to help your sniffer dog get to where they need to be to find odor, rather than hindering them by blocking or pulling them away from scent. Therefore, understanding scent theory is critical to the success of your team.

Scent theory refers to how scent moves, in different environmental conditions: including wind, elevation/depression, humidity, temperature, etc. Whether you’re indoors or outdoors, there’s almost always wind (aka air flow). Wind carries and distorts scent cones, making it harder or easier to detect. In the field, carrying flagging tape, baby powder or smoke pencils can help show you how scent travels.

Since humans are visual and scent is usually invisible to us, this course will show you the basics of what scent looks like in many common searches. Examples include not only videos of smoke grenades, but also research on how prey animals use the wind to hide, increasing their chances of surviving by avoiding detection by predators. Many of the concepts are explored in more detail in the excellent resources listed at the end of this article.

How Dogs Smell

Wondering how dogs smell? They detect chemicals (target odor) carried in the air to their noses. By comparing whether the scent is stronger at their right nostril or the left, they can navigate through the odor plume to find source (the highest concentration of target odor). Watch Ted-Ed’s “How do dogs “see” with their noses?” for a 5-minute introduction: https://www.ted.com/talks/alexandra_horowitz_how_do_dogs_see_with_their_noses/transcript?language=en

No Wind

Imagine searching for a hide on the ground when there is no wind. Theoretically, scent spreads out in all directions equally, forming a primary scent pool around the hide in an approximately spherical shape:

The scent is only detectable in a small circular area around the hide. Therefore, when the air is calm, searching dogs will need to wander very close to the hide to pick up its scent.

Having said that, outside the laboratory, there are almost always winds/airflows. Whether you search inside or outside, expect wind.

Basic Scent Cone

When there’s a slow, constant wind, scent will be carried away from the hide in the direction of the wind, in approximately the shape of a cone (Fig. 2). This airborne scent is also known as a scent plume. This video captures a scent cone in slow motion. It is most concentrated near source, and less intense further away.

The longer the aging/soak time (between when a hide is placed into the environment and when the sniffer dog starts their search), the larger the cone gets, making it easier to detect far away. How far scent is carried varies (depending on the winds, elevation of the hide and barriers in the environment), but eventually scent will fall to the ground due to gravity. Experience can help improve a dog’s ability to detect scent farther away from source (Conover, 2007, pg. 60).

This video shows the scent cone from a smoke grenade on top of a boulder in real time.

Human Scent

It’s virtually impossible to travel through an environment without contaminating the search area (15, 16). With every breath you take and every word you say, you distort scent in the search environment. For example, all humans leave behind tiny flakes of skin, which are rubbed off by friction with our clothes. This debris (aka “scurf”) falls from our exposed heads, the openings of our shirts and pants (Fig. 3).

Professor John Crimaldi, a fluid mechanist at the University of Colorado Boulder uses lasers to visualize what scent looks like as it moves through space (17). In this video, 3:28 shows what a single breath looks like, moving around one nostril (used with permission from Nsikan Akpan, 18).

Next, air currents and gravity come into play, messing up this tidy model. One test collected dandruff from volunteers, then dropped it from 6 feet above the ground. Flakes took between 1 and 10 seconds to hit the ground, travelling up to 9 feet away, when the breeze was barely noticeable (15, 16).

The branch of forensic science dealing with identification of suspects from lineups by police scent detection canines is called “osmology” or “odorology”. Collecting evidence of scent left by suspects at the crime scene is critical because “it is difficult for people to avoid leaving scent molecules on objects touched or in places where they were present, even if a human as the source of the scent did not directly touch the object but was above it” (2).

Containers & Residual Odor

When scent is inside a container, scent fills the container first, then exits from all holes, leaks and seams. To illustrate, here’s a video of a smoke grenade inside a container.

After smoke exits all the holes in the container, I move the container outside. Observe how the scent cone is distorted while I carry it, and when the door opens and closes. Many handlers are surprised to learn that their sniffer dogs can smell the scent as soon as they enter the room. In reality, sniffer dogs frequently pick up scent from adjacent rooms via drafts under the bottom of doors. Note the residual that remains in all the rooms of the building, minutes after the hide was moved outside.

In Police K-9 Magazine, Ted Daus explains: “The term residual means: the quantity remaining after most of something has been removed. Odor is a distinctive smell. Thus, “residual odor” is what remains of a distinctive smell after the original object has been removed”. He goes on to comment that: “Dogs alert to odor and direct the handler to the source of the odor, and then the officer searches for drugs… Every handler has had the experience of his dog alerting to a vehicle in which odor was present, but no drugs were found. That should not shock anyone when it happens, nor should it shock the judge or the lawyers involved in the case once they learn that a dog does not find drugs but merely alerts to drug odor” (19).

From the dog’s perspective, after a hide is removed from a container or vehicle, residual scent persists. Nobody vacuumed up the cloud of scent in the air! In a messy world, you won’t always know when residual odor is present, because you won’t see it like this smoke grenade. 

As far as sniffer dogs are concerned, searches start well before the start line. Sometimes they detect odor at your car. Enthusiastic green dogs may lie down when they first encounter the scent plume. Patience is required by the dog and their handler to find the precise location of source, when scent is detectable throughout the building. To pinpoint source, most handlers think about checking front, top, and sides of the object. Don’t forget to search all 6 sides, including behind/under, especially for containers like mailboxes and lockers (see Fig. 4).

Dog Path

In a search containing one hot box, a stereotypic dog path is shown in yellow in Fig. 5). Notice:

• Ideally, the team performs a systematic search e.g. box 1, box 2, etc. At position #1, the dog is searching for the plume.

• When the dog encounters the scent plume (shown in black), watch for changes of behavior.

• It’s important to notice where the dog does a head snap (position #2), an abrupt change of direction (2). The head snap does not always happen, but when it does, it’s important to notice the location. In ideal conditions, after the head snap the dog will go to source next. If your dog has difficulty pinpointing source, go back to areas suggested by the head snap.
• According to Conover (pg. 37), “Scientists are disadvantaged when studying how predators use olfaction because humans lack the olfactory acuity of olfactory predators. The method most commonly used by scientists to ascertain when a predator has detected the odor plume of prey is watching for a change in the predator’s behavior. Hunting dogs change their behavior immediately when they detect the odor plume of a bird. The dogs become excited; their ears perk up; they run or walk faster, turn directly into the wind, and move in a straighter line than before” (2).
• Between positions #2-4, the dog is sourcing (trying to pinpoint the highest concentration of target odor). They bracket, zigzagging back and forth, comparing the difference in odor intensity between nostrils as they travel up the scent cone.
• The dog reaches source at position #4. This is where the indication (aka trained formal response) occurs.

Resources

1. Andrew Rebmann, Edward David. Cadaver Dog Handbook: Forensic Training and Tactics for the Recovery of Human Remains. 2000. CRC Press, Boca Raton, FL. 
2. Conover, Michael. (2007). Predator-Prey Dynamics, The Role of Olfaction (pp. 36-7). CRC Press, Boca Raton.
3. Carla Simon, How Long Should Hides be Aged? – Scent Work Competition FAQ, 2018. Download at https://nosework.huntersheart.com/2018/02/22/how-long-should-hides-be-aged-soak-time-faq/
4. https://www.cbc.ca/news/canada/calgary/calgary-weather-2019-marks-fourth-coldest-february-1.5037342
5. https://www.provincialheating.ca/blog/perfect-room-temperature-winnipeg/
6. https://www.leisurepro.com/blog/explore-the-blue/difference-thermocline-halocline/
7. https://en.wikipedia.org/wiki/Thermocline
8. https://secure.execuread.com/facts/
9. https://www.thoughtco.com/leeward-and-windward-sides-of-mountain-3444015
10. William Sanders (2017). Modern Enthusiastic Tracking: The New Step-by-step Training Handbook (pp. 302-307). Stanwood, WA.
11. https://www.sciencedirect.com/topics/materials-science/convection
12. http://dronenodes.com/firefighter-drones/
13. https://study.com/academy/lesson/wind-shear-definition-lesson.html
14. NJR ZA (2015). [Video file] retrieved from https://en.wikipedia.org/wiki/File:20150226-08h40_to_09h45_-_JBay_-_Wind_shear.ogv
15. Milo Pearsall and H. Verbruggen, Scent – Training to Track, Search, and Rescue. Alpine Productions, Colorado, USA, 1982. Page 15.
16. Chris Brown. “Meet Angus, the C. difficile-sniffing dog trained to detect superbugs”, 2016. Downloaded from http://www.cbc.ca/1.3665351.
17. Nsikan Akpan and Matt Ehrichs. “What a smell looks like”, 2016. PBS News Hour. Retrieved from https://www.pbs.org/newshour/science/what-a-smell-looks-like.
18. Nsikan Akpan (2016). What a Smell Looks Like [Video file]. PBS News Hour. Retrieved from https://youtu.be/58U52lDTuvk.
19. Ted Daus (2013). “When does drug dog odor become ‘residual’?” Police K-9 Magazine [web page]. Retrieved from: https://www.policeone.com/police-products/k9/k9-training/articles/when-does-drug-dog-odor-become-residual-5MI27HxiHFyk9afl/.
20. Canine Olfaction Science & Law (Advances in Forensic Science, Medicine, Conservation, and Environmental Remediation) Ed. Tadeusz Jezierski. 2016, pages 279-284.