1 Scent Movement
1.1 Introduction
This chapter addresses basic information necessary to understand scent movement. While a technical treatment of the subject is beyond the scope of this book, it is desirable to have a basic understanding of scent and air movement and how weather, terrain, vegetation, and any intervening medium influence that movement. This information can be used to deploy a SD in an efficient manner that is most likely to result in detection and location of the source. These involve both art and science developed through personal observation, experience, and study of scent movement. Do not be discouraged by failures because scent movement and transport of scent by moving air is complex. It is impossible to understand all the situations faced by SD teams due to continuously changing weather and environmental settings (vegetation, terrain) that control scent movement.
The movement of scent molecules occurs by chemical diffusion, gravity,
buoyancy, and transport by wind, water, and other fluids. Water transport
refers to scent carried by water movement in above ground streams and water
bodies, in underground streams, on the ground surface, and in thin water films
on soil particle surfaces. Chemical diffusion is the movement of scent molecules
from regions of higher to lower chemical concentration in any medium.
Concentrations are highest at a source and decrease with distance from the
source so that the movement is radially away from the source in all directions
unless prevented by obstacles. This movement is so slow that it would require
about 2 hours to move scent 1 f by diffusion while a wind speed of 1 f/sec or
0.7 mph would require 1 sec to move scent the same distance. This means that
chemical diffusion is not a factor in scent transport in moving air or water. It
can be a factor in scent transport in fine-grained soils and snow where long
times or very short distances exist.
Gravity flow of scent is the downward settling of scent molecules that are heavier than air in settings (soils, enclosed spaces, buildings, vehicles) where there is little or no wind. Scent can also be transported (carried) by wind in the downslope and down valley gravity flow of colder air at night. Buoyant flow refers to upward movement of scent molecules that are lighter than air (ammonia, methane) where there is little or no wind. Scent can also be carried by the convective transport of warmed buoyant air that transports scent molecules and scent plumes upward (Figure 1). Tus, scent transport by the gravity flow of air, convection, and wind are the primary methods of scent transport in the atmosphere. These processes create scent plumes that are thought to behave like smoke plumes from a chimney or a camp fire. The analogy is not precisely correct since smoke consists of particles and scent consists of molecules, but it appears to be a good approximation.
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| Figure 1 A dark cedar tree in calm air, warmed by the sun, caused air in contact with it to warm. The warmer air is lighter and buoyant which caused it to rise carrying smoke or scent upward. A small gust was observed to bend this plume toward the ground. |
1.2 Scent Plumes
Scent plumes carried by the wind are subject to all the conditions
that influence wind behavior including the type of fow, atmospheric stability
and instability (thermal turbulence), and mechanical turbulence. The fow of air
can be laminar (layered without vertical mixing) but is generally turbulent
with eddies in the plume swirling and moving with the wind (Schroeder and Buck
1970) as shown in Figure 2. Eddies vary in size from tiny ones that are
barely visible to huge ones produced by large terrain elements such as mountains
and can rotate in any direction independent of the prevailing wind direction.
Much of the information on scent plumes comes from studies of
insects (Carde and Willis 2008). Some of it may not apply to the typically much
larger sources that search dogs are tasked to find using their relatively large
noses. Since our interests are in the behavior of wind at the height of the
dog, interactions of wind with features that influence wind behavior at this
height are of primary importance. The plumes appear to be continuous close to
the source and about the same size as the source. The effective size of a source
can be much larger when it is in a turbulent location (e.g. interior to shrubs
and trees, behind obstacles in the wind). Plumes increase in size with distance
downwind, primarily because of lateral turbulence that causes them to spread or
disperse when the eddies are smaller than the plume. If the size of the eddies
is larger than the plume, they can transport the plume long distances.
As the plumes move downwind, turbulence eventually deforms
(stretches, twists, bends, breaks) them which creates gaps of clean air within
the plume as it expands and disperses downwind (Figure 2). At this stage,
plumes may be described as patchy distributions of scent clouds that move with
the wind. The regions between the scent clouds consist of relatively clean air.
The distance between scent clouds increases downwind, which indicates that dogs
encounter an increasingly intermittent scent signal in the plume. At large distances,
the separation between scent clouds becomes much larger than the size of the
dog. The plume may also meander at very low wind speeds and, at high wind
speeds, is influenced by turbulence elements and random wind gusts in the fow.
If the dogs detect scent at this stage, they cast about trying to acquire the
plume and may or may not be successful.
This scent plume structure has several implications for search
dogs. Most scent is a mixture of multiple VOCs, and when turbulence is present,
different compounds will be influenced similarly. The ratios of compounds in the
scent clouds should be the same as the ratios emitted by the source. This has
the effect of preserving the scent picture for the dogs and helps them to
recognize it. Intermittent scent plumes with scent concentrations which are
relatively undiluted can move significant distances downwind before they disperse
to the point where dogs can no longer detect them. This picture of scent plumes
is one where the scent is distributed in scent clouds separated by relatively
clean air. It indicates that the use of changes in scent concentration (scent
gradient) by a dog as a directional guide to the source may not be reliable
unless they are close to the source.
Discounting visual means, successful location of a scent source
requires at least two sensory inputs: detection of the presence of scent and
the wind direction bearing that scent. Dogs detect the presence of scent with
their noses and may determine the direction of scent concentration gradients at
small scales due to scent discrimination between the nostrils or by moving
along the plume. Wind direction may be determined by differential cooling. A
dog’s wet nose and your fnger wetted and held in the air are sensitive indicators
of wind direction. However, in many cases, proceeding into the wind afer
detecting a source may not enable the team to fnd it.
The rest of the book explores the influence of weather, terrain,
vegetation, and intervening media on scent and scent plumes.
1.3 Wind
Wind occurs in response to changes in buoyancy caused by heating
or cooling air in contact with the earth’s surface materials such as soil, vegetation,
and water (Schroeder and Buck 1970). Irregular heating or cooling of the
earth’s surface produces temperature diferences in the adjacent air. Convection
occurs when warmer, lighter, buoyant air rises. Horizontal winds occur as
colder and heavier air moves to replace the warm air that moved upward.
Wind is usually thought of as the horizontal motion of air with
respect to the earth’s surface. The wind reported on the evening news is the prevailing
horizontal wind high above surface obstacles. We are primarily interested in
winds that carry scent to the dog’s nose. These winds are the prevailing
horizontal winds modified by obstacles such as terrain, trees, vegetation,
buildings and by processes such as uneven surface heating (convection).
SD handlers obtain information about wind speed and direction from the feel of the wind on their body, visual observations (waves, vegetation, dust, smoke, clouds, soaring birds), and handheld devices (powder puffers, surveyor tape). Handheld anemometers are useful for determining wind speed and direction at the height of the dog. The Beaufort wind scale (Table 3.1) is an approximate method that allows rough estimates of wind speed based on visual observations. Observed items are near the ground surface (dust, leaves, paper), exposed skin, and much higher (leaves on trees, wind vanes, fags, smoke). These estimated wind speeds are not usually those at the height of the dog because wind speed typically increases with elevation and is modified by surface obstacles.
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| Source: Modified from https://en.wikipedia.org/wiki/Beaufort_scale#Modern_scale. The scale is approximate because of the differences between observers and motion of leaves and branches depend on the type of tree. Personal observations indicate that a wind speed of 1 to 2 mph can be felt on damp skin. |
True laminar flow is relatively rare in the air near the surface but does occur (Figure 2). Low wind speed over fat, smooth, and gently sloping terrain favors laminar flow. Rough surfaces with obstacles and frequent changes in vegetation and topography favor turbulent flow. Atmospheric turbulence is ofen large at the leading edge of a cold front.
Laminar flow produces continuous layered scent flow not broken by
scent clouds. A dog that detects scent from a source in laminar flow should move
directly upwind toward the source, although there are few observations to
confirm this conclusion (they may also do this in light wind conditions when
turbulence is low). Turbulent flow consists of eddies in a swirling, gusting,
chaotic flow with rapid changes that redirect, fragment, and dilute the scent
which increases the diffculty of detecting and locating a source. A dog that
detects scent from a source in a small scale turbulent flow will usually quarter
upwind to find the source.
The distance that scent can be transported in a relatively short
time by even light winds is large. For a wind speed of 3 mph or 4.4 f/sec,
scent would move 264 f in 1 min, almost the length of a football field.
The wind can transport particulates such as airborne skin fakes,
soil particles with attached scent, and small scent particles. Gravity causes
these tiny particulates to settle, but at very slow rates. These particulates
suspended in the air by the exhalation jets from a dog’s nose could remain in
the air for up to several minutes and create a plume many tens of yards long.
1.4 Air Stability
1.4.1 Stability Conditions
Air is a mixture of gases, primarily nitrogen and oxygen. A one
square-inch column of dry air weighs 14.7 lb at sea level and its density is
about
0.08 pounds per cubic foot at 32°F. Warm air is lighter than
cooler air (e.g. air density at 90°F is about 0.072 pounds per cubic foot) and
water vapor molecules are lighter than air molecules so that air containing
water vapor (humid air) is lighter than dry air.
Solar radiation passing through the air typically warms it <1°F
in a day. Air is heated or cooled primarily by contact with surfaces such as
soil, water, vegetation (leaves, grass, tree trunks), rocks, and others. Darker
surfaces absorb more heat from the sun than lighter ones and become warmer
than lighter ones. (Put your hand on the same horizontal surface of light and
dark colored cars in sunlight and feel the temperature differences.) Since this
warm air is lighter than the air above, it rises in a process called convection
(Figures 1 and 3). Convection cells have been observed to rise from ground
surface areas that were about a square yard at speeds of about a 1 f/ sec at
intervals of about 4/min. At night and when a surface is shaded from the sun,
it cools by radiating to the sky and cools the air in contact with the surface.
Since colder air is heavier than warmer air, colder air remains on the surface
and cannot rise through the warmer air above, a highly stable configuration
called an inversion.
Air stability is an important factor in searches with search dogs
because it influences our success in detecting and locating a scent source in
the natural environment. Scent from a source in unstable air is subject to
convective turbulence that causes it to rise with the air above the height of
the dog and disperse, which makes it difcult or impossible for a dog to detect.
Scent from a source in stable air remains closer to the surface where it is
possible for a dog to detect it.
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| Figure 3 Microscale development of convective structures associated with instability. Surface heating (a) causes tiny bubbles of heated air to form (1) which lift off the surface in a mushroom shape (2) and grow vertically (3 and 4). Wind (b) mixes these structures into a thin layer at the surface that can grow into large convection cells. |
The change in temperature with elevation above a surface determines air stability or instability. This information is not normally accessible to a handler. Stability must be determined by alternate methods. For calm conditions, stability is sensitive to the amount of solar radiation incident on a surface, which depends on the angle of the sun above the horizon, latitude, time of day, time of year, slope of the surface and the direction that the surface faces. While these factors can vary substantially on the earth’s surface, it is convenient to describe their effects by three categories of radiation: strong, moderate, and slight. Fortunately, the factors influencing radiation incident on a surface and the corresponding radiation categories can all be determined in the field by measuring the length of your shadow (Lavdas 1976). For a person who is 6 f tall, a shadow <3½ f long indicates strong radiation, a shadow between 3½ f and 8½ f long indicates moderate radiation and a shadow >8½ f long indicates slight radiation. While shadow lengths can be prorated for persons of any height, the above results can be used by persons between 5½ and 6½ f tall without significant error.
However, air stability also depends strongly on wind, cloud cover,
and whether it is day or night. Meteorologists have combined these factors into
a set of six stability classifications: Class A—very unstable, B—moderately
unstable, C—slightly unstable, D—neutral, E—slightly stable, and F—stable. The
factors influencing atmospheric stability, except for the color of the surface,
are combined in Table 3.2. Shadow lengths are for a person 6 f tall and wind
speed is estimated with the Beaufort scale. The stability classes in the three
columns for day use hold for up to 4/8 cloud cover or any high clouds. For 5/8
to 7/8 cloud cover, use class C. Class D should be used for overcast
conditions, day or night, independent of wind speed. Cloud cover is the
fraction of sky covered by clouds. Night is defined as the period from 1 hr
before sunset to 1 hr afer sunrise.
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| Source: Modified from Lavdas, L.G. 1976. J. Air Pollution Control Assn 26(8):794, and |
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| Source: Data from Graham, H. 1994. NASAR Response Mag Winter. |
1.4.2 Effects of Stability on Searching
Graham (1994) developed an empirical method based on air stability
for choosing grid/lane widths for a desired probability of detection (POD) and
for estimating the POD of canine teams when searching during the day for live
subjects. This method is based on stability estimates (Tables 3.2 and 3.3) and
his experience with canine training and searches and includes the effects of
wind for all stability classes. There is little published information for other
above ground sources in the outdoor environment.
While there are limitations to this empirically obtained stability
method for estimating POD, it is more valuable than guesses by handlers or incident
command (IC). Unfortunately it is not generally taught or required of handlers,
their support, and/or search personnel.
The stability method of Graham (1994) can be used by teams to
select grid/lane widths for a desired POD by consistently using the method
during training. The results would be site specific for the conditions that exist
in the training areas and specific to the individual dog and handler. This
information on detection distances (hits and misses) can provide a starting
point for selecting grid/lane widths. For example, when searching for a source
under similar conditions, if the handler has information that their dog rarely
detects a source beyond a certain distance, then the grid/lane width should be
less than that distance. Also, if the handler has information on POD for a
certain detection distance, then this distance would be a starting point for
selecting grid/lane widths that would result in that POD. The method has the
added benefit of making the handler acutely aware of the effects that wind and
air stability have on the success of searches in their local environmental
settings.
In addition to air stability, POD for canine teams also depends on
the handler, dog, and other factors. Estimates of wind speed using the Beaufort
scale are only approximate for field conditions. Stability classes for different
observers using Table 3.2 can easily vary by a class or more. Efforts to categorize
atmospheric conditions by stability classes as in Table 3.2 take the effects of
thermal turbulence into account but do not address the effects of mechanical
turbulence produced by wind and local site conditions. Since the scent plumes
associated with typical scent sources are normally close to the ground, their
plumes are likely to encounter various obstacles in the airflow and result in
mechanical turbulence. This can have a major impact on the scent plume, which
makes searches and success of the search team dependent on the site
conditions.
The primary utility of Tables 3.2 and 3.3 is not so much for making
precise estimates of POD but rather for determining favorable and less
favorable times to search. Favorable times exist for stability classes D, E,
and F that occur at night (as defined in Table 3.2), on completely overcast days
all year, and during early morning and late affernoon hours all year, especially
when wind is present. Dificult times exist for stability classes A, B, and C
that occur during the middle of the day during late spring and summer,
especially when there is little or no wind. For these dificult conditions, it is
necessary to reduce normal grid spacing or lane widths to achieve PODs
comparable to those for favorable conditions. While searching in the natural
environment is not an exact science, handlers can develop a sense of what works
for them and their dogs by paying attention to the factors noted in Tables 3.2
and 3.3 during training.
1.4.3 Effects of Stability on Plumes
Information on air stability can also be obtained from
observations of dust movement and smoke plumes. The presence of dust devils
indicates a high degree of convective instability. Table 3.4 shows the effects
of three stability classes with five possible smoke (scent) plume patterns and
common conditions when these occur. Other more complex patterns have been
observed. Strong instability produces a looping plume pattern typical of
daytime and windy conditions during late spring and summer (Figure 4). This
type of plume is a result of wind that moves the plume horizontally and thermal
convection cells that move it up and down over alternating warm and cooler
surface temperatures. Looping can repeatedly bring the relatively undiluted
plume in contact with the ground downwind from the source. When a dog
encounters the scent plume near the ground, it will offen look up, whine in
frustration, alert or self reward. If possible, the team should move upwind and
try to detect multiple places where the scent plume loops to the ground. The
source should be upwind along a line connecting these places. Hilly,
mountainous, and forested terrain can redirect the wind and confound such
efforts.
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| Sources: Bierly, E.W. and Hewson, E.W. 1962. J Appl Meteorol 1:383–390. Graham, H. 1979. NASAR Search and Rescue Dogs Tech. Note 2:1–4. |
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| Figure 4 Looping and coning scent plume patterns (see Table 3.4 and text) and their effects on dog teams. Looping plumes are carried upward with rising air over a warm surface and downward with falling air over a cooler one. Wind causes the rising and cooling air and plume to angle downwind. Coning plumes expand horizontally and vertically and, if the source is elevated, will contact the ground downwind. (Modified from Graham 1979.) |
For stability close to neutral, a cone type of plume develops that expands about equally in both the horizontal and vertical directions as it moves downwind, unless the source is on the ground where it can only expand horizontally and upward. If the scent source is elevated, it will contact the ground at some distance downwind (Figure 4). Coning is characteristic of windy and cloudy conditions that can occur during the day or night in all seasons. When a dog encounters this type of scent plume, it can usually follow it to the source.
Inversions are highly stable air conditions that occur when warmer
air exists over colder air. The radiation type tends to occur in valleys,
gullies, and depressions on clear nights with little wind when radiational
cooling of the ground cools the air in contact with it and produces a layer of
colder air with warmer air above. They are shallow initially, tend to increase
in thickness through the night, and are very stable. These inversions are
destroyed when the morning sun heats the ground or surface vegetation and
causes convective turbulence that results in fumigation, and by prevailing
winds that produce mechanical turbulence. They may persist during the day under
cloudy and relatively calm conditions in depressions, shade and valleys, especially
during the winter at high latitudes and low sun angles.
Radiation inversions are important for SD teams because upward
movement of scent is effectively eliminated, trapping it below the inversion.
Dogs working above an inversion cannot detect scent from a source below it and
dogs working below the top of the inversion cannot detect scent from a source above
it (Figure 5). Inversions can be detected when the handler passes through
them, by a distinct change in temperature over a short distance from colder to
warmer (moving uphill) and from warmer to colder (moving downhill). The behavior
of a smoke plume can indicate the presence of an inversion. When a smoke or a
scent plume above an inversion encounters it, the bottom of the plume becomes
fat. When a rising plume from below encounters the top of an inversion layer,
it fattens and spreads horizontally. Rising smoke from a campfire or house is an
example.
Strongly stable air at night (common with an inversion present)
with light winds and little turbulence, produces a vertically thin plume that
spreads horizontally downwind in a fan shape (fanning, Figure 5) or a
straight or meandering ribbon when it encounters the top of the inversion. If
the source is above the inversion, scent would flow downslope and fan out on top
of the inversion and a dog worked just above the top of the inversion could
detect it. The dog may also be able to detect a source on the far side of the
valley if it is not too wide but note that the source may be above the level of
the inversion. Lack of vertical mixing above the inversion allows scent plumes
of this type to be transported long distances without much change in
concentration so that fanning conditions can be favorable for distant alerts.
Late in the day when an inversion is building up from the surface,
the atmosphere above it may still be unstable. This allows vertical mixing of
the plume upward while the stable inversion layer below prevents downward mixing.
For a source above the inversion layer, this process (lofting, Figure 5)
creates a plume with increasing thickness downwind and a fat bottom on the
developing inversion layer. A dog working within the inversion layer cannot detect
the source above the layer. If there are terrain changes downwind so that the
scent plume intersects the ground, a dog working above the inversion layer may
be able to detect it.
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| Figure 5 Fanning, lofting, and fumigating scent plumes in the presence of valley inversions (see Table 4 and text - Scent and Wind) and their effects on dog teams. Wind is to the right. For sources above the inversion, fanning plumes spread downwind on top of the inversion in a fan shape and lofting plumes occur when conditions are favorable for vertical mixing. Fumigation is the reverse of lofting. (Modified from Graham 1979.) |
Fumigation (Figure 5) is the reverse of lofting and occurs with
the reverse transition in stability in early morning. When the sun strikes and
warms the ground surface, it warms the air in contact with the ground and
causes buoyant mixing upward while the inversion layer still exists. If the
source is elevated but below the top of the inversion, the air above the source
is stable and prevents transport upward while the unstable layer below mixes
the plume vertically downwind, likely bringing it in contact with the ground
where a dog can detect it. Fumigation is transitory and ceases when the buoyant
convecting air layer exceeds the inversion depth.
Valley inversions behave differently when the rising sun contacts
one or both side slopes. The air in contact with the slope, including that below
the inversion level, is warmed, becomes bouyant, and flows upward along the
slope. This means a dog would have to be above the source to detect it.
2 Turbulence
2.1 Characteristics
Turbulent flow is common near the ground, and as the turbulence
increases the difficulty in detecting and finding a source increases. Since the
primary task of a SD handler is to get the dog into the scent plume from the
source, recognizing and evaluating the presence, characteristics, and effects of
turbulent flow are necessary skills for them. A high degree of skill at this
task will increase the team’s probability of success.
Turbulent flows may be viewed as a spectrum of eddies with a wide
range of sizes. Mechanical turbulence depends on surface roughness, obstacles
in the flow, and wind speed and direction (Schroeder and Buck 1970). The general
roughness of an area’s surface contributes to the formation of larger eddies
that move over the landscape. Like a boulder in a stream, an obstacle in the
airflow produces a recirculating mix of eddies behind it with eddies that break
of and move downwind from the obstacle. Every obstacle, including porous ones
like vegetation, produces turbulent eddies. The sizes, shapes, and motions of
eddies are determined by the characteristics of the obstacle and the speed and
direction of the wind.
An important feature of these turbulent eddies is their size
relative to the size of the scent plume. If the eddies are smaller than the
scent plume, they dilute the plume; if they are larger than the scent plume,
they can transport the plume or parts of it with little change for relatively
long distances. Thus, the size of the eddies determines detection distances for
a scent plume and is important for distant alerts.
Thermal turbulence associated with air instability consists of
convection cells with warm buoyant upward air movement and adjacent cooled downward
air movement (Figure 4). Since it is the result of surface heating, thermal
turbulence increases with the intensity of surface heating. Mechanical and
thermal turbulence frequently occur together and create a mixed convection.
Turbulent flow can also bring higher wind speeds from aloft down to the surface,
usually in spurts and gusts (large scale looping). There is also small-scale
turbulence produced by the dog’s movement and by exhalation during sniffing which
appears to help the dog find sources.
Eddies may form with their axes of rotation in any plane. Dust
devils and thunderstorms are vertical eddies. Rotation speeds in eddies are
often much greater than the average horizontal wind speed. Eddies associated
with an obstacle tend to hold a stationary position in the lee of the obstacle,
although secondary eddies may break of and move downwind. A rule of thumb is
that an obstacle influences the airflow for a downwind distance 5 to 10 times the
height of the obstacle. An appreciation of eddy characteristics can be gained
by observing the flow of water over and past obstacles in a river and the flow of
smoke plumes and clouds moving over the terrain (especially with time lapse
photographs).
Variable heating and cooling across the terrain causes typical
daily cycles in wind behavior. Daytime surface winds in fat terrain increase to
their highest speeds and turbulence (primarily thermal turbulence) about the
time of maximum heating. With the onset of nighttime cooling, surface winds
normally decrease in speed and turbulence.
2.2 Thermal (Convective) Turbulence
Prevailing winds often dominate near the surface but when these
winds weaken in the presence of clear skies that produce strong daytime
heating, local convective winds become important. Convective winds can enhance
or oppose prevailing winds, and interactions between the two can result in
local variations in speed and direction over distances of a few yards in
complex terrain.
2.2.1 Surface Temperatures
Microclimatic conditions that influence scent movement vary
tremendously over the arctic, temperate, and tropical zones. These variations
are made more complex by seasonal effects and changes in vegetation, topography,
moisture, and other factors that produce different ecosystems. This global
complexity in physical settings and conditions produces tremendous variability
in the behavior of the wind that carries scent to the dogs. Most of the
following material focuses on temperate zone conditions unless noted otherwise.
The sun warms the exposed earth surfaces and these surfaces warm
the air in contact with them. The resulting convection can transfer scent well
out of reach of SDs (like looping in Figure 4). Tus, surface temperatures can
influence the ability of a dog to detect a source and may suggest where and when
a dog should be deployed for the highest probability of success.
Daily temperature variations of the earth’s land surface are typically 10 to 30°F but can be much smaller or larger. Small variations are associated with light colored and wet earth surfaces, low sun angles, coastal regions, and cloudy days. Large variations are associated with dry, dark colored surfaces, high sun angles, high altitudes, desert regions, and sunny days (Figure 6).
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| Figure 6 Schematic of earth surface temperatures across a varied landscape on a summer day. The difference between the road surface and the lake surface can exceed 50°F. Hot air over the road and island would likely cause convection cells. |
2.2.2 Non-Vegetated Surfaces
Common non-vegetated surfaces include water, snow, ice, soil,
sand, rock, and manufactured ones (concrete, asphalt). It requires much more
radiative heat from the sun to increase the temperature of water, snow, and ice
surfaces compared to other surfaces. Water, snow, and ice allow some of the
radiation to penetrate which reduces their impact on surface temperatures.
Moving water can carry some of the radiant energy away from the surface. Snow
and ice surfaces cannot exceed 32°F (the equilibrium temperature of water and
ice in contact). At night and when in shade, snow surfaces cool quickly and,
under favorable conditions, develop strong inversions at the surface. Daily
surface temperature variations for water are small and limited for snow and ice
compared to the relatively large variations for other non-vegetated surfaces.
Bare soil, sand, rocks, and asphalt tend to dry and warm quickly
when exposed to solar radiation. Some soils, vegetation, and asphalt are
relatively dark and good absorbers of solar radiation. These properties lead to
large daily variations in their surface temperatures that favor formation of
small-scale surface convection (Figure 3) and large-scale convection cells
(thermals) (Figure 4, looping) when the overlying air is unstable. The impact
of solar radiation on air over surfaces depends on whether the surfaces are
level or sloped and on the characteristics of the surfaces. From sunset to
sunrise and when a surface is in shade, the surface radiates to a cold sky
causing it to cool which cools the air in contact with it and creates a layer
of colder air near the surface with warmer air above it. This is a stable
configuration for level surfaces since the colder air is heavier than the warmer
air above (lighter, warmer air “floats” on the heavier, colder air below it).
Under calm conditions, an inversion begins to form and scent from a source on
or emitted from such a surface will concentrate in a layer near the surface
without mixing with the warmer air above (Figure 7). Dogs may need to get
their noses down to detect scent close to the surface, especially from buried
sources. Wind may reduce or prevent the formation of these night inversions.
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| Figure 7 Scent from a source on or emitted from the surface of level ground with an inversion. The coldest air is at the surface and scent concentrates there. Temperatures increase up to the inversion and decrease above it. |
When the sun is shining on a level, horizontal surface (roughly sunrise to sunset), it absorbs solar radiation and causes the surface to warm and to warm air in contact with it. Since the warmer air at the surface is lighter than the colder air above it, the warmer, buoyant air can rise. This initially creates vertical air currents in the form of tiny convection cells (Figure 3) that form a thin turbulent surface layer. If the atmosphere is stable, the thickness of this layer of convection cells will be limited or disappear. If it is unstable, the convection cells can grow into thermals (looping, Figure 4) and eventually become large cells (thermals). Thermals can grow thousands of feet high and can transport air from near the ground surface to these high elevations. Glider pilots and soaring birds use thermals to gain or maintain altitude and vultures use them to sample air from near the ground to detect the scent of food. Thermals carry scent out of reach of dogs and make it difficult or impossible to find the source.
The effects of radiation on sloping surfaces differ substantially
from those on horizontal surfaces (Mahrt et al. 2001). When in the shade and at
night, sloping surfaces also cool by radiating to a clear sky, which makes the adjacent
air cooler and heavier than the air above and gravity causes it to flow
downslope. Under these conditions, scent is transported downslope with the cold
air in a thin layer adjacent to the surface (Figure 8) and possibly pools in
depressions. Under these conditions, SD teams should search at the bottom of
the slopes or below the expected position of the source.
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| Figure 8 Thin downslope flow about 15 min after the slope went into shade. |
East facing ridges go into shade some time before sunset (Figure 9), allowing canine teams to take advantage of them during daylight hours. This setting is reversed in early morning when the west facing ridges remain in shade for some time after sunrise, again allowing canine teams to take advantage of them during daylight hours. In summary, “Sun up, scent up and sun down, scent down,” but remember shade.
In the sun, sloping surfaces are also heated and warm the adjacent
air which makes it lighter than the air above. Buoyancy causes it to flow
upslope and into the atmosphere at the top of the slope where it may continue
to move upward into the unstable air or be carried away by prevailing winds.
This upslope flow is typically thicker and more turbulent than the nighttime
downslope flow. Under these conditions, SD teams should search on ridges or hill
tops or at elevations higher than where the source may be.
These upslope and downslope winds may not be continuous but may
start and stop during the day and night. Downslope gravity flow can occur in
areas with hardly noticeable slopes. They may occur with prevailing wind and flow
from side slopes that can enhance, eliminate, or redirect them sporadically or
continuously creating complex scenting conditions. Figure 10 shows downslope
gravity flow that consists of a primary down valley flow and flow from both side
slopes.
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| Figure 9 Downslope gravity flow of wind begins when the slope goes into shade. Canine teams searching the left slope need to be at the valley bottom or below the source while teams searching the right slope need to be on top the ridge or above the source. (Modified from Schroeder and Buck 1970.) |
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| Figure 10 Downslope night time gravity flow consisting of a primary down valley flow and flow from both side slopes. Prevailing winds can enhance, redirect, or eliminate gravity flows resulting in complex scenting conditions. (From Schroeder and Buck 1970.) |
Canine searches during the day can be frustrated by the depth and turbulence associated with upslope flow and prevailing winds. At night and in the shade, prevailing winds tend to decrease and become less turbulent. Downslope gravity flow is in a thin layer and the scent is closer to the ground, which results in more favorable scenting conditions than during the day.
2.2.3 Thermal Reversals
Air movement caused by radiant cooling and heating of sloping
surfaces produces downslope flow of cold air at night and upslope flow of warmer
air during the day. This results in thermal reversals (reversals in wind
direction) every morning and evening under stable weather conditions. Thermal
reversals can occur in any terrain with sloping surfaces even if the slopes are
very small. Under stable weather conditions, the usual wind is downslope or
down valley at night, as noted. When the sun rises, it warms surfaces and the
adjacent air at higher elevations, with the warming progressing downhill as
the sun gets higher. This warm air moves upslope and reverses the nighttime flow
progressively down valley until it has the usual upslope daytime flow. If an
inversion is present, it may take some time for it to dissipate.
As sunset approaches, the lower elevations go into shadow first and
start to cool by radiating to the sky. This cools the adjacent air and the
colder air moves downslope and down valley which reverses the daytime flow progressively
up the valley until it has the usual downslope nighttime flow. This downslope flow
(katabatic wind) can travel at very high speeds, especially over long slopes
and on glaciers.
When and where the thermal reversal occurs, there is a cessation
of the airflow followed by a variable calm period of minutes to an hour or more
and then the reversal. The timing of these reversals depends on the incoming and
outgoing radiation on the valley slopes and bottom, which depends strongly on
local conditions, especially slope orientation. It is expected to vary seasonally,
earlier each morning and later each evening from winter through spring and
later each morning and earlier each evening from summer through fall. The
morning reversal may occur well after sunrise because of the buildup of a cold
air pool or inversion in the valley. The strong dependence of these reversals
on local conditions indicates that handlers must become familiar with thermal
reversals in their search areas in order to be at the right place at the right
time to take advantage of them. Thermal reversals are also associated with sea
and land breezes.
2.2.4 Sea and Land Breezes
A sea breeze is a wind that flows from the sea to the land while a
land breeze flows from the land to the sea (Figure 11). The amount of heat
energy required to change the temperature of water is much larger than the heat
required to change the temperature of earth materials like soil, rock, and
vegetation. As the day begins, the sun heats both the land and water but the
land surface temperatures increase faster and their increase is greater than
the water. Air in contact with the land is warmed and becomes lighter, less
buoyant and rises. The colder air over the water flows toward and over the land,
called a sea breeze, replacing the rising air. The strength of the sea breeze
depends on the temperature difference between the land and the water. If there
is no prevailing wind, a convective cell may form with air falling over the
water and rising over the land. A prevailing wind can enhance, redirect, or
prevent the sea breeze from forming. At night, the land cools faster, more than
the water, and the process reverses, creating a land breeze (Figure 11).
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| Figure 11 Sea breezes are from the sea and land breezes are from the land. (From Schroeder and Buck 1970.) |
When a sea breeze is present, a dog searching along the shore can detect sources somewhat offshore and when a land breeze is present it can detect sources somewhat inland. It may be possible to search a difficult shoreline with a dog in a boat close to shore when a land breeze is present. Similar breezes may be produced by lakes and ponds but modified by their smaller size compared to the sea and influenced by nearby terrain and vegetation.
If conditions are favorable, convection cells can be observed when
the colder air from a land breeze or from gravity flow contacts warmer water and
causes fog to form over water bodies such as swamps, ponds, lakes, and sea.
These fog layers are common during the transition to colder night temperatures
in fall. On ponds they are typically a few feet thick but can be much thicker
(hundreds of feet) on large water bodies or when temperature differences
between the air and water are large and can persist well into the day.
Initially, the upward movement of warm moist air in convection cells from the
surface is marked by patches of fog and the downward movement of cold air by
the absence of fog. At the onset, the upwelling fog cells are usually less than
a yard square and a light wind can destroy them.
2.2.5 Vegetated Surfaces
Vegetation (grasses, bushes, trees) that is darker than its
surroundings and the dark bark of some deciduous trees without leaves can
produce convective flow during sunny days (Schroeder and Buck 1970), which can
carry scent upward and out of reach of search dogs. This type of convective flow
can occur with any dark horizontal or vertical surface (clump of grass, single bush
or tree (Figure 1), grasslands, forests, and buildings). It exists when these
darker surfaces are in the sun and disappears when they go into shade and at
night. It can also exist in forest clearings where sunlight can reach dark
ground and warm it which causes the air in contact with the ground to warm and
rise (Figure 12). This draws cooler air from the adjacent forest so that SDs
worked in the shade around the clearing can detect sources deeper in the
forest. For clear and relatively quiet conditions at night, the ground cools by
radiation and causes the flow to be from the clearing into the forest. SDs can
then detect sources in the clearing. Fields that are darker than the
surrounding fields and islands of darker soil or vegetation in fields can also
create isolated convection cells.
The presence of a vegetative canopy (grass, weeds, crops, shrubs,
trees) on or over the ground surface causes significant changes in the
microclimate. These changes vary seasonally because of seasonal changes in
vegetation, foliage, and other factors (e.g. snow). Canopies range in depth
from grass prairies and short shrubs to tall forests and differ substantially in
characteristics (structure, type of foliage, density, etc.). Grasslands and
crops typically have a single layer structure. Managed forests and some mature
forests may have two layers (foliage and tree trunks). Forests with mixed tree
types of differing ages are more complex.
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| Figure 12 Openings in forests where sunlight can reach the ground and where the adjacent forest shades the ground can act as chimneys under conditions of daytime heating and light winds. Cooler air from the forest is drawn to the opening and replaces the heated air that rises. (Modified from Schroeder and Buck 1970.) |
Canopies influence the microclimate by their impacts on moisture, temperature, wind, and radiation. Moisture is intercepted and used by canopies. Canopy foliage shades the ground day and night which alters the ground and air temperatures and soil moisture. During the day, it shades the ground and stem or trunk layer from the sun and makes the ground cooler. This can create an inversion beneath the canopy in the trunk layer of a forest. At night, the upper canopy cools by radiating to the cold sky and cools the air in contact with it. If the canopy blocks radiation from the trunk layer it will be warmer than the air in the upper canopy. Heat from the soil and woody mass in the trunk layer also helps to keep this layer warmer. This is an unstable configuration that makes it possible for cooler (heavier) air in the canopy to drain downward and the warmer air carrying scent to move upward, which possibly results in difficult scent conditions for the dogs. Wind blowing over the forest and variations in terrain can dramatically change these conditions.
Thus, forest canopies may produce lower maximum temperatures during
the day and higher minimum temperatures at night in the air near the ground
than adjacent grasslands or forest clearings. Lower maximum temperatures may
induce flow of colder air from the forest into the grasslands by day. However,
this process may compete with upper canopy heating that would draw air from
adjacent fields toward the forest, possibly with restricted exit flow (from the
field) caused by the dense vegetation commonly found at forest edges. Handlers
need to observe air flow at the forest edge close to the ground to determine if
conditions are favorable for dog teams to search along the edge of the forest
and detect sources within the forest close to the edge. Higher minimum
temperatures at night may induce flow from colder grasslands and clearings into
the forest although this may be modified by cold air draining down through some
canopies. Handlers must assess local conditions to determine an effective search
strategy.
2.3 Mechanical Turbulence
2.3.1 Surface Roughness
Mechanical turbulence results from the interaction of wind with
surface roughness elements in the flow. These roughness elements can be any obstacle
that projects into the flow including both porous and solid ones such as
pebbles, rocks, grass, crops, trees, buildings, and terrain features such as
hills and valleys. Important characteristics are their size, especially height,
number, and spacing. Their impact on the flow can extend upward several times
their height and many times their height downwind. Turbulence and reverse flow
(recirculation) can develop immediately upwind and downwind of an obstacle in
steep terrain and at sharp changes in vegetation.
2.3.2 Terrain Effects
Modification of wind by roughness elements makes the behavior of
the wind unique in every landscape and impossible to predict in detail.
Generally, terrain elements redirect the airflow, accelerate it over their tops
(even when the maximum slopes are small), and create turbulence and
recirculation in the flow. These changes strongly influence scent movement for
short distances upwind and to the sides and long distances downwind in the wake
of the elements. These areas require special care and consideration when
conducting canine searches.
The following discussion of terrain effects assumes that there is
level homogeneous terrain upwind of the elements, neutral atmospheric
stability, and wind direction perpendicular to the terrain elements. The results
differ if these assumptions are not true. Nevertheless, these simple examples
can help in developing an improved understanding of terrain effects on airflow
and turbulence. There are two broad classes to consider: moderate terrain with
slopes typically <17° (rise/run < 3/10) and steep terrain with slopes >17°
(rise/run > 3/10).
2.3.2.1 Moderate terrain For the gentle topography of large
terrain elements (hills, valleys, ridges) and low wind speed, the wind can be
expected to flow over the terrain with relatively little turbulence except for
that produced by much smaller surface elements (boulders, gullies, ditches).
However, high wind speeds may lead to turbulence similar to that found in steep
terrain at lower wind speeds. Wind speeds can be expected to be higher at the
top of terrain elements and in terrain constrictions and lower at the toe of
slopes and in valley bottoms.
2.3.2.2 Steep terrain Significant turbulence near the ground
surface can be generated by steep terrain elements that influence search
strategies and the ability of dogs to detect and locate sources. An
understanding of this turbulence can help the team find sources in steep
terrain. The location and the characteristics of the turbulence for some simple
terrain elements are shown in Figure 13; however, changes in air stability,
wind velocity, variable terrain, angle of wind to the element, and vegetation
can make flow over these elements more complex.
Wind accelerates along a vertical cross section through the
centerline of a hill or island or perpendicular to a ridge when it encounters
the slope and is maximum as it passes over the crest. A recirculation zone
(bolster eddy) may form near the toe of the slope with surface winds that are weak,
variable, intermittent, turbulent, and downslope. A much larger near-surface
recirculation zone (lee eddy) may form downwind of the crest. The winds are
light, variable, intermittent, turbulent, and upslope. The effects of this lee
eddy may extend downwind many times the height of the ridge or hill.
Consequently, it may be possible for dogs to detect a source downwind of the
toe of the slope by working them in the lee eddy along the slope or even near
the top of the slope. On the upwind slope, it may be possible for dogs to
detect a source somewhat upslope by working them in the upwind eddy along the
toe of the slope. For a clift with a sharp break at the top, an eddy may also
form at the top slightly back from the edge.
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| Figure 13 Effects of steep terrain elements on wind flowing over them. These effects will be the same for any features with the same density and geometry independent of their size. (From Oke, T.R. 1987. Courtesy of Dr. Timothy R. Oke.) |
The above discussion holds for other changes in steep topography associated with airflow into and out of a wide valley, river channel, lake, and on the approach to or leaving a steep change in topography.
An eddy may be created in a depression, gully, valley, road cut,
or narrow canyon with light, variable, intermittent, and turbulent winds near
the ground with flow opposite to the direction of the flow aloft.
The plan view of a symmetrical hill or island shows the lee eddies
in more detail. Airflow increases in speed on the front and sides of the hill
and the lee eddies extend farther past the hill than shown.
A plan view of a sharp constriction in river channels, valleys,
mountain passes, and road cuts shows the turbulent recirculation zones that
form at the upstream and downstream sides of the constriction. The eddies in the
recirculation zones shown in vertical sections rotate in a vertical plane but
those in the plan views rotate in a horizontal plane.
The results for airflow and turbulence over the above terrain
features are general in the sense that they can be applied to a wide range of
scales provided the cross-sectional form is similar. For example, the results
for ridges can be applied to levies and those for valleys to ravines and road
cuts. If the airflow is less than perpendicular to the long axes of
two-dimensional features, the strength and persistence of the flow features will
be reduced and modified. If the air is unstable, formation of eddies is enhanced
relative to neutral stability. If the lee slope is in sunshine, the upslope flow
will strengthen the lee eddy.
2.3.3 Effects of Vegetation
Isolated trees and bushes produce mechanical turbulence on their
upwind sides, and for 5 to 10 times their height in the downwind wake. This turbulence
depends on the type of vegetation, its size and density, and wind direction and
speed. At low wind speeds, scent can be channeled around dense isolated trees
and bushes, and at higher speeds turbulent eddies form on their windward and
lee sides. An isolated tree trunk, or one in an open forest, can trap scent in
turbulent recirculating eddies on the downwind side (Figure 14). Scent from a
scent plume upwind accumulates in the eddies and on the bark; dogs typically
alert there.
The behavior of scent in islands of trees and bushes may be
expected to be like that of individual vegetation if the islands are small
(width a few times the vegetation height) or like forests for large islands
(width many times the vegetation height).
2.3.3.1 Shelterbelts Shelterbelts are long, narrow barriers that
provide shelter from the wind. They are commonly porous features (fences,
bushes, hedges, trees) but can also be nonporous (berms, stone walls).
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| Figure 14 Smoke from a source placed on the upwind (right hand) side of the tree was caught in recirculating eddies on the downwind side of the tree. Scent accumulates in the eddies and on the bark on the downwind side and dogs typically alert there. The sticks leaning against the nearside of the tree created a smaller eddy. |
A high-density shelterbelt is like a ridge (Figure 13), where similar eddies can be produced on the upwind and downwind sides. The magnitude of these eddies is determined by the height of the canopy and its porosity to airflow. An upward flow of air on the sunny side of the canopy may occur when it is in the sun and winds are calm or light. The upwind eddies influence scenting conditions immediately in front of the shelterbelt while the influence of the downwind eddies extends from the shelterbelt to many times its height downwind. Wind incident at an angle (up to about 25°) tends to flow perpendicularly through a shelterbelt and to mix downwind with the air flowing over it which produces lateral turbulence. The areas upwind and in the turbulent eddies downwind of shelterbelts require special attention when searching.
A shelterbelt that is open at its base (Figure 15) may allow a
substantial flow of air through it. This air can act as a “cushion” that may
persist many times the height of the vegetation downwind and inhibits downward
mixing. This makes it difficult to detect elevated sources in the vegetation of
the shelterbelt when downwind from it. An elevated structure or vehicle
(tractor trailer) that is open underneath may have a similar effect. It may be
desirable to start the search from a distance downwind rather than close to it.
2.3.3.2 Forests Wind approaching, leaving, and blowing through forests
(and clearings in them) produces turbulence in the form of updrafts, downdrafts,
frequent changes in direction, recirculation, sweeps, ejections, and low speed
zones that make it dififcult to find sources in and adjacent to the forest. The
effects of a forest on wind depend on the air stability, wind velocity, forest
density, presence or absence of leaves, and other characteristics of the
forest.
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| Figure 15 Vertical profile of a shelterbelt showing a possible cushion on the downwind side which inhibits downward mixing of scent from above for many times the tree height downwind. (Modified from Oke 1987.) |
Wind incident on a forest edge produces turbulence and updrafts in front of the edge and some distance downwind of it in the forest. If the forest is sufficiently dense, a recirculation zone may develop immediately in front of the edge like that upwind of a steep change in terrain or dense shelterbelt (Figure 12). If the forest is sparse in the trunk space, a jet of air into the forest may develop near the ground (Figure 16). Deep in the forest, wind directions are highly variable, wind speeds are greatly reduced, and turbulent eddies are much smaller compared to those in an open field.
A comparison of wind in a field and adjacent forest (Table 3.4)
showed that in the field above the vegetation, a relatively steady wind
direction and speed of 5 to 10 mph existed for more than an hour. In the forest
(below the canopy), the wind direction was highly variable with long periods of
very light wind (<½ mph) and then several minutes of 2 to 5 mph wind gusts. These
intermittent wind gusts are energetic sweeps of air that move downward through
the canopy from above. Weaker ejections of air upward and out of the top of the
canopy occur in association with the sweeps. Sweeps and ejections can also
exist in other types of vegetative canopies such as grasses, weeds, crops, and
bushes. These sweeps and ejections would be expected to cause significant
vertical mixing of scent and make scenting conditions difficult while they
occur (Table 3.5).
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| Figure 16 Schematic drawing of a jet of air penetrating into a sparse forest near ground level. Distance is scaled according to forest height and wind speeds; 1 to 5 are relative speeds. A low speed wind jet, 1, penetrates almost five times the forest height. (Modified from Belcher et al. 2012.) |
Immediately downwind of a forest edge, wind speed increases and moves downward in response to the presence of grassland. If the forest is sufficiently dense, a recirculation zone may develop in the lee of the edge like that downwind of a steep change in terrain or dense shelterbelt (Figure 12). These effects may not occur or would be substantially modified for deciduous forests without leaves.
The above differences between wind in fields and forests indicate
that scent plumes will behave differently. The relatively constant wind direction
in a field indicates that a dog that detects a scent plume there and moves
upwind should be able to follow the plume to the source. However, the variability
of wind direction in a forest indicates that a dog that detects a scent plume
there and moves upwind at that instant will not likely be moving in the
direction of the source. This makes it increasingly difficult to find sources
beyond 100 yards distance in forests and indicates that closer grid spacing may
be required in forests compared to fields. However, as the wind speed increases,
the variability in wind direction decreases (i.e. the path of the scent plume
becomes straighter), which should reduce this difficulty somewhat.
Managed forests are usually uniform in the sense of having trees
of the same type, height, and density while natural forests, especially old
growth, have canopies with irregular upper surfaces that contain openings and
clearings as a result of the death of old trees, windstorms, and selective
cutting. These gaps in a forest may have downdrafts carrying scent which increase
the likelihood of sweeps and ejections. Gaps in the forest canopy may be barely
noticeable from the ground.
Wind over forest openings and clearings is sensitive to their size
and orientation. Gaps about the width of the tree heights in a dense forest may
develop a recirculation zone within them that can cause air flowing over the
forest to sweep down into the openings along with any scent in the air (Figure 12). This recirculation zone may exist until the gap widths are many times the
tree heights. Flow over very wide gaps (10 times the height of the trees or
more) may behave like the flow on the lee side and upwind side of a wide valley
(Figure 13).
Patterns of snow accumulation in clearings are a result of the
airflow in the clearings and the adjacent forests (Gary 1974). Snow
accumulations are greater in clearings and less in the forest downwind of
clearings. Where the width of the clearing is about the same as the height of
the trees, snow may accumulate on the upwind side of the clearing with reduced
accumulations on the downwind side and some distance into the forest. This
pattern of snow accumulation reflects the average direction and duration of
airflow rather than the values at any instant of time.
The above patterns of wind behavior at forest edges, within forests
and in clearings vary with the physical settings (e.g. air stability, type of
trees, old growth, new growth, tree density, etc.) so that somewhat different
results may occur depending on the setting. Handlers need to be familiar with
the effects of wind behavior in forests in their local training and search areas
and this is best done during training.
2.3.3.3 Combined terrain and forest effects Airflow and turbulence
are influenced by even gently undulating topography and forest canopies. When a
forest canopy covers large variations in topography, the effects on flow and
turbulence are even greater (Belcher et al. 2012). Typically, wind velocity in
the canopy increases on an upwind slope, is largest near the crest, and
decreases in the lee of a hill or ridge. If the slope is sufficiently large or
the canopy sufficiently high, a recirculation zone may exist in the canopy in the
lee of the hill and a smaller one on the upwind slope of the hill. With
increasing slope, the depth of the lee recirculation zone can increase to where
it spans the depth of the canopy. The presence of a recirculation zone on the
lee side indicates that, under some conditions, it may be possible for a dog to
detect a source that is in the forest far down the slope in the prevailing wind
direction by working the dog in the recirculation zone across the slope or near
the top of the zone.
3 Summary
Scent movement is concerned with the interactions among weather
and the physical environment (vegetation, terrain) and any intervening medium
(air, soil, ground cover, snow, water) that influences scent movement from the
source to the dog’s nose. These interactions determine our search strategies and
favorable times and places to search; ultimately, the success or failure of our
search efforts.
Processes that cause scent molecules to move are chemical
diffusion, gravity, buoyancy, and transport by wind, water, and other fluids.
Chemical diffusion is the movement of scent molecules from regions of higher to
lower chemical concentrations in any medium. The movement is radially away from
the source in all directions, very slow and not significant for scent transport
in calm air. It can be a factor in fine-grained soils and snow for long times or
very short distances. Gravity flow is the downward settling of scent molecules
that are heavier than air when there is no significant wind. Buoyant flow refers
to the upward movement of scent molecules that are lighter than air (ammonia,
methane) and to convective transport of warmed air that carries scent
molecules and scent plumes upward (Figure 1). Scent in air, water, and other
fluids can be transported by the movement of these fluids.
Wind is the primary method of transporting a scent in the
atmosphere where scent plumes behave like smoke plumes from a chimney or
campfire. The initial size of a scent plume is about the same as the source.
Close to the source, scent plumes appear continuous, but turbulence creates
scent clouds with gaps of clean air as the plume expands and disperses downwind
(Figure 2). This means the use of scent gradients by a dog as a directional
guide to the source is unreliable except close to the source; wind direction is
the primary method used to find the source.
Successful location of a scent source requires detection of scent
and wind direction. A dog’s wet nose is a sensitive indicator of wind direction
but proceeding into the wind after detecting a source may not enable the team
to find it.
Wind occurs in response to changes in buoyancy caused by heating
or cooling air in contact with the earth’s surface materials. Vertical
convection occurs when this warmer buoyant air rises. Horizontal winds occur as
colder and denser air moves to replace warm air.
Wind flow can be laminar or turbulent (Figure 2). It is usually
turbulent, consisting of eddies of all sizes that cause rapid changes which
redirect, fragment, and dilute scent. This increases the difficulty of detecting
and locating a source. Observations of smoke plumes suggest scent plume patterns
of looping, coning, fanning, lofting, and fumigating (Figures 4 and 5).
The impact of radiation on or from surfaces depends on whether the
surfaces are level or sloped. At night, outgoing radiation cools level
surfaces that cool the adjacent air which causes scent to remain on the surface
where it may be detected by dogs (Figure 7). This is a highly stable
configuration called an inversion (Figure 5). During the day, dark surfaces
absorb more incoming solar radiation than lighter ones and become warmer,
sometimes several tens of degrees above the air temperature (Figure 6). Warm
surfaces warm the adjacent air which causes it to rise (convection) and
transport scent above the height of the dog where it is impossible for the dog
to detect.
The effects of instability caused by radiation on search strategies
can be determined in the field by measuring the length of your shadow. The difficulty
in locating a source increases as your shadow length decreases. Taking air
stability into consideration, favorable times to search occur at night, on
completely overcast days all year, and during early morning and evening hours
all year, especially when wind is present. Difficult times occur during the
middle of the day and afternoon during late spring and summer, especially when
there is little or no wind (Tables 2 and 3).
At night and when in shade, a sloping surface cools by radiation
and cools the adjacent air, which makes it heavier and less buoyant than the
air above. This causes it to flow downslope in a thin layer carrying scent with it
(Figures 8 and 9). Dogs must be lower than the source to detect it. In
sunlight, a sloping surface warms and warms the adjacent air, which makes it
lighter and more buoyant than the air above. This causes it to flow upslope carrying
scent with it. Dogs must be above the source to detect it (Figure 9). The
upward flowing layer is thicker than the downward flowing layer which makes the
downward flowing layer (at night and in shade) easier for dogs to detect.
Sea and land breezes occur as a result of heating and cooling of
land surfaces adjacent to the water. A sea breeze allows a dog searching along
the shore to detect sources offshore and a land breeze allows it to detect
sources inland (Figure 11). Similar breezes are associated with other bodies
of water (ponds, lakes, rivers, swamps) but modified by their size.
Air flow at forest and field edges is strongly dependent on site
conditions. Handlers must assess local conditions to determine an effective
search strategy. Openings in forests where sunlight can reach the ground and
where the adjacent forest shades the ground tend to act as chimneys under
conditions of daytime heating and light winds (Figure 12). Cooler air from
the forest is drawn to the opening replacing the heated air that rises.
For terrain with slopes <17° and low wind speed, wind can be
expected to flow over the terrain with relatively little turbulence. In steeper
terrain, an eddy forms near the toe of the upwind slope and a much larger lee eddy
forms downwind of the crest (Figure 13). Surface winds in these eddies are
weak, variable, intermittent, turbulent, and opposite to the direction of the
prevailing wind. The effects of this lee eddy may extend downwind many times the
height of the ridge or hill. Canine teams can use these eddies to search parts
of the slopes.
On the upwind slope, dogs can detect a source somewhat upslope by
working in the upwind eddy along the toe of the slope. On the downwind slope,
dogs can detect a source downwind of the toe of the slope by working in the lee
eddy across the slope or even near the top of the slope.
An isolated tree trunk or one in an open forest traps scent in
eddies on the downwind side. Scent accumulates in the eddies and on the bark and
dogs typically alert there (Figure 14). A high-density shelterbelt is like a
ridge; in that similar eddies can be produced on the upwind and downwind sides.
Low density shelterbelts (Figure 15), elevated structures, and
tractor trailers that are open underneath may prevent scent from reaching the
ground for some distance downwind. Starting to search them from downwind
rather than close to them may be desirable.
Wind direction in a forest is highly variable, which indicates
that a dog that detects a scent plume there and moves upwind at that instant
will not likely be moving in the direction of the source (Table 3.4). This makes
it more difficult to find sources in forests than in fields and indicates that
closer grid spacing is required in forests compared to fields.
A jet of air near the ground level can penetrate a sparse forest
almost four times the forest height (Figure 16) so that it may be possible to
search the area adjacent to the forest edge from well into the forest.
Tom Osterkamp
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