When it comes to understanding this program and the
daily authorizations, one of the most important things to realize is
that the main intent of this program is the control of smoke.
On any given day, we are concerned with what is going to happen to the
smoke that results from crop residue burning. Daily
authorizations restrict or prohibit crop residue burning on days when
the smoke dispersion capabilities of the atmosphere are limited or
poor. Unfortunately, conditions that are the most favourable for
smoke dispersion are often the least favourable for burning crop
residue (for example, a storm event).
![](/web/20061121033554im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbd09s03j.jpg) Good smoke dispersion
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![](/web/20061121033554im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbd09s03i.jpg) Poor smoke dispersion
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How well smoke is dispersion on any given day is
dependant on three factors:
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Mechanical Mixing
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Convective Mixing
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The Volume of Clean Air Available to Mix the Smoke (‘Dilution is the
Solution’)
In the atmosphere, the wind acts like a giant
spoon, mixing the smoke with clean air. Wind can carry smoke 10’s to
100’s of meters up into the atmosphere. How well the wind mixes the
smoke with clean air is dependant on the velocity of the wind and the
underlying topography (you will have greater mixing in areas of uneven
topography).
![](/web/20061121033554im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbd09s03a.jpg)
Wind can help to mix smoke with clean air on days when we have good
dispersion or a large volume of clean air available with which to mix
the smoke. However, on days when dispersion is poor or the volume of
clean air is limited, wind will transport the smoke over great distances,
resulting in more adverse effects from the smoke.
Convective Mixing
Although wind is a factor in smoke dispersion, it
is not really a very effective mechanism by which to mix smoke with
clean air. Convective mixing is by far the most dominant mixing force
in the atmosphere.
To help explain convective mixing we can use the
analogy of heating a pot of water on the stove. Usually when you boil a
pot of water you simply put it on the stove and walk away, you don’t
have to stand there physically mixing the water. Obviously the water at
the bottom of the pot will heat up first as it is closest to the heat
source, the element. So how is it that the water at the top of the pot
is also heated up?
As the water heats up, it becomes less dense than
the water above it and rises to the top of the pot. The cooler, denser
water on top, sinks to the bottom. This results in what is called
convection currents which carry the hot water from the bottom of the pot
to the top and the cool water from the top to the bottom.
![](/web/20061121033554im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbd09s03b.gif)
The redistribution of heat within the pot of water
is called convective mixing. Even if we leave the pot alone, the
water mixes itself, because the temperature variations generate their
own turbulence.
A similar process occurs on a daily basis in the
atmosphere. The sun heats the ground and the ground in turn heats the
air directly above it. This results in the temperature of the Atmosphere
decreasing with height. As the air near the ground heats up, it
begins to rise. This air will continue to rise as long as it is warmer than the surrounding air. As this warmer air rises, cooler
air from above moves in (sinks) to take its place near the ground. The
greatest amount of convective mixing occurs during the warmest part of
the day. On any given day, convective mixing can carry smoke from 100’s
to 1000’s of meters into the atmosphere, which makes it a far more
effective mixing mechanism than wind.
![](/web/20061121033554im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbd09s03c.gif)
Smoke will continue to rise into the atmosphere as
long as it is warmer than the surrounding air.
Day Versus Night Atmospheric Conditions
The concept of convective mixing should give
you some indication of why burning is not permitted at night. Remember,
convective mixing is the dominant force in the atmosphere
mixing/diluting smoke with clean air. On any given day we want as much
convective mixing as possible. Convective mixing is dependant on the
SUN heating the ground which in turn heats the air directly above it.
At night, however, little to no convective mixing
can occur because there is no heat source. Furthermore, the earth tends
to cool more quickly than the atmosphere. Just as during the day the
ground heats the air directly above it, at night the ground will cool
the air directly above it. As a result, the temperature of the
atmosphere tends to increase with height.
![](/web/20061121033554im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbd09s03d.gif)
This results in what is called an inversion.
An inversion is the result of warmer air lying over top of cooler air.
Inversions can be expected to form 90% of the time when the sun goes
down.
How does an inversion inhibit smoke dispersion?
Smoke will only rise as long as it is warmer than
the surrounding air. At night a pocket of smoke will rise a bit, but as
the air around it becomes warmer, it will actually sink back towards the
ground and spread out horizontally.
![](/web/20061121033554im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbd09s03e.gif)
This is what is called a ‘Diurnal’ or daily cycle,
basically this process repeats every day. During the day, we often
experience what is termed ‘unstable’ atmospheric conditions and at night
we have much more ‘stable’ atmospheric conditions.
Why is the amount of convective mixing not
constant from one day to the next?
There is one other factor that will affect how well
smoke is mixed or diluted in the atmosphere on any given day.
‘Synoptic’ weather patterns affect how much clean air is available with
which to mix the smoke. These weather patterns change from day to day
and thus the volume of clean air available to mix with the smoke changes
from day to day.
The Volume of Clean Air Available – The Effect
of High and Low Pressure Systems
Convective mixing is capable of carrying smoke
100’s to 1000’s of meters into the atmosphere. On any given day, how
high smoke is actually carried into the atmosphere will be determined by
the type of pressure system, high or low, that we are experiencing.
Daily pressure systems dictate the volume of clean air that is
available on any given day with which to mix the smoke.
The Influence of a High Pressure System
With a high pressure system you have a lot of air
pressing down or the air is subsiding. High
pressure systems are often characterized by clear,
blue skies and light winds. They are
essentially very nice, calm days. Atmospheric
conditions are very stable, there is little to no
activity in the atmosphere. On days like this,
the volume of clean air with which to mix the smoke
is really quite small. Smoke may only rise a
couple of hundred metres before it
starts to spread out horizontally. When there is a lot of burning going
on, this small volume of clean air can quickly become inundated with
smoke and smoke dispersion will be minimal.
High pressure systems are usually indicative of a
layer of warmer air over a layer of cool air. That is, the temperature
of the atmosphere is increasing with height, which inhibits the
smoke from rising very high into the atmosphere.
![](/web/20061121033554im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbd09s03f.jpg)
This is one of the most frustrating aspects of this program for
producers. From a producer point of view this would be considered a
really good day for burning. There is little to no wind, so there is little chance of the fire getting
out of control. From a smoke dispersion perspective, however, this is a
very bad day for burning.
The clear skies and fair weather indicate that the atmosphere is very
stable, that air is subsiding. On days like this, due to the enormous
amount of pressure from the air pressing down, convective mixing is
seriously limited. That is, air closer to the ground is unable to rise
very high into the atmosphere.
The air closer to the ground is cooler than the
air aloft. This is what is called a capping
inversion. This change in temperature ultimately acts like a
lid/cap on the atmosphere, preventing smoke from rising very high into
the atmosphere. This inversion is so strong that it actually inhibits
convective mixing.
The Influence of a Low Pressure System
With a low pressure system, warm air is rising.
Low pressure systems are usually characterized by cloudy skies (clouds
tend to have a lot of vertical development), strong winds and rain. The
atmosphere tends to be very unstable and there is a large volume of
clean air available with which to mix the smoke. The temperature of the
atmosphere decreases with height, which means that the smoke may rise
1000’s of metres into the atmosphere. On days like this, there is a lot
of activity or mixing occurring in the atmosphere and smoke dispersion
conditions tend to be very good.
Low Pressure System:
characterized by cloudy skies, wind, rain
![](/web/20061121033554im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbd09s03g.jpg)
![](/web/20061121033554im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbd09s03h.jpg)
This is a good example of what would happen to the
smoke on a day with a low pressure system. As you can see there is smoke being generated from this one fire. However, because there
is a lot of convective mixing occurring, the smoke is being lifted high
up into the atmosphere. There is actually very little horizontal
movement at all. There is a large volume of clean air with which to dilute/mix the smoke. The temperature of the atmosphere is decreasing
with height, which means that the smoke will continue to be carried high
into the atmosphere (probably 1000’s of metres into the atmosphere).
There is a very light wind to aid mixing.
When does most of the burning take place?
When it comes to burning of crop residue, the days that
are the most suitable for the actual burning (clear, sunny, calm days)
tend to be the worst for smoke dispersion. The
best days for smoke dispersion are often those when weather
conditions (windy, rainy) are such that burning cannot occur in a
safe and controlled manner. As a result the majority of burning tends
to occur on days when we are experiencing moderate smoke dispersion
conditions. It is on days such as this that burning may be restricted
to a certain portion of the day (usually later in the afternoon) or
certain rural municipalities may be closed. We always have to be
careful when we authorize burning on days when the smoke dispersion
capabilities of the atmosphere may be limited. It is
during days such as these that the atmosphere can become inundated with
smoke if there is a substantial amount of burning taking place.
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