Category: Boom Sprayers

Main category for sprayers with horizontal booms

  • Nozzle Selection for Boom Sprayers

    Nozzle Selection for Boom Sprayers

    Picking the correct nozzle for a spray job can be a daunting task.  There is a lot of product selection, and a lot of different features.  We try to break the process down into four steps.

    1. Identify Your Needs

    Before making any assumptions about the right nozzle for you, review your needs and objectives. Are you trying to reduce drift? Do you want better coverage? Are you moving towards more fungicide application? Do you need a wide pressure range?

    It’s always a good idea to review your experience with your previous nozzle. What, if anything, would you like to change?

    2. Identify Flow Rates

    Most spray operations fall into one of three categories, (a) pre-seed burnoff (3 to 7 US gpa); (b) in-crop early post-emergence (7 to 10 US gpa); (c) late season application to mature canopies (10 – 20 US gpa).

    To find the right nozzle size, you need to know the application volume, the travel speed, and the nozzle spacing. Most sprayers have 20” nozzle spacing, but some have 15” spacing. Use these metric or US units charts to find the right flow rate for common nozzle spacings. Various on-line calculators from Hypro, Greenleaf / Agrotop, Lechler, or Wilger or their apps, are also helpful.

    If you use our chart, the top row lists water volumes. The columns contain travel speeds. Travel speed is somewhat flexible and can change throughout the field.

    Let’s assume the water volume is 7 gpa, and the desired application speed is 13 mph. Move down the “7 gpa” column, searching for 13 mph. You will encounter 13 mph about 5 times: 02 nozzle @ >90 psi, 025 nozzle @ 60 psi, 03 nozzle @ 40 psi, and 035 nozzle @ 30 psi (the 035 size is only offered by some manufacturers) and the 04 nozzle at about 25 psi.

    Nozzle chart, in US units, solving for 7 gpa at 13 mph. Five nozzles can produce the required flow, each at different pressures.

    Note that for the smaller nozzle sizes, the spray pressure is perhaps too high, and for the larger sizes, it is too low. Select a size that allows optimum nozzle performance and travel speed flexibility. In this example, the 025 size is optimal, producing an expected pressure of about 60 psi. The column for the 025 nozzle can now be used to predict the travel speed range from 30 psi to 90 psi, about 9 to 16 mph. For the 03 nozzle, the minimum speed would be 11 mph, too fast for some.

    For Pulse Width Modulation (PWM), slightly different rules apply. See here for instructions.

    3. Select the Nozzle Model

    For general spraying, we recommend intermediate spray qualities ranging from Medium to Very Coarse.

    These intermediate spray qualities offer good coverage at reasonable water volumes and good drift control. Their spray quality can be tailored with pressure adjustments to suit specific needs. For images, see here. In alphabetical order:

    Air Induced:

    There is plenty of selection in this popular category, all manufacturers offering similar specs and performance.

    Pulse Width Modulation:

    PWM nozzle selection is improving, but some gaps in availability remain.

    All nozzles should be operated near the middle of their pressure range, for air-induction this is 50 to 60 psi or higher, a bit less for non air-induced types. This allows maximum flexibility when travel speeds change or when spray quality is adjusted with pressure.

    For fusarium headblight, consider a twin fan nozzle.

    Keep your booms no more than 15” to 25” above the heads for best results.

    Air Induced:

    There is an excellent selection of twin fans from most manufacturers.

    Pulse Width Modulation:

    Relatively poor selection, limited flow rate ranges or spray qualities available for some models.

    For finer sprays (lower water volumes), simply increase spray pressure or consider a non-air-induced design.

    There has always been a large selection of finer sprays on the market, remnants from a time when drift was less important. Very few offer flow rates above 06 or 08, decreasing utility for PWM systems.

    Notice that conventional flat fan tips and most pre-orifice tips are absent from these lists. These nozzles are not recommended for herbicides because they produce sprays that are too fine for acceptable environmental protection (ASABE Fine and Medium). The added coverage afforded by such sprays only has value with low water volumes, and in those instances is more than offset by their higher drift and evaporation. An exception is the use of insecticides with contact mode of action targetting small insects such as flea beetles or aphids. In thes cases, finer sprays (ASABE Fine or Medium) may be required to provide effective tragetting.

    Very high flows are sometimes needed (11010 and above, usually for PWM). When this occurs, conventional flat fans have merit because the higher flow rates of any nozzle usually create coarser sprays, and even conventional tips will create sufficient coarseness to prevent drift.

    For the best drift protection, consider these tips.

    The advent of the dicamba-resistant trait in soybeans has spawned interest in very low drift tips that comply with the label requirements for these products. Although superior for drift control, they are not well suited for low volume or low-pressure spraying, nor for contact herbicides or grassy weeds, as spray retention and coverage may be poor. But they are very valuable when drift control is paramount and when higher volumes can be used to maintain adequate coverage.

    The following advice is based on the rules at the time it was written. These may be suitable for 2,4-D application in Australia under the newest APVMA guidelines (check spray quality to be sure it is VC or coarser). Many are also suited for Dicamba in Canada (must be XC or coarser), or dicamba in the US (must be on approved lists such as this one for Xtendimax or this one for Engenia, but caution is advised, some low pressure limits make them impractical. Always check that spray quality can be achieved at pressures that offer travel speed flexibility.

    Air Induced:

    Excellent selection. This market has received much attention in recent years.

    Pulse Width Modulation

    Before making a selection, check the nozzle’s recommended pressure range and the spray qualities within that range from the manufacturer info. The target pressure for these tips may differ from your expectations.

    4. Tweak and Confirm

    Under field conditions, the spray pressures which produce the desired water volumes can vary from the charts. Make sure you trust your pressure gauge reading and know the pressure drop from the gauge signal to the nozzles, particularly with PWM, where the solenoid adds additional drop. Add the pressure drop to your target pressure reading. If using a rate controller, use the pressure gauge as your speedometer to ensure optimal nozzle performance. Adjust travel speed until the nozzle pressure meets with your spray quality and pattern goals. If that speed is too slow or fast…you have the wrong size nozzle and/or water volume.

    Spray pressure is more important than travel speed – make your pressure gauge your speedometer.

  • Pesticide Safety for Student Workers

    Pesticide Safety for Student Workers

    This article is based on a presentation by Dr. Melanie Filotas, who delivered it as part of the 2019 agriculture summer student orientation day.

    Most crops are sprayed with organic or synthetic pesticides at some point during the growing season. Use caution before entering any area where crops are grown (e.g. corn field, nursery, greenhouse, orchard etc.). Always confirm that it is safe to enter.

    Most crops receive some form of chemical input during growth. Be aware of what has been applied.
    Even organic operations apply controlled products that may make it unsafe to enter for a period of time.

    You can be exposed to pesticides if you enter a treated area before pesticide residues break down and vapours dissipate. The minimal time that must elapse before being permitted to enter is called the Restricted Entry or Re-entry Interval (REI).

    REIs are data-driven and established by the federal government. They are defined as: “The period of time that agricultural workers, or anyone else, must not do hand labour in treated areas after a pesticide has been applied.” Hand labour can be any task involving substantial contact with treated plants, plant parts or soil, including planting, harvesting, pruning, and scouting.

    Things you should know about REIs:

    • REIs can range from one hour to several days
    • If a pesticide label does not indicate a REI, the default is 12 hours
    • REIs can vary with the product, crop and type of activity (e.g., scouting, harvesting, etc.)
    • REIs can change over time so always refer to the most recent label
    • If a tank mix (multiple products) was applied, observe the most restrictive REI

    Before visiting an operation to work in the field:

    • Tell your supervisor where you will be that day
    • Ask the grower or spray applicator what was sprayed. Records may be posted, but verbal confirmation is preferred
    • Look up the REI for the product on the crop you will be entering
    • Check with your supervisor on any products with special instructions beyond the REI

    Do not enter the field until the REI has ended. Pesticide REIs can be found in local production guides, or on pesticide labels.

    Local production guides summarize REIs.
    Local production guides list REIs by crop, by product applied, and by activity.

    If local production guides are not available, registered pesticide labels can be found using Health Canada’s Pesticide Label Search service online. In the United States, most labels can be found on the EPA’s Pesticide Product and Label System website.

    Health Canada’s online pesticide label search.

    Miscommunication can sometimes happen. Learn to recognize the signs of spraying. When in doubt, leave the planted area and call the grower to confirm or call your supervisor.

    • In some cases you can look for fresh tracks in the operation, but be aware they may not have been made by a sprayer
    • Some products have a distinctive odour
    • It can be difficult to see a sprayer operating, particularly in orchards, but they can be heard. Do not wear earbuds or headsets while in a production area
    • Look for foliar residue. This is an indicator, but does not always mean it is unsafe to enter
    Fresh wheel tracks may indicate recent spraying.
    Some products have a distinctive odour.
    It may be difficult to see a sprayer operating in the vicinity, such as in this orchard. However, they can often be heard. Do not wear a headset or earbuds in a production area.
    Residue on leaves may indicate a recent application, as in the left photo. However, it could also be unrelated. On the right is calcium magnesium precipitation from irrigation water. (Photo credit [right]: Jennifer Llewellyn)

    There are many potential symptoms of pesticide exposure: headache, fatigue, irritation of the skin, eyes, nose or throat, loss of appetite, dizziness, nausea or vomiting, diarrhea, decreased muscle coordination, and blurred vision. Each product has a Material Safety Data Sheet (MSDS) that will provide details on exposure symptoms and treatments.

    While sometimes confused with symptoms arising from sun stroke or dehydration, if you suspect pesticide exposure it is always best to be prudent and get medical help immediately. Contact your local poison centre or 911.

    Summer work in crop production can be rewarding and enjoyable, but always use caution and be safe.

  • Tipping Sprayers and Spills

    Tipping Sprayers and Spills

    This short article is a reminder for sprayer operators to respect the possibility of tipping a sprayer. Every spring I catch wind of someone tipping over. When I can ask the operator questions I start with “Is everyone alright?” and “Was the sprayer full?“. Hopefully the answers are “Yes” and No“, but not always.

    The following factors are always involved:

    • Driving too fast. Usually entering a field at road speed.
    • Entering the field on a downhill slope and/or catching a pothole or soft shoulder.
    • Turning in a tight radius, usually 180 degrees. This is made worse when the sprayer is towed.
    • Sprayer is not completely full and “slosh” changes the centre of gravity.
    • Narrow tires and a narrow base.
    Fortunately the sprayer wasn’t damaged and the spill was minor.
    A tight turn at high speed coupled with a depression in the entryway and tank slosh was enough to tip the unit. They had it righted and hauled out soon after. No one was hurt.

    I’ve heard as many cases involving seasoned operators as new operators. The next few pictures are of a veteran operator’s sprayer carrying 28%/ATS. Just like the images above, a tight turn at high speed sloshed the load just as a deep pot hole caught the outside front wheel. This sent the sprayer into a lane of traffic before it tipped back and over into the field. No one was hurt.

    Fortunately for the operator, the spill was contained in their field (not the road or ditches). The 90′ boom had to be cut off before the sprayer could be towed back to the yard to be sold off as parts. While the operator has looked at the bright side (an opportunity to upgrade) it has left them relying on a custom operator for spring spraying and making a hasty in-season equipment purchase.

    Lost a tire during the tow back to the yard.
    Crumpled boom after having to be cut from the sprayer.
    Not the way anyone wants to see their sprayer.

    Major Spill

    What follows are generic steps for what to do if there is a major spill. Always defer to the process outlined by your regional authority.

    1. If you do tip the sprayer, first protect yourself, then others, then animals in that order.
    2. Stop any exposure by removing clothing and washing as best you can.
    3. Stop people from entering the area.
    4. If it is safe to do so, try to prevent the spill from spreading.
    5. Contact your local spill centre. In Ontario, the Spills Action Centre will receive calls 24 hours a day at 1-800-268-6060. Consult with your municipality for their spill reporting contact numbers.

    Take home

    Of course we’d rather avoid this problem altogether. Be sure to slow down before turning into a field. Take the turn as gradually as possible. Remember that soft spring ground and new pot holes can become serious obstacles – consider scouting the entry before the first spray or at minimum getting out of the cab and checking before entering.

  • Reducing Selection Pressure for Herbicide Resistance

    Reducing Selection Pressure for Herbicide Resistance

    Herbicide resistance has been called the number one threat to conventional herbicide-based weed management strategies.

    Since the 1970s, the number of cases of herbicide resistant weeds has shown a linear increase both globally (currently at about 500 documented unique weed species x mode of action cases) and within Canada (at about 70 such cases), according to the herbicide resistance website WeedScience.org. The rate of increase has been constant, and there is not yet any reason to believe that growth in the number of cases will slow.

    Figure 1: Growth of global herbicide resistance cases (Source: WeedScience.org)

    By using herbicides, we select for weed biotypes that, for some reason, can tolerate the product. Mutations which confer herbicide resistance are rare, but present at very low levels in most weed populations. Repeated use of the same mode of action will increase the relative frequency of the resistant biotype until it becomes noticeable, and shortly thereafter, problematic.

    The best-known forms of resistance involve single-gene mutations that alter herbicide target sites (target sites might be enzymes that produce essential plant cell building blocks) so that herbicide binding is reduced, resulting in reduced control. As a result, the target pathway keeps working, and the plant grows normally after herbicide application. Other forms of resistance involve the overproduction of the target enzyme by the plant, mechanisms that either metabolize or sequester the herbicides, or changes in uptake of the herbicide. The main mechanisms are summarized in this table:

    Table 1: Mechanisms of herbicide resistance*

    Resistant ClassMechanism
    Target SiteTarget site mutation
    Increased gene copy number
    Enzyme over-expression
    Non Target-SiteEnhanced metabolism
    Differential uptake
    Differential redistribution
    Sequestration
    Delayed germination
    Rapid necrosis / defoliation

    *Source: Bo AB, Won OJ, Sin HT, Lee JJ, Park KW. 2017. Mechanisms of herbicide resistance in weeds. Korean Journal of Agricultural Science 44:001-015.

    The simple act of using a herbicide can select for resistance to that herbicide. While we can’t predict or prevent resistance entirely, we can slow its onset by reducing the frequency of herbicide use, for example by integrating cultural controls such as crop rotation, seeding rate, cultivar competitiveness, and other factors into our agricultural systems.

    A powerful option to slow resistance development is to reduce our reliance on a single mode of action, either by rotating modes of action in successive sprays, or, more importantly, by tank mixing multiple effective modes of action (MEMoA) whenever we make an application.

    Let’s not kid ourselves. The recent discovery of glyphosate (e.g. Roundup) -resistant wild oats in Australia, and glufosinate (e.g. Liberty) -resistant ryegrasses in several countries is sobering.  Relying more on these herbicides will only increase selection pressure.

    If we decide to use herbicides, we need to look at the situation from the perspective of delaying the onset of resistance. What we’re trying to do is buy some time, so that new strategies can be developed.

    How can spray application methods slow the onset of resistance?

    The use of herbicides will continue to select for resistance. The best we can hope to achieve within a herbicide system is to delay that eventuality.

    To better understand our options, we need to talk about a specific type of herbicide resistance called polygenic resistance. This refers to accumulation of additive genes of small effect over time, a process that is more efficient in plants that share genetic material among plants in a population, i.e., they outcross.

    Outcrossing plants receive genetic material from others, increasing their genetic diversity, and therefore their ability to adapt.

    In a field, a population of any specific weed may contain some individuals that have slightly greater tolerance to a herbicide than others. If we apply a slightly lower than label herbicide dose to those individuals, they might survive the application and eventually cross with other survivors and set seed. Their offspring may be as tolerant or even more tolerant than their parents. If this repeats itself over successive generations, the additive effects build until finally, low-level resistance becomes full-blown resistance and even label rate herbicides no longer work. This resistance isn’t a single gene mutation, it’s simply an accumulation of tolerance due to several genes which impact how much of the herbicide active ingredient reaches the target site.

    In a recent study at the University of Arkansas, susceptible Palmer amaranth (P. amaranth has both male and female plants and is therefore an obligate outcrosser) was treated with a range of dicamba doses to identify individuals that survived the higher doses. The researchers allowed the survivors to cross, and then grew out their seed, then repeating the procedure. After just three generations, the experiment produced individuals with a three-fold increase in LD50 (compare LD50 at P0 (111) to P3 (309) in Table 2). Recall that LD50 refers to the dose required to observe 50% of the full effect.

    Table 2: Dicamba doses (g ae/ha) required for 50% (LD50) and 90% (LD90) control of Palmer amaranth populations selected following sublethal doses of dicamba in the greenhouse.*

    HerbicideSelected PopulationLD50LD90
    DicambaP0111213
    P1198482
    P2221546
    P3309838

    *Source: Tehranchian, P. et al.  2017.  Recurrent sublethal-dose selection for reduced susceptibility of Palmer amaranth (Amaranthus palmeri) to dicamba. Weed Science 2017 65:206–212.

    The lessons are three-fold:

    • Herbicide resistance cannot be prevented if herbicides are applied.
    • To prevent polygenic resistance, we need to apply the full label rate and avoid repeated sublethal doses, so that all weeds are killed;
    •  We need to apply Multiple Effective Modes of Action (MEMoA) whenever possible so that when one fails, the others have its back;

    How can this be achieved?

    1. Prevent application practices that result in less effective dosing. Larger weeds, or weeds growing in difficult environmental conditions, may require higher herbicide doses. Early application is helpful because small weeds are easier to control. In addition, crop canopy shading at later staging leads to dose reduction and increases dose variability. Spraying under windy conditions also reduces dose, and can increase deposit variability. For some herbicides such as glyphosate or diquat, the dust generated by wind or fast travel speeds can reduce effectiveness.
    Figure 2: Smaller, exposed weeds require lower doses to control
    Figure 3: Crop canopies provide valuable competition to help suppress weeds, but they can also intercept spray, reducing the dose received by weeds.
    • Get Pulse Width Modulation (PWM) with turn compensation. If your sprayer makes the same turn around the same feature year after year, then the outer boom region will under-dose the same part of the field over and over. This is the breeding ground for polygenic resistance. Look for this in field corners, around water bodies or tree bluffs, rock piles, etc.
    Figure 4: PWM correction of under-dosing during a turn
    • Prevent boom sway and yaw. Boom movements result in uneven application, which results in lower control. Pull-type sprayers with supporting wheels are best, but these are becoming rare. Suspended boom performance depends on the manufacturer and the levelling technology they use.  However, boom movement is usually more consistent with slower travel speeds.
    Figure 5: Boom yaw causing over- and under-application (Source: Farmonline.com.au)
    • Minimize air turbulence. Large sprayers, and those moving at fast speeds, create aerodynamic turbulence that can displace spray. The main problem spots are wheels, in whose tracks measurably less spray is deposited.  The exact dynamics of turbulence is still unknown, but we do know that its magnitude can be reduced with slower travel speeds.
    Figure 6: Turbulence due to sprayer speed (Source: Dr. Hubert Landry, PAMI)
    • Consider spot spraying. The use of optical spot spray equipment, such as the new WEEDit Quadro, or Trimble’s WeedSeeker II, save product during burnoff or post-harvest. These savings can make the use of more elaborate, expensive tank mixes containing multiple effective modes of action, affordable.
    Figure 7: Optical Spot Spraying (WEEDit Quadro) (Source: WEED-it.com)
    • Avoid spray drift. Field margins that harbour weeds rarely receive a full dose of herbicide. Exposing these weeds to spray drift won’t kill them. But it will, over time, select for weeds that are more able to tolerate the herbicide.

    Implications

    Aside from specific technology such as PWM, improved booms, or a spot sprayer, the most effective fix for variable application doses is slower travel speed.

    While this may seem problematic when timing is critical and greater productivity is required, there is a way to drive more slowly and still get more done. We simply need to look at productivity differently.

    We tend to equate productivity with speed. Travel speed. But a spray day is filled with many hours of non-spray time – filling, cleaning, transporting, repairing, fueling, record-keeping, etc. How much time is lost to these activities depends on the operation, but for everyone, it’s useful to do time accounting.

    Record how a spray day’s time is spent. Pay attention to activities during which you can save time without much expense.

    ActionActual TimeTarget Time
    Fuelling, lubing  
    Loading jugs and totes  
    Checking label (rates, rainfastness…)  
    Filling tender tanks  
    Loading sprayer (in yard)  
    Transport to field  
    Entering field data into monitor  
    Checking, recording weather  
    Checking for pest, stage  
    Changing nozzles  
    Spraying load  
    Unplugging / replacing nozzles  
    Replacing nozzle body  
    Making turn  
    Filling sprayer  
    Getting sprayer unstuck  
    Driving to tender truck  
    Waiting for tender truck  
    Spraying out tank remainder  
    Cleaning tank  
    Cleaning filters  
    Flushing boom ends  
    Loading sprayer (in field)  
    TOTAL

    On any given spray day, less time spent filling, or transporting, is credited to spray time.  Our analysis shows that time lost to driving slower can more than be made up with these changes. The productivity gain gives more opportunity to spray under more ideal conditions that save yield and also ensure more uniform application.

    Using productivity analysis, spraying can become more uniform and help delay the onset of resistance.

    Note: The assistance of Dr. Charles Geddes, Research Scientist at AAFC Lethbridge, in drafting this article is appreciated.

  • Thermal Inversions for Sprayer Operators

    Thermal Inversions for Sprayer Operators

    In April 2014, NDSU extension published an excellent factsheet explaining what thermal inversions are, how to detect them and how they affect pesticide spray drift. That factsheet inspired this article.

    The Atmosphere

    The Earth is surrounded by a layer of air called the atmosphere. Think of it as a sheet of liquid percolating and flowing over the Earth’s surface. Seems a bit precarious, doesn’t it?

    We define “layers” of atmosphere based on their distance from the Earth’s surface (see image below). We’ll focus on the lowest part of the Earth’s atmosphere: the Surface Boundary Layer. As it drags along the Earth’s surface it experiences rapid changes in wind speed, temperature and humidity (on a time scale of an hour or less).

    The Earth’s Atmosphere. The illustration of the Earth is to scale, but the landscape is not. Our focus in on the Surface Boundary Layer.
    The Earth’s Atmosphere. The illustration of the Earth is to scale, but obviously the landscape is not. Our focus in on the Surface Boundary Layer.

    Atmospheric temperature

    In relatively calm, clear and dry conditions (e.g. a nice afternoon), air cools with elevation at a rate of about 1°C per 100m. This change is called the Adiabatic Lapse Rate and it’s caused by pressure changing with elevation. If your ears have popped when driving down a steep hill, you’ve experienced pressure change with elevation; there is more atmosphere overhead and the weight pushes down.

    With higher elevation, there is less atmosphere overhead. Less weight means less pressure and this gives air room to expand. Expansion takes work and work costs energy, which creates a cooling effect. See how simple thermodynamics are?

    In the graph below, the red line shows the Adiabatic Lapse Rate of air cooling with elevation. The blue line indicates wind stirring and homogenizing the atmosphere, reducing the degree of temperature change with elevation (more on that later).

    Day and night

    When we add the effect of daytime solar heating and nighttime cooling, the rate of temperature change is affected. Let’s consider how this works on a clear, relatively calm day:

    Early morning

    The morning sun emits short wave radiation, which is absorbed by the Earth’s surface. The surface conducts some of this energy deeper into the ground and also heats the air near the surface. This creates a temperature gradient wherein the surface is warmest and the air gets relatively cooler with elevation (remember the red line in the graph above).

    As the air near the surface warms, that energy causes air molecules to vibrate and push away from one another. Parcels of air become less dense and rise just like the gloop in a lava lamp. The cooler air around it falls to fill in the space left behind, and air begins to circulate in a Convection Cell. The rising parcel of air will eventually cool and shrink as it rises through the relatively cooler air above it.

    These convection cells create Thermal Turbulence, which is a very effective way for airborne particles, such as pesticide vapour, to be rapidly diluted. This is also how the atmosphere disperses pollution. More on the process of dispersion, later.

    Mid to late afternoon

    As the sun passes over and the wind starts to rise, the convection cells get disrupted by the wind and experience mechanical turbulence (remember the blue line in the graph above). So, mechanical turbulence also mixes warmer air near the ground with cooler air above it, but suppresses thermal turbulence.

    Mid-afternoon to night

    As the energy from the sun lessens, the soil begins to cool and so does the air next to it. Once the air cools enough to be colder than the air above it, we have the beginning of a Radiation Inversion, which is a specific kind of Thermal Inversion (see the green line in the graph below). It is called that because we now have the reverse of the typical day-time temperature profile. The height of the inversion (the ceiling) grows with time, and can reach a maximum of about 100m by sunrise. Within the inversion layer (before the green line bends back at 100m), turbulence is suppressed. We have a stable air mass. More on that below.

    How inversions affect dispersion

    The rising portion of a convection cell carries whatever particles are in the air with it. Suspended particles become much less concentrated at ground level thanks to the thermal turbulence.

    Thermal Turbulence allows particle-laden warm air to rise and clean cool air to fall. This disperses air-borne particles like dust or pollution.

    Now let’s imagine we are in a thermal inversion. The cooler, particle laden air near the ground cannot rise and the cleaner air above, which is now relatively warmer, cannot sink. Thermal turbulence is suppressed, and so is any vertical dispersion.

    Thermal Turbulence is suppressed during a Temperature Inversion. Particle-laden cool air at the surface cannot rise, and warm, clean air cannot fall. No dispersion occurs, and the concentrated, particle-laden air tends to move downhill or laterally with light winds.

    When spraying, the smallest spray droplets fall slowest, staying airborne for long periods of time. If spraying occurs during an inversion, those particles accumulate beneath the inversion layer. Remember we said our atmosphere behaves like a liquid? The colder, denser (pesticide-laden) air drains downhill into low-lying areas. It can also move laterally over great distances, in unpredictable directions, when light winds begin.

    Clouds

    If the morning were overcast instead of clear, the clouds would intercept much of the sun’s short-wave radiation, absorbing or reflecting it back into space. The Earth’s surface would still warm, but more slowly, suppressing thermal turbulence. As an aside, if clouds form in the evening, they reflect long-wave radiation from the Earth’s surface back down. This Greenhouse Effect is why overcast nights are warmer than clear ones.

    Therefore, extended periods of mostly clear skies in the evening or night means a high probability of strong temperature inversions. Conversely, cloud cover usually means a near-neutral atmosphere, so no strong inversion.

    Wind

    Inversions are only mildly affected by light wind (e.g. 6 to 8 km/h), but as the wind increases and mechanical turbulence mixes the air, the strength of the inversion will be reduced and the atmosphere will approach a neutral condition (see the blue line). In this condition, airborne particles are not dispersed by thermal turbulence, but some mixing will occur. So, there may not be a thermal inversion, but spraying would still be inadvisable if the wind got too high.

    Humidity

    Inversions form more rapidly when there is less water vapour in the air to absorb radiation. Once humid air has cooled to the dew point, water condensation gives off energy and warms the air a little. This slows the formation of the inversion. Be aware that inversion conditions can exist long before fog, dew or frost forms, so they are not a good indicator for the beginning of an inversion – you’re already in one!

    If you see fog, dew or frost, you’re already in an inversion. The air has become cold enough to condense or even freeze water.
    If you see fog, dew or frost, you’re already in an inversion. The air has become cold enough to condense or even freeze water.

    Soil conditions and topography

    This is a complex issue, but soil conditions that make inversions more intense include low soil moisture, freshly tilled soils, coarse soils, heavy residue and closed crop canopies. Topography matters, too. We’re discussing radiation inversions in arable regions, and the kind that form on mountains or deep valleys. Nevertheless, inversions in shaded areas (e.g., behind windbreaks) start sooner, and last longer. See the NDSU factsheet for more detail.

    Spray timing

    Inversions, once formed, persist until the sun rises and warms the Earth’s surface, or until winds increase and mix the stationary layers of air together, re-establishing a more neutral temperature profile.

    Sunset is not a good indicator of the beginning of an inversion – it can start a few hours before. Therefore, evening spraying may be just as risky as night spraying. Very early mornings (e.g. around sunrise) are not much better. Remember, at sunrise, the inversion will be at its maximum height.

    The rising sun will warm the earth and create turbulent conditions, starting near its surface (e.g. a few metres). Most inversions will have dissipated two hours after sunrise, which may be the best choice for spraying.

    Detecting an inversion

    The only sure way to know if you are in an inversion is to take two air temperature readings: one near the ground and one about three metres higher. If the surface air temperature is cooler, you are in an inversion. The magnitude of the difference indicates how strong the inversion is.

    Accurate measurements are difficult to manage with conventional thermometers, but SpotOn now makes a hand-held detection unit. If you have one, be sure to let it acclimate before you use it. Leaving it in a hot, or cold, truck or sprayer cab prior to use means it may give a false reading.

    Inversion forecasting is getting better, but it’s still location-specific and not entirely reliable. Sprayer operators should learn to watch for the following environmental cues:

    • Large temperature swings between daytime and the previous night.
    • Calm (e.g. less than 3 km/h wind) and clear conditions when the sun is low.
    • Intense high pressure systems (usually associated with clear skies) and low humidity where you intend to spray.
    • Dew or frost indicating cooler air near the ground (fog may be too late).
    • Smoke or dust hanging in the air or moving laterally.
    • Odours travelling large distances and seeming more intense.
    • Daytime cumulus clouds collapse toward the evening.
    • Overnight cloud cover is 25% or less.

    Note: If you suspect a temperature inversion, don’t spray.

    For more information on how weather affects drift, download this pamphlet from the Australian Government Bureau of Meteorology.