This shows glyphosate drift onto a grain field. A major part of the field was affected as
shown in the green outlined section. This problem was caused by an air temperature
inversion.
Many incidents of spray drift are available that was caused by an air temperature
inversion. Currently, Arkansas state law requires that an application cannot be made
until the field temperature increases 3 degrees F. from the low field temperature. This
increase in air temperature causes air to rise which causes air inversions to dissipate.
Some basic information is needed to understand how an air temperature inversion
occurs. First thing to understand is the heating from the sun is due to intense short wave
radiation. The sun warms the earth’s surface and objects on the earth. As this happens,
the air near the surface warms and begins to rise.
At night or late in the afternoon, the warm earth’s surface begins to cool as the sun goes
down. The cooling is due to long wave radiation carrying the heat from the surface out
into space. On clear cloud free nights temperature inversions are usually the strongest
or what is called a very stable condition.
When heating from the sun ends late in the afternoon, all objects and the earth’s
surface radiate heat in all directions.
As cooling of the atmosphere occurs late in the afternoon and at night, the coolest air is
located near the earth’s surface and warm air above. This is an air temperature
inversion.
On a clear and calm day, an air temperature inversion will begin before sunset, increase
overnight (become stronger) and continue until after sunrise when the sun begins
heating the earth’s surface which warms the air near the earth’s surface. With the
warming of the air, the air begins to rise dissipating the temperature inversion.
Research at NDSU (North Dakota State University) has shown with low wind and bright
sun, the temperature at 4 inches above a grassed surface is 105 degrees F. and at 5 feet
above the surface, the temperature is 90 degrees F. This indicates an unstable condition
as warm air near the surface is rising. Any small spray drops will evaporate or rise up into
the atmosphere and will dissipate and usually not cause a problem due to spray drift in
the mentioned wind conditions. Caution still must be observed if a susceptible crop is
downwind.
But, if the air temperature early in the morning is significantly cooler at the earth’s
surface than the air above, a strong inversion is occurring. As shown in this example, a 6
degree F. temperature difference is shown in a 5 foot height.
This shows a study starting about 6 or 7 pm and continuing until about noon the next
day. As seen, The lowest blue line (3 in. level) late in the evening is about 40 deg. F. (5
deg. C. is similar to 40 deg. F. and 10 deg. C. is close to 50 deg. F.). The red line is near 45
deg. F. and the top blue line is about 5 deg. warmer than the red line. This indicates
about a 10 deg. F. difference in 20 ft. above the earth’s surface. This would indicate a
strong inversion overnight. As shown, after sunrise as the sun warms the surface, the
lowest temperature (3 in. level) sensor reading indicates a considerable rise in
temperature above the other sensors. This is due to the warming of the earth’s surface
which warms the air at the earth’s surface which rises and dissipates the temperature
inversion.
Some indicators of an inversion is moisture or frost forming on plant leaves if sufficient
moisture is present.
If enough moisture is in the air, fog will form.
If a layer of fog occurs, the thick fog is at the dew point temperature. The clearer air
above is slightly warmer or contains less fog. This would indicate a gradual warming with
elevation.
This indicates a temperature inversion has cooler air near the earth’s surface and
gradually increases in temperature with elevation. An inversion does not contain a mass
of cold air near the surface and a mass of warm air above. It is a gradual change in
temperature with elevation as shown. The layer of fog is due to the air temperature and
dew point temperature being equal.
Previous information has indicated that a temperature inversion can occur at winds of
less than 6 mph. If higher winds occur, mixing of the air will usually destroy the inversion
or reduce the intensity of the inversion.
Clouds (water vapor in the clouds) absorb heat or long wave radiation and radiates the
heat back to the earth. Partial cloud cover will absorb some heat, but some will move
off into space causing a weak inversion to occur.
This indicates a weak inversion occurring on a windy or cloudy day. This weak inversion
will usually occur during the same time period as a strong inversion.
On a hot and windy day, the surface temperature may be slightly cooler at the 5 ft. level.
Wind tends to mix the air but the air near the earth’s surface will usually be slightly
warmer than the air above due to the sun warming the earth’s surface. This would
indicate an unstable condition.
During a cloudy and windy condition, early morning temperature profiles indicate a fairly
uniform temperature gradient.
Cool air flows from higher elevations to lower elevations causing fog and is similar to
flowing water. Fog in low areas is an excelent indicator of an inversion.
This indicates cold moist air flowing into a low area and the moisture in the air
condenses, causing fog.
Often high pressure areas are associated with cool/dry, clear skies, low wind and is often
a good indicator for a temperature inversion.
Humidity affects the formation and dissipation of temperature inversions. This is due to
the heat capacity of the air due to the moisture present in wet climates. Moisture in the
air will absorb long wave radiation very well. Dry climates will cause inversions to form
faster and dissipate faster as water vapor is lower. Dry air will allow terrestrial radiation
to escape out into space faster than in moist conditions. Desert areas usually will be
extremely warm during the day and be very cold at night if cloud free.
Bare soil or almost bare soil will have a warmer surface as compared to mulched
covered soil or a crop canopy which will have a cooler surface. The later is due to the
lower heat capacity of the mulch or closed canopy. This causes temperature inversions
to form quicker and stronger.
Open rows allow warming of the air from the soil and will be slightly cooler than bare
ground. A closed crop canopy will have a much cooler temperature above the crop due
to the lower heat capacity of the crop as compared to the almost bare soil.
Trees interfere with wind which will break up temperature inversions allowing them to
form sooner in the afternoon allowing cool air to be trapped near the shelterbelt. Tree
shadows also cause inversions to form earlier in the afternoon and will prevent
dissipation later in the morning.
Fog forming in low lying areas is an excellent indicator that a temperature inversion is
present. Spraying should not be done. Most chemical labels state that an application
should not be done when an air temperature inversion is present.
Another good indicator that an inversion is present is smoke hanging low to the ground.
Cool air is near the surface and at higher elevations, air is warmer. This reduces small
spray drops from rising (less than 150 to 200 microns) and evaporating.
An air temperature inversion will usually form slightly before sunset, last all night unless
wind dissipates the inversion, become strongest slightly before sunrise and as the sun
begins to warm the earth’s surface, the inversion will usually become less. Heat from the
sun and winds above 5 to 6 mph will help dissipate the inversion.
Some exceptions exist to the previous mentioned things, such as large stagnant air
conditions.
If a spray application is completed during a temperature inversion, large spray drops will
deposit on the target, but drops smaller than 150 to 200 microns will take more time to
fall and deposit on the target. If a gentile wind is present, the small drops may move out
of field and cause damage to a susceptible crop as they will continue to fall or move
with the wind. Studies have shown that temperature inversions can occur in winds up to
about 5 to 6 mph.
As can be seen, fine drops are being produced from both nozzles. This is especially bad
in an inversion where the fine drops may stay suspended in the cool air near the soil or
crop surface.
The cool air near the soil surface and warmer air above, will keep the fine drops from
rising and dispersing.
Some visible affects of an inversion is dust particles hanging in the air. Many spray drops
are often smaller than dust.
This shows spray drops hanging in the air after a spray plane made a pass across a field.
A filter was applied to the previous picture to enhance the color of the spray drops that
are hanging in the air.
If a gentle breeze blows the fine spray drops from the field, trees or any susceptible crop
can be affected by the pesticide.
Most pesticide labels state that pesticide application should not be done during an
inversion. If weather conditions are changing to disseminate the inversion, use the
correct nozzle that will provide the proper drop size based on the what is stated on the
chemical label. The drop size produced by the nozzle is provided by the nozzle
manufacturer which is listed in company product information.
Applicators need to be aware of the of vaporization of the pesticide. The pesticide may
hit the target, but later vaporize and move off target.
A light wind will move vaporized chemical off target.
Applicators must also be aware of the solvents that can vaporize and move downwind.
Some clues to a temperature inversion are:
You can smell smoke from a distant fire or odor from some other source.
Dust from vehicles hanging in the air indicates an inversion is present.
You can hear sounds from distant sources that you do not normally hear.
An inversion often exists during a cloud free night or with only light clouds and low
winds.
Inversions often occur late in the afternoon, intensify overnight, be strongest slightly
before sunrise and usually dissipate a few hours after sunrise when the earth’s surface
warms.
When applicators have been shut down due to high winds or other poor spraying
conditions for a number of days, they may make an application and lose track of
changing weather conditions and spray during a developing temperature inversion. Then
a drift problem could result.
Spraying during the late afternoon with an inversion present could present a serious
problem. Studies have shown that fine drops, can move several miles downwind. The
best way to determine if an inversion is present is to measure the air temperature at 2
to 3 in. above ground level and at the 10 to 15 ft. above ground. Use a thermometer
while shielding the sensor with something other than your hand. The heat from your
hand can affect the reading. If the temperature at the 10 to 15 ft. height is higher than
the lower reading, an inversion is present. Spraying should not be done. As mentioned
previously, chemical labels usually state that an application should not be made during
an inversion.
This chart (an example) shows the weather conditions that show when there is a good
chance for a temperature inversion to occur. The information in the box shows the air
temperature in red from 7 pm until 7 am the next morning. At night, radiation from the
earth’s surface is moving out into space due to little or no cloud cover as shown in blue.
The temperatures in green are the dew point temperatures which is close to the air
temperature. When they are equal, fog starts to form. With high humidity, (green line)
moisture will often form on plant material which is another indicator of a temperature
inversion.
The previous chart is available for anywhere in the country from the “weather.gov”
website. All is needed is the city or town and state as indicated.
Page down and click on “Tabular Forecast”. Forecast information on an hourly basis is
available for the next 48 hours. One must remember that this information is obtained
from weather reporting stations that may be many miles from the field to be sprayed.
Also, the temperature information is measured 5 ft. above ground level and may be
considerably different at ground level. This information may not be reliable for spraying
in a field miles away. It is best to measure the temperature conditions in the field to be
sprayed.
National weather station information is never a substitute for in field information. It is
also advised to take readings in the field some distance away from the entrance into the
field.
An indicator for aerial applicators to determine if an inversion is present is to apply
smoke at spraying height. If the smoke dissipates or rises, it may be safe to spray.
Shelterbelts help reduce wind across fields, but they can cause other conditions adverse
to spraying. Conditions need to be observed closely.
These environmental conditions can make matters worse for spraying.
These conditions have been discussed previously but must be closely watched. The
environment is constantly changing and will often have a major affect on spray
conditions.