Weather or Not To Spray
Scott Bretthauer, Extension Specialist, Dept of Ag. Engineering
Of all the factors that can influence a pesticide application,
one of the most important is entirely outside your realm of control:
the weather. When it comes to weather and spraying, the only decision
you have is if you can make an application safely and effectively.
It is critical to make sure that you avoid spraying during conditions
that increase the risk of drift. Understanding how weather affects
spraying and how you can monitor the weather will improve your
decision making process. There are two kinds of drift. Particle
drift is the movement of the liquid spray particles, or droplets.
Vapor drift is the movement of vapors, after evaporation (or volatilization)
has occurred. Weather conditions strongly influence both types
of drift, but in somewhat different manners. Remember to always
consult the pesticide label for specific instructions related
to weather conditions. There are three main components of weather
that directly affect spraying: temperature, humidity, and wind.
Temperature primarily affects the evaporation of a pesticide.
The warmer it is, the quicker evaporation occurs and the greater
the risk of vapor drift. Humidity also affects the evaporation
of a pesticide formulation. Humidity refers to how much water
vapor is in the air. Low humidity means there is a low level of
water vapor in the air, indicating that conditions are dry. Low
humidity increases evaporation, thus increasing the risk of vapor
drift. Some pesticides, particularly ester or oil based formulations,
are especially volatile, and care must be taken to avoid spraying
with these formulations on hot, dry days.
Wind has a strong influence on the risk of particle drift. High
wind speeds carry spray particles longer distances, increasing
the chance of off-target damage. It is generally recommended not
to spray if wind speed is greater than 10 miles per hour (mph).
A 100 micron droplet (see the January 2004 issue of Illinois
Pesticide Review, available at http://www.pesticidesafety.uiuc.edu/,
for a review of droplet measurement) can drift 3 feet in a 2 mph
wind when released 18 inches above the ground. In a 10 mph wind,
that 100 micron droplet can travel over 17 feet before reaching
the ground. Wind speed increases as altitude increases, so the
wind is usually blowing faster several feet above the ground.
This is why releasing the spray as close to the target as possible
is important, but remember to maintain proper nozzle overlap for
spray uniformity. Wind direction is also important. Since drift
occurs down wind, it is important to know which direction the
wind is blowing, and where areas that might be sensitive to the
pesticide you are applying are located. Do not spray if the wind
is blowing towards a sensitive area. Remember that even nozzles
designed to reduce the number of small spray droplets, such air-induction
nozzles, still create some small droplets which can drift.
Very calm conditions can also cause problems. Under calm conditions,
when the air is very still with little or no wind, small spray
droplets can remain suspended in the air in a concentrated group.
These droplets can then be blown off-target later in the day when
the wind speed increases, or evaporate while suspended and lead
to vapor drift. Also problematic are shifting winds, when wind
speed, direction, or both are changing during the application.
The ideal time to spray is usually when the wind is blowing steadily
between 3 and 10 mph and in a safe direction, away from sensitive
Inversions are particular instances of very calm weather. Inversions
occur when the temperature profile of the air becomes inverted,
or turned upside down. In a normal temperature profile, the warmest
air is at the earth’s surface and the air temperature decreases
as altitude increases. Since warm air rises, the air at ground
level rises and is replaced by cooler air, which in turn is warmed
and rises. This creates a continuous vertical mixing of air, which
disperses small spray droplets and prevents them from remaining
suspended in the air and being blown off-target in a concentrated
mass later during the day. During an inversion, the air at ground
level is cooler than the air above it. Since it is warm air that
rises, the cool air is essentially trapped below the layer of
warm air above it. This prevents vertical air mixing, creating
very calm conditions and allowing small spray droplets to remain
suspended in the cooler air at ground level, like fog early in
the morning. Inversions typically occur early in the morning or
late in the evening. Spraying should be avoided when an inversion
For small spray droplets, temperature, humidity, and wind speed
can all interact to create situations involving both particle
and vapor drift. Spray droplets begin to evaporate immediately
after being released, and small droplets can be blown off target
as particle drift but evaporate completely downwind while still
airborne. For instance, when droplets with a diameter of 50 microns
are released from a height of 18 inches in a 10 mph wind, they
can drift over 50 feet before finally evaporating, never reaching
the ground. In high temperatures and low humidity, the distance
these particles travel is less because the droplets evaporate
quicker. Lower temperatures and high humidity decrease the evaporation
rate and allow the particles to drift greater distances. Since
neither situation is ideal, it is important to reduce the amount
of small droplets created during an application.
How do you determine if weather conditions are suitable for spraying?
For temperature, any accurate thermometer will work fine. Measuring
wind speed is critical not only for determining if an application
should be made, but also for maintaining accurate records of the
application. Wind speed can be easily and accurately measured
with a handheld wind meter. Wind meters are available that measure
a variety of additional weather factors, such as temperature,
humidity, heat index, and dew point. More expensive models store
data for later recording. A benefit of wind meters is their small
size and portability, which allows you to check and record wind
speed and other weather factors throughout an application. Use
a compass to precisely determine and record wind direction. Complete
weather stations are also available, but at a higher cost. For
more information about the various types of products available
for measuring weather conditions, visit http://www.ambientweather.com.
To determine if an inversion exists, you can use a smoke bomb
or other similar device to create a cloud of smoke. If the smoke
hovers close to the ground and fails to disperse, there is an
inversion and you should not spray.
However you choose to measure wind speed and other weather factors,
remember that monitoring and recording them periodically throughout
an application is important. Weather conditions can change during
an application, requiring spraying operations to be halted. A
written record of wind speed, direction, and other factors throughout
an application, not just at the beginning, can be beneficial during
a drift complaint. It also demonstrates your professionalism and
dedication to making safe pesticide applications.