Weather Conditions Affect Spray Applications
PDF Version - 529 KB.
and Relative Humidity
A Handheld Weather Station
conditions such as wind, temperature, relative humidity and precipitation influence
the effectiveness of spray applications and the potential for wastage by run-off
and drift. This Factsheet describes the impact of weather conditions on spray
applications and how to change application methods to match weather conditions,
improve accuracy and reduce wastage. It also describes how to use simple tools
to measure and monitor weather conditions and provides information on where to
purchase these tools.
Wind direction determines
whether droplets travel toward the target or toward unintended downwind areas
such as open water, sensitive crops or areas of human activity. Wind speed affects
the distance a droplet will travel before it is deposited on the target.
- Spray only when wind direction is consistent and between 215 km/h, or
as indicated on the product label.
- The impact of wind is particularly
significant when performing directed (e.g. airblast) spraying, so spray with a
crosswind and always orient nozzles and deflectors to direct the spray into canopies,
not over them.
Table 1, Wind Conditions and Spraying Recommendations,
describes various wind conditions and the potential for drift and advises whether
or not to spray. Figure 1 shows the wind speed or force at which there is reduced
risk of drift and the wind speed or force at which caution is advised. Cut out
this scale and carry it in the spray cab for quick and easy reference.
can be applied at the high end of this scale by using:
- larger droplets
- slower forward speeds
or deflectors and/or
- by reducing the distance to the target.
and Relative Humidity
- Spray when temperatures are low and relative
humidity is high. In general, do not spray when relative humidity is less than
40 per cent and air temperature is above 25°C. This reduces the chance of
drift due to temperature inversions or evaporation. It also increases target deposition
- Hot and dry conditions increase drift because droplets
rapidly evaporate and become fine droplets, vapour or particles of concentrated
pesticide. Few countries specify optimal temperature or relative humidity because
several other factors affect drift and on-target deposition. These factors include
formulation, spray method, and droplet size.
- Optimum spraying conditions
are early mornings following overcast nights. However, the best time to spray
to avoid disrupting bee activity is evening or at night.
1. Wind Conditions and Spraying Recommendations
Speed / Beaufort at Boom-height||Visible Signs||Spraying|
|Still||May lead to vapour drift where finer droplets
remain suspended in the air, prone to evaporation and drift long after spraying
is completed||02 km/h |
|Smoke rises vertically||Do
|Gusty||These conditions make wind
direction unpredictable and may indicate an inversion||Not applicable||Direction
keeps changing||Do not spray|
conditions||23.2 km/h |
|Direction shown by smoke||Spray
|Light to gentle breeze||Ideal conditions||3.29.6
|Leaves rustle, wind felt on face, twigs in motion||Spray|
|High||Higher wind speeds pose the most obvious risk
of drift through, around or over target||9.614.5 km/h |
|Small branches move, raises dust||Spray with caution or Do not
1. Drift Scale Cut-out
Rain can have both a positive or negative effect on spraying. Some products
work best when rain water carries them into the soil after application, but not
far enough to enter the water table before they do their work and break down.
Depending on the rain-fastness of the product, rain soon after application may
also wash the product from leaves and reduce the level of protection. While rain
can also redistribute certain products over the target, do not rely on this for
- Monitor weather forecasts and understand the impact
on the product being applied.
- Avoid spraying when foliage is still wet
from rain or dew unless indicated by the label. A leaf can retain only a limited
volume of spray, and therefore a limited amount of product.
- Once wetted,
deposition will not increase beyond the tank concentration and the surplus will
run off to the lower leaves and onto the soil.
Measure weather conditions when planning to spray, or
during spraying if a change in conditions is suspected. Limitations on application
temperature and wind speed may be on the pesticide label. Weather Innovations
Incorporated (WIN) now offers an online spray advisory forecast for boom sprayers
in Ontario at www.weathercentral.ca. This information is a prediction based on
wind speed and is therefore only one factor in the decision to spray or not to
It is the responsibility of all applicators to monitor and record
local conditions using a combination of weather forecasts, a standard compass
or windsock, and a fixed or handheld weather station.
2. Reading a Conventional Compass
A handheld weather station is an option for measuring temperature,
relative humidity and wind speed.
- To measure temperature or relative
humidity, take readings in the shade and wait a minimum of 15 seconds for accurate
- To measure wind speed, hold the meter 1.5 m above the ground
or the height of the spray boom, whichever is greater.
- Depending on the
fullness of the canopy, wind speed is higher at the outer rows and the top of
an orchard or vineyard canopy, so use a pole to lift the meter to canopy height
and then check the recorded average.
- Monitor wind speeds over 23
minutes to determine the maximum and average wind speed and direction.
a basic orienteering compass to determine prevalent wind direction.
away from the sprayer, face square into the wind, and hold the compass level at
waist height. (See Figure 2).
- Turn the whole compass so that the
arrow in the centre of the base is facing away from you (into the wind). Turn
the bezel, the rotating ring surrounding the compass face, until the N
is centred over the north (red) end of the compass needle.
- Read the
bearing of the wind, which is in degrees, from the circular scale directly over
the centreline of the compass. The bearing is a value between 0 and 360 degrees.
Sourcing A Handheld Weather
At the time of publication, the average price of a handheld weather
station is about $175, but the price varies depending on the features required.
Features to look for include a durable body, a hard case and lanyard, a backlit
display and the ability to measure wind speed, relative humidity and temperature
(see Figure 3). Retailers are listed in Table 2, Sourcing Handheld Weather
3. Anatomy of a Generic Handheld Weather Station
2. Sourcing Handheld Weather Stations
|Green Lea Ag. Center||1-800-661-5019|
|Phillips Farm Supplies||1-800-811-6238|
discrimination is intended and no endorsement by the author or OMAFRA is implied.
Information subject to change.
planning to spray, consider local forecasts and consult the label for product
specifications such as optimal application conditions, drying time, absorption
rate and retention time. Use a handheld weather station to adapt your application
method to changing weather conditions. If conditions become too adverse it is
sometimes necessary to stop spraying until they improve. You can't change the
weather, but with accurate information you can work with it to achieve the best