How Weather Conditions
Affect Spray Applications
Table of Contents
Weather 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.
Spray can be applied at the high end of this scale by using:
- drift-mitigating nozzles
- larger droplets
- slower forward speeds
- shrouds or deflectors and/or
- by reducing the distance to the target.
Temperature 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 and coverage.
- 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
- 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.
Table 1. Wind Conditions and Spraying
||Wind Speed / Beaufort at Boom-height
||May lead to vapour drift where finer droplets remain suspended
in the air, prone to evaporation and drift long after spraying
|Smoke rises vertically
||Do not spray
||These conditions make wind direction unpredictable and may
indicate an inversion
||Direction keeps changing
||Do not spray
|Direction shown by smoke
|Light to gentle breeze
|Leaves rustle, wind felt on face, twigs in motion
||Higher wind speeds pose the most obvious risk of drift through,
around or over target
|Small branches move, raises dust
||Spray with caution or Do not spray
Figure 1. Drift
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
- Monitor weather forecasts and understand the impact on the product
- 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
Measuring Weather Conditions
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 spray.
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.
Reading a Conventional Compass
Handheld weather station
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 readings.
- 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.
- Use a basic orienteering compass to determine prevalent wind
- Step 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
- 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 Station
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 Stations.
Anatomy of a Generic Handheld Weather Station
Table 2. Sourcing Handheld Weather Stations
|Green Lea Ag. Center
|HJV Equipment Ltd.
|Phillips Farm Supplies
|Forestry Suppliers Inc.
No discrimination is intended and
no endorsement by the author or OMAFRA is implied. Information subject
When 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