WEATHER GUIDELINES FOR
PESTICIDE SPRAYING
- Read the product label and follow all label instructions.
- Spray when wind is steady and ideally 3–15 km/h.
- Avoid variable or gusty wind conditions.
- Avoid calm conditions - small droplets may remain
suspended for long periods.
- Spray when wind blows away from sensitive areas.
- Avoid spraying in temperatures above 28 °C.
- Aim to spray when Delta T is between 2 and 8 and
not greater than 10.
- Do not spray when inversion conditions exist.
- Aim to spray when the atmosphere is neutrally stable.
- Most chemicals require a rain free period – check
the label.
- Be aware of local topographic and convective
influences on wind speed and direction.
- Record on-site weather conditions at spray time.
The Bureau gratefully acknowledges the
Primary Industries Standing Committee "Spray Drift Management" publication, the
Queensland Government DPI publication "How to prevent spray drift", the University
of Queensland (Pesticide Application
Information / Fact Sheets), Nufarm, Grains
Research and Development Group (GRDC)
While the guidelines above are useful, more in-depth
knowledge of how wind, temperature and humidity might
vary over the target area will enhance the opportunity for
successful spray application.
WIND
Wind values mentioned in Bureau forecasts and observations
refer to the average wind over a 10-minute period at a height
of 10 metres – crop level winds may be different. On weather
maps wind is strongest where isobars (lines joining points of
equal pressure) are close and light where isobars are far apart.
Wind speed and direction may differ significantly from that anticipated
from the weather map. This is because wind at ground
level tends to flow in much the same way as water flows in a
stream. Local wind may be deflected or blocked by obstacles and
become chaotic in speed and direction. Daily variations of wind
speed occur, generally with a minimum early in the morning
increasing to a maximum sometime between late morning and
mid afternoon. A sea breeze during the day and a katabatic* wind
at night may completely mask ‘weather map’ inferred wind. Local
convective cloud also has the potential to generate gusty wind.
Virga (rain falling from cloud but not reaching the ground) warns
of potential gusty conditions at ground level.
Beware of deflected winds over and around obstructions.
Check drainage wind potential and anticipate wind direction with
respect to ground slope.
TEMPERATURE AND HUMIDITY
- Temperature. In meteorology, air temperature and humidity
are measured in the shade at 1.25m above the ground. On hot
days ground temperature may be up to 20°C higher. Volatile
herbicides exposed to high temperatures and low humidities are
inclined to vapourise, releasing damaging vapour even when the
observed air temperature is measured to be within acceptable
limits for spraying.
- Humidity affects evaporation rate. Humidity of greater than 45% is
often recommended for spraying – but very high humidity can
suppress droplet evaporation, leading to extended life and unacceptable
spray drift. On the other hand, if humidity is too low,
droplets – especially small ones – quickly evaporate leading to a
high risk of drift. In extreme cases where humidity is low and temperature
high, pesticides convert to crystalline form and settle on
the target, only later being activated by additional moisture. Late
take up of pesticide may lead to unacceptable residue levels.
Optimal temperature and humidity conditions frequently
occur in the early morning and into mid morning when conditions
are not too hot nor too dry (See in Delta T section).
- Temperature Inversions are layers of air in which temperature
increases with height (as opposed to the normal decrease with
height). A cold drainage wind (katabatic) can contribute to the
strength of inversions (A) and add to the risk of drift.
Inversions frequently form in the late evening and strengthen
overnight (being strongest near sunrise) before being eroded
by mid morning as the air near the ground heats up.
Inversion layers are stable (see stability section) and are characterised
by calm, light or variable winds, that make it difficult to
predict the movement, both vertically and horizontally of spray
droplets (most drift complaints involve spraying under inversion
conditions).
Within inversion layers droplets may rise and be trapped at
some higher level before being carried out of the target area (B).
On the other hand droplets may remain suspended at a lower
level and drift off target in light winds (C).
A: Characteristics of a temperature inversion layer.
B: Entrapment of rising droplets near the top of an inversion
layer.
C: Suspended droplets at low level.
*Overnight, when air over a slope is cooled by cold ground it
becomes dense and heavy and drains to lower levels. |
