PDF《PESTICIDE》

STABILITY Stability refers to the atmosphere's ability to resist or assist vertical motion of air.

A stable atmosphere will resist upward motion of air, an unstable atmosphere will assist it, while a neutral atmos- phere will neither assist nor resist it. A neutral atmosphere is therefore best for spraying since droplets will fall solely under gravitational force C rather than being swept up by air currents in unstable conditions or concentrated within stable inversions. Smoke plume generating devices are available to help deter- mine stability. A rough smoke behavioural guide is shown below. THERMAL DRIFT Thermal drift occurs as a result of unstable meteorological condi- tions. Chemical droplets can be carried on thermal eddies and deposited some distance from the target site. WEATHER INFORMATION The Australian Government Bureau of Meteorology provides a wide range of weather information at http://www.bom.gov.au/ Information specific to ground sprayers is available at the Bureau's Registered Users Internet site. Please email [email protected] or phone (03)96694984 for information about accessing this site. Seven day computer generated forecasts of temperature, humidity, precipitation and wind are available for any location around Australia through the Bureau's SILO web site for a small annual fee. For details, please visit the SILO home page: http://www.bom.gov.au/silo/products/MeteoGrams.shtml WEATHER for PESTICIDE SPRAYING smoke or dust plume likely behaviour in neutral condition Release point Height Neutral Neutral Conditions, most likely early morning and early evening. Smoke or dust will spread out in an even pattern and fall under gravity, reaching the ground a short distance from the release point. smoke or dust plume likely behaviour in stable condition Release point Inversion layer Wind direction Stable Height Stable conditions, most likely overnight. Smoke or dust may initial- ly rise but will spread out at the inversion level and may slowly descend at some distance from the release point. Unstable conditions, most likely mid-morning to late afternoon. Smoke or dust will rise vertically. Wind direction Unstable smoke or dust plume likely behaviour in unstable condition Height Height Hot Ground Chemical droplets Buoyant air parcel Film of hot air Bubble breaking away Chemicals caught up in an air parcel and lifted above target zone DELTA T Delta T is becoming one of the standard indicators for acceptable spray conditions. It is indicative of evaporation rate and droplet lifetime. Delta T is calculated by subtracting the wet bulb temper- ature from the dry bulb temperature. The diagram below relates air temperature and relative humidity to values of Delta T. When applying pesticides, Delta T should ideally be between

2 and 8, Cover image courtesy of Goldacres Pty Ltd. Published by the Bureau of Meteorology

2004 PO Box 1289K Melbourne Victoria

3001 Australia ? Commonwealth of Australia

2004 Material No

507993 ABN

92 637

533 532 Delta T diagram courtesy of NUFARM LTD. 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 C 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 antic- ipated 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. 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 C but very high humidity can suppress droplet evaporation, leading to extended life and unac- ceptable spray drift. On the other hand, if humidity is too low, droplets C especially small ones C quickly evaporate leading to a high risk of drift. In extreme cases where humidity is low and tem- perature 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 condi- tions 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 strength- en 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 char- acterised 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). Check drainage wind potential and anticipate wind direction with respect to ground slope. *Overnight, when air over a slope is cooled by cold ground it becomes dense and heavy and drains to lower levels. Beware of deflected winds over and around obstructions. C: Suspended droplets at low level. B: Entrapment of rising droplets near the top of an inversion layer. A: Characteristics of a temperature inversion layer. WEATHER GUIDELINES FOR PESTICIDE SPRAYING ? Read the product label and follow all label instructions. ? Spray when wind is steady and ideally 3C15 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