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ARS Home » Southeast Area » Stoneville, Mississippi » Crop Production Systems Research » Research » Publications at this Location » Publication #272033
Title: Atmospheric stability intervals influencing the potential for off-target movement of spray in aerial application

Author
item Thomson, Steven
item Huang, Yanbo
item Fritz, Bradley - Brad

Submitted to: International Journal of Agricultural Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2017
Publication Date: 2/1/2017
Citation: Thomson, S.J., Huang, Y., Fritz, B.K. 2017. Atmospheric stability intervals influencing the potential for off-target movement of spray in aerial application. International Journal of Agricultural Science and Technology. 5(1):1-17.

Interpretive Summary: Aerial application of crop protection materials must be made under atmospheric conditions that do not favor off-target movement of spray. Spraying should be avoided while a temperature inversion has taken place, which has been shown to occur during the evening and early morning hours. An inversion occurs when the air temperature increases with height, under which a parcel of air cannot rise or disperse. A temperature inversion is one indication of a ‘stable’ atmosphere, along with wind speed. If spraying occurs during a stable atmosphere when the wind is calm, the material can move laterally when the wind picks up, sometimes for miles. This can cause damage to crops or create potential health hazard. This study determined times of day when stable atmosphere occured. These conditions were indicated by measurement of air temperatures and wind speeds at several heights. Results showed that during clear days in the hot summer months, conditions changed from stable atmosphere (unfavorable for spraying) to unstable atmosphere (favorable for spraying) between 0600 to 0700 and back to stable atmosphere between 1800 and 1900 hrs. Wind speed as an indicator of atmospheric stability showed a threshold of about 1.60 m/s (3.6 mph) below which stable atmosphere was likely to occur and spraying should be avoided. When cloud cover or cool temperatures prevailed, time in the morning where it was safe to spray was delayed by about one hour. A regulation developed in Arkansas was designed to reduce the incidence of off target movement of spray due to spraying under stable atmospheric conditions. This recommendation states that if the temperature has increased three (3) degrees Fahrenheit from the morning low at the time of application or has not decreased more than three (3) degrees Fahrenheit from the afternoon high, then it is safe to spray. Our data corroborates this rule when spraying on clear days in the summer months.

Technical Abstract: This study was designed to determine atmospheric conditions favorable for long-distance movement of spray from aerial application that can have deleterious consequences to susceptible crops. A tall meteorological monitoring tower was equipped with six precision thermistors at six heights to 27.4 m and wind speed anemometers at four heights to 27.4 m to monitor weather conditions needed to quantify atmospheric stability throughout the day during the growing season. Wind speed and temperature data were used to calculate atmospheric stability ratios using temporal and temperature gradient patterns at two different altitudes. Periods when wind patterns and stable air conditions occur known to favor off-target drift were determined from tower data. Temperature inversion periods favorable for drift were examined for strength, time of occurrence, and duration. Data presented in this study showed consistent patterns of atmospheric stability occurring primarily between 1800 and 600 hrs during summer months on clear days. Wind conditions in the morning transitioning from stable atmosphere to unstable atmosphere ranged between 1.25 to 1.60 m s-1 (2.8 and 3.6 mph). During periods of cool temperatures or cloud cover, stable atmospheric conditions primarily caused by temperature inversions persisted for longer periods in the morning and began earlier in the evening thereby reducing the effective time window for ‘safe’ spraying by about two hours. A regulation developed in Arkansas was designed to reduce the incidence of off target movement of spray due to spraying under stable atmospheric conditions. This recommendation states that if the temperature has increased three (3) degrees Fahrenheit from the morning low at the time of application or has not decreased more than three (3) degrees Fahrenheit from the afternoon high, then it is safe to spray. Our data corroborates this rule when spraying on clear days in the summer months.