ARS | Publication request: Temporal indications of atmospheric stability affecting off-target spray drift in the midsouth U S

Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: March 31, 2011
Publication Date: March 31, 2011
Citation: Thomson, S.J., Huang, Y., Fritz, B.K. 2011. Temporal indications of atmospheric stability affecting off-target spray drift in the midsouth U S. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE). pp. 8.

Interpretive Summary: Aerial application of crop protection materials must be made under atmospheric conditions that do not favor drift long distances from the target crop. Spraying should be avoided while a temperature inversion is taking place, which has been shown to occur during the evening and until the 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 preliminary study was designed to determine the times of day when stable atmosphere occur. 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. These results were similar to a companion study conducted in Texas where a wind speed threshold of 1.8 m/s was indicated.

Technical Abstract: This study was designed to determine atmospheric conditions favorable for long-distance spray drift deleterious to susceptible crops. A tall meteorological monitoring tower equipped with six precision (and periodically calibrated) thermistors at six heights to 27.4 m and wind speed anemometers at four heights to 27.4 m was logged continuously throughout the day during the growing season. Wind speed and temperature data were used to calculate atmospheric stability ratios, and temporal and temperature gradient patterns at different altitudes were determined. Periods when wind patterns and stable air conditions occur that are known to favor off-target drift were determined from tower data. Temperature inversion periods influencing atmospheric stability were examined for strength, time of occurrence, and duration. The data presented in this study showed stable conditions (unfavorable for spraying) occurring primarily between the hours of 6:00 PM and 6:00 AM during clear conditions in the hot summer months. Wind conditions in the morning transition from stable to unstable atmosphere ranged between 1.25 to 1.60 m/s (2.8 and 3.6 mph). The ultimate goal is to incorporate this information into new guidelines for pilots on weather conditions and times of the day not to spray to reduce the potential for far-field off-target drift in the Mid-south.