Which of the following external factors can affect droplet size and drift during pesticide application?

Humidity is a crucial external factor that affects droplet size and drift during pesticide application. Higher humidity levels can increase droplet size because the water vapor in the air can promote the coalescence of smaller droplets into larger ones. This can make the application more effective, as larger droplets are less prone to being carried away by wind, thereby reducing drift. Conversely, in low humidity conditions, smaller droplets may form, which are more susceptible to evaporation and drift due to wind turbulence.

In contrast, factors like soil type, crop type, and elevation are more related to the characteristics of the application environment or the target area rather than directly influencing droplet dynamics during the spray process. While these factors may play a role in overall application effectiveness or pest control, they don't inherently impact the size of droplets being produced or the likelihood of drift occurring as humidity does. Understanding the critical relationship between humidity and droplet behavior is essential for successful aerial applications and minimizing environmental impact.