In order to better understand the cause of the spray inefficiency, it is useful to reflect on the implications of the large range of droplet sizes produced by typical (hydraulic) spray nozzles. This has long been recognized to be one of the most important concepts in spray application (e.g. Himel, 1969), bringing about enormous variations in the properties of droplets.
Historically, dose-transfer to the biological target (i.e. the pest) has been shown to be inefficient.  However, relating "ideal" deposits with biological effect is fraught with difficulty, but in spite of Hislop's misgivings about detail, there have been several demonstrations that massive amounts of pesticides are wasted by run-off from the crop and into the soil, in a process called endo-drift. This is a less familiar form of pesticide drift, with exo-drift causing much greater public concern. Pesticides are conventionally applied using [http://www.dropdata.org/DD/noz_data.htm#hydraulic hydraulic atomisers], either on hand-held sprayers or tractor booms, where formulations are mixed into high volumes of water.
Different droplet sizes have dramatically different dispersal characteristics, and are subject to complex macro- and micro-climatic interactions (Bache & Johnstone, 1992). Greatly simplifying these interactions in terms of droplet size and wind speed, Craymer & Boyle concluded that there are essentially three sets of conditions under which droplets move from the nozzle to the target. These are where:
* sedimentation dominates: typically larger (>100 µm) droplets applied at low wind-speeds; droplets above this size are appropriate for minimising drift contamination by herbicides.
* turbulent eddies dominate: typically small droplets (<50 µm) that are usually considered most appropriate for targeting flying insects, unless an electrostatic charge is also present that provides the necessary force to attract droplets to foliage.  (NB: the latter effects only operate at very short distances, typically under 10 mm.)
* intermediate conditions where both sedimentation and drift effects are important. Most agricultural insecticide and fungicide spraying is optimised by using relatively small (say 50-150 µm) droplets in order to maximize “coverage” (droplets per unit area), but are also subject to drift.