## 1 Introduction  
Grape production represents one of the most important agricultural businesses worldwide, with 7.5 million hectares and five countries representing 50% of the world vineyard harvested area: Spain (13%), China (11%), France (10%), Italy (9%) and Turkey (7%) [1]. However, considering the total grape production, Italy ranks second after China, with 8.4 and 13.7 Mt respectively, followed by USA, France and Spain [1]. Considering wine grapes, only the EU ranks first with 61.3% of total world production [1]. Producing high-quality food in a safe manner is one of the most important goals in modern agriculture [2]. Indeed, the high number of spray applications during the vegetative season in intensive production systems determines a massive Plant Protection Product (PPP) use that can cause undesirable effects related to pesticide residues on food and adverse effects to the environment. At present, PPP spraying is a commonly used technique to control pests and diseases in commercial crops. Nevertheless, spraying is a complex process in which several factors interfere [3]. For that reason, controlling all factors is nearly impossible, and as a result PPP applications could be inefficient. This fact led the European Authorities to develop Directive 2009/128/EC on sustainable use of pesticides [4]. This Directive specifies that pesticide applications should always consider the principles of integrated pest control strategies. Among these, efficiency in the pesticide application necessarily plays a major role that is not completely matched nowadays. Among the main parameters to be considered regarding spraying efficiency, liquid flow rate, airflow rate and droplet size are three of the most important [5; 6; 7; 8; 9; 10; 11; 12; 13; 14], along with the forward speed [15; 16]. Indeed, during spray applications in bush/tree crops with conventional sprayers, only a limited fraction of the total amount of PPP is deposited on the intended target according to the canopy characteristics [7; 17]. Therefore, part of the applied PPP can be transported outside the sprayed area by the action of air currents during the application process as spray drift [18]. Some authors have quantified that, during a spray application, up to 50% of the total applied PPP spray mixture can be lost to the air from the targeted site to a non-target receptor site [19; 20; 21; 22; 23; 24; 25; 26; 27]. In addition to the more localized movement of agrochemical residues in turbulent air masses downwind of the application area, residues can also become concentrated in inversions or stable air masses and be transported at long distances [28]. Another fraction could end up as spray deposition on the ground, directly in the field tractor path and underneath target tree rows or indirectly in the adjacent area [5; 6; 7; 29; 30]. Among spray losses, spray drift remains the most troubling, as it is really difficult to control [31; 32; 33; 34; 35; 36], especially in 3D crops such as vineyards [37]. Due to the importance to minimize PPP spray drift generation, strong efforts have been undertaken to properly study this phenomenon and to give adequate advice to farmers around Europe [38]. Among the different factors influencing spray drift, wind speed and direction are the main ones [39]. The higher the wind speed, the higher carrying effect it will have for droplets; therefore, the spray drift risk increases [40; 41]. A solution to avoid spray drift is necessary, and it relies on not spraying when the wind is present. Nonetheless, reality is not so simple, as there are many situations in which pesticide needs to be applied a certain day or in a very narrow time window, so the farmer needs to spray even if the wind conditions are not favorable. Consequently, they must act on technical factors (those controlled by the applicator) through a proper adjustment of the spraying equipment [19; 42]. Droplet size has proven to be the most effective factor in reducing spray drift [12; 23; 24; 25; 28; 33; 43].