Many important agricultural chemicals are applied as sprays of fine droplets. Spray application minimizes mechanical damage to the crop and leads to good coverage of the target foliage by the chemical. During the time that spray droplets are suspended in the air they are susceptible to unintended lateral movement away from the target, a process termed drift. Meteorological variables with significant effect on drift are atmospheric stability and wind speed, and to a lesser extent, humidity. Stability refers to the tendency of adjacent volumes of air to mix together. The ideal atmospheric condition to apply chemicals by spraying is slightly unstable. In this situation most of the applied chemical can settle onto the crop, and the rest will mix vertically and dissipate. Under highly unstable conditions, strong vertical and lateral motions of wind make drift a problem because the chemical is not given an opportunity to settle onto the crop.