The transfer of acid gases and particles from the atmosphere to the earth's surface, where they exert adverse environmental effects, is called acid deposition. Relevant removal processes occur during dry weather conditions, during the formation of clouds or by scavenging of falling rain or other hydrometeors. The first removal process is called acid dry deposition while the last two, known as rainout and washout, respectively, are lumped together into the phenomenon of acid wet deposition.