In a letter condemning the Montgomery County police antidrug ''jump-out'' squads {July 7}, Cary Clennon complained about the police tactic of hiding to observe locations known to be frequented by drug users and then -- when the police officers' observations convince them there is probable cause to believe someone is using drugs -- ''jumping out'' and making an arrest. He objected to the tactic as ''questionable,'' and said that since The Post's article {June 28} did not say how many are arrested and never charged, the 90 percent conviction rate for arrests is not justification.

Had Mr. Clennon asked someone before finding probable cause to condemn the tactic, he would have learned that almost all of the ''jump-outs'' have resulted in arrest. Including the ''not charged'' along with the ''not proven guilty,'' the conviction rate is still far above that for almost every crime from theft to murder.

Mr. Clennon also asserts without any apparent basis that judges are reluctant to enforce constitutional requirements for searches. Experienced defense attorneys concede that most searches conform to these requirements. If that were not so, the practice of suppressing evidence from illegal searches would be pointless. Police today know what search-and-seizure law requires them to do. Mr. Clennon's publicly condemning the police and judges without evidence is unfair -- and hardlylawyerlike. ANDREW L. SONNER State's Attorney for Montgomery County Rockville