Section 121.599 - Familiarity with weather conditions(a)Domestic and flag operations. No aircraft dispatcher may release a flight unless he is thoroughly familiar with reported and forecast weather conditions on the route to be flown.(b)Supplemental operations. No pilot in command may begin a flight unless he is thoroughly familiar with reported and forecast weather conditions on the route to be flown.Doc. No. 6258, 29 FR 19222, Dec. 31, 1964, as amended by Amdt. 121-253, 61 FR 2614, Jan. 26, 1996