This set of groups shall be repeated as many times as necessary to describe the changes in the meteorological conditions forecast in a given area, due attention being given to the need for strict economy in the number of groups used. The first group 1GDFmWm in which G 1-8, and the following optional group(s), if used, then shall refer to the forecast weather commencing at the time given in the group YYG1G1 and continuing through the period indicated by G. Subsequent groups 1GDFmWrn (G 1-8) shall give the period of time that the described weather is forecast to persist, commencing at the end of the period covered by the preceding group 1GDFmwm (G m 1-8). If a phenomenon is forecast to occur occasionally in the same period any set 1GDF (2VST Tr) (3DKPwHwHw) (G 1-8) shall be followed by a group 1GDFmWm (G 9) NOTE: The specific value of any of the elements given in the forecast should be understood to be necessarily approodmate and the value of the element in question should accordingly be interpreted as tepresenting the most probable mean of a range of values which the element may assume during the period of forecast concerned and over the area concerned