An Effective Downwind Message (EDM) or Nav Effective Downwind Message contains the effective downwind fallout speed and direction for each of the seven weapon yields. You must use the downwind direction and speed that correspond to the weapon yield of a burst, should one occur, to determine the zones of the most hazardous fallout. The EDM transmitted to shore units is in the following format:

The line ZULU contains the date (DD) and time (TTTT) (GMT or UTC) of the upper-wind observation used in determining the effective downwind directions and speeds for each yield. The effective downwind direction (ddd) is the direction toward which the wind is blowing, to the nearest whole degree. The EDF speed (fff) is in kilometers per hour. A direction and speed are given for each yield group except when the effective downwind speed is less than 8 kilometers per hour for a particular yield. When this occurs, the downwind distance to Zone I is transmitted instead of a dddfff group. The downwind distance is transmitted as a three-digit group only.

A three-digit group within parentheses may also appear in the EDM beside the effective downwind direction and speed: for example, FOXTROT 230030 (060). The group within parentheses represents the expansion angle to be used in constructing the RADFO diagram. Normally, the RADFO diagram is constructed using the information in the EDM and a 40-degree warning angle (20 on either side of the effective downwind direction). When the 40-degree warn-ing angle is insufficient for a particular yield, the required expansion angle is transmitted within parentheses following the dddfff group. The expansion angle may also be transmitted as an additional digit added to the dddfff group (FOX-TROT 2300306). When a seventh digit is used in any one yield group, all of the yield groups will contain seven digits. See figure 6-9-11.

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