Radiological fallout from large surface bursts creates a major operational problem. Potentially, it may extend to far greater distances than the immediate blast area and may cause more casualties than any other nuclear weapon effect. Another problem with fallout is that it may exist for a considerable time after detonation, especially overland. Overland, fallout remains a hazard until the radioactive material decays. Fallout at sea is not as great a factor as it is overland, because the ocean absorbs fallout and acts as a shield against radioactive material. Ships can avoid RADFO by moving beyond the fallout zones.

Areas of hazardous RADFO are determined and diagramed using data based on weapon yield and the atmospheric winds in the vicinity of the blast. The winds determine the direction in which and the speed at which RADFO will travel. Two types of RADFO messages are used in diagraming fallout areas. They are the Basic Wind Data Message and the Effective Fallout Data Message. In the absence of either of these messages, Aerographers Mates must work from raw wind-data taken from upper-air reports.

Learning Objective: Diagram the zones of the most hazardous fallout using the Data Message.

The Basic Wind Data Message provides mean winds between the surface and 30,000 meters (100,000 feet) in 2,000-meter increments.

See figure 6-9-1. Note that each layer is designated by a code figure. For example, the surface-to-2,000-meter layer is encoded as 2; the 3,000-to-4,000-meter layer as 4, and so forth.  

The wind direction (ddd) and speed (fff) for each layer are entered using six digitsthree digits for direction and three digits for speed. The wind direction and speed may appear as one 6-digit group or two 3-digit groups. For example, the

surface-to-2,000-meter layer may appear as follows:

Wind direction for each layer is the direction from which the wind is blowing, and it is entered to the nearest 5 degrees. Mean wind speeds are entered in kilometers per hour or in knots. Kilometers per WIND DATA MESSAGE.

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