COMPUTING EFFECTIVE FALLOUT WINDS USING STANDARD-PRESSURE-LEVEL WINDS

When RADFO messages are not available (that is, when there is no Basic Wind Data

Message or Effective Downwind Message with which to work), fallout predictions are still possible. Effective downwind direction and speed can be computed using a pre-determined or an estimated weapon yield and wind data obtained from upper-air soundings.

The use of standard-pressure-level winds to compute effective downwind data assumes that these winds represent mean winds for the layers of the atmosphere. Each standard pressure level applies to a certain layer of the atmosphere.

See table 6-9-4. For example, the wind at 700 millibars is assumed to be the mean wind in the layer between 7,400 feet and 14,100 feet, and the wind at 500 millibars is assumed to be the mean wind for the layer from 14,100 feet to 21,000 feet. Table 6-9-4 also provides the nuclear-cloud-bottom parameters for the seven standard weapon yields (Alpha through Golf).

The standard-pressure-level winds used to compute the effective downwind direction and speed for a given weapon yield are those winds representing the layers up to and including the height of the nuclear cloud base. In other words, if the nuclear cloud base is estimated to be at 30,000 feet, only those layers up to and including this height are used.

The standard-pressure-level winds must be weighted and vectorially added. The weighting is required to account for atmospheric layer thickness and the various densities of the layers.

Weight factor tables are available for wind speeds in knots and in kilometers per hour. See tables 6-9-5 and 6-9-6. The weight factors are applied to the wind speed for each level. To obtain the weighted wind speed, you simply multiply the wind speed for each standard pressure level by the appropriate weight factor.

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