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Estimating Trapping Conditions The trapping estimation procedure uses air-temperature- and sea-surface-temperature (SST) readings, as well as the saturation vapor pressure at the sea surface and at the observation level. The procedure is as follows: 1. Obtain the current air temperature (TA) and sea-surface temperature (TS) in degrees Celsius. 2. Determine the difference between TA and TS Remember, a 1C increase in temperature causes a decrease in N by 1 unit, while a 1C decrease causes an increase in N by 1 unit. 3. Using TA and TS, determine the saturation vapor pressure for each. Saturation vapor pressures can be computed or taken from a table. Since the use of a table is much easier, refer to table 6-3-1. Find the saturation vapor pressure for an air temperature of 27C and a SST of 20C. You should come up with 36-millibars and 23-millibars (rounded off), respectively. Table 6-3-1 is a partial table taken from the Smithson-ian Meteorological Tables. 4. Determine the difference in vapor pressure between the sea surface and the air-temperature observation level. Remember, the vapor pressure to N -unit ratio is 1 to 5. 5. Algebraically add the N -units as deter-mined in steps 3 and 4. For example, lets assume that the temperature difference causes a 4 unit increase in N (+4) and that the vapor pressure difference causes a 15 unit decrease in N ( 15). By adding +4 and 15, you should come up with 11 N -units. 6. Determine the refractive condition based on the N-gradient. For example, if the height difference between the sea surface and the observation level is 100 feet and we determine that N decreases 11 units over this distance, the gradient is 11 N -units per 100 feet, or 110 N -units per 1,000 feet. Since trapping conditions occur when N decreases at a rate of 48 N -units per 1,000 feet or greater, we can assume that trapping conditions are occurring. This method provides an ESTIMATE ONLY because of certain limitations and assumptions. We assume that the SST is the same as the air directly above the sea. If an air temperature reading could be taken directly at sea level, this reading would be used. However, obtaining an air temperature reading at sea level while on a moving ship is not possible. Also, we are limited to a true surface reading (no injection readings). Sea-water injection temperatures should not be used, because they can differ from true surface temperatures by several degrees. Another assump-tion concerns vapor pressure. The difference in vapor pressure between the observation deck and the sea surface is assumed to be linear (a straight-line difference), but in reality, vapor pressures can be quite varied over these distances. Ductogram One very accurate method of determining the Since trapping criteria differ based on various wavelengths and radar types, separate diagrams are required for each different frequency band. The ductogram and diagrams, along with instruc-tions on their use, can be found in Appendix III. Learning Objective: Define anomalous propagation. |
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