

Click here to Order your Radar Equipment Online Extrapolating UpperLevel Heights A common analysis practice is extrapolation of upper heights from sealevel reports. The scarcity of upperairreporting stations in many regions of the world and the absence of upperlevel data from stations that do report require that this procedure be used. This is a common practice in computeranalyzed constantpressure charts. Extrapolated heights are plotted enclosed in parentheses. The thickness of a stratum or height of an upper level is computed using a known sealevel pressure and temperature and an assumed mean virtual temperature for the stratum. Tables and nomograms, derived from the hydrostatic equation, are also used for this purpose. Figure 811 contains a nomogram for computing the height of the 700 or 500mb level using a known surface temperature and an estimated upperlevel temperature. The upperlevel temperature is usually estimated from past analysis and compensated for any changes which may have occurred since the last analysis; i.e., any cold or warm air advection. A stepbystep extrapolation procedure follows: 1. Estimate the upperlevel temperature. 2. Determine the height of the 1000mb level. 3. Determine the thickness of the stratum, following nomogram directions. 4. Algebraically add the 1000mb height and the thickness of the stratum obtained in step 3. This is the height of the upper level above sea level. The 1000mb height must be determined, because tables and nomograms use 1000 mb as the standard surfacepressure value. To compute its height, we assume 7 1/2 mb equals 60 meters of height, or 8 meters per millibar. Take the difference between the actual sealevel pressure (SLP) and 1000 mb and multiply this figure by 8. For example, a ship reports a sealevel pressure of 1015.0 mb. The difference between the reported value and 1000 mb is 15. Multiply this difference by 8. 15 x 8 = 120. The height of the 1000mb level is 120 meters above sea level. SLP(mb) – 1000 mb x 8 m/mb = 1000mb height Minus (–) values indicate 1000mb heights below mean sea level, and plus ( + ) values indicate 1000mb heights above mean sea level. Given the following parameters, compute the height of the 700mb level, using the nomogram shown in figure 811: Estimated 700mb temperature – 5.0°C; sealevel temperature, 25.5°C. REMEMBER—the nomogram contains 700mb heights and 500mb heights. The 700mb heights are to the left of the center line. In step 4, algebraically add the thickness of the stratum just computed and the height of the 1000mb level. For computational purposes, use the 120meter height previously computed. The thickness of the 1000700mb stratum as computed from the above information should read approximately 2,957 meters. Because the reported sealevel pressure in this example is greater than the standard, you should have added the 120 meters to the height of the 1000700mb stratum. Your answer should read 3,077 meters. If the reported sealevel pressure is less than 1000 mb, subtract the 1000mb height from the height of the stratum. Since we are working with assumed or estimated temperatures and not those of an actual upperair sounding, inversions anywhere in the stratum or nonrepresentative temperatures at the surface or upper level will result in incorrect height computations. In the case of marked inversions of any type, the estimated height will be less than the "true" height, because the computed mean virtual temperature will be less (colder) than the true mean virtual temperature. Compensate for such inversions by using a higher estimate of the upperlevel temperature. Higher estimates may be required in the following situations: 1. In the vicinity of highpressure cells where subsidence inversions are present 2. Where surface inversions are indicated by stable weather phenomena such as fog 3. When a frontal surface exists below the level to be extrapolated A study comparing the true thickness of the 1000700mb and 1000500mb stratums to extrapolated thicknesses using temperature averaging showed the extrapolated thicknesses

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