FORECASTING ALTIMETER SETTINGS

LEARNING OBJECTIVES: Discuss the basic considerations in forecasting altimeter settings. Determine altimeter setting errors due to surface pressure variation and nonstandard temperatures. Describe the forecasting of altimeter settings.

Under certain conditions it may be necessary to forecast or develop an altimeter setting for a station or a location for which an altimeter setting is not received. There is also a possibility that an altimeter setting may be required for an area not having a weather station. A forecast of the lowest altimeter setting (QNH) for the forecast period is required. For these reasons it is import ant that forecasters have a basic understanding as to the importance of correct altimeter settings and a knowledge of procedures for forecasting altimeter settings.

The altimeter is generally corrected to read zero at sea level. A procedure used in aircraft on the ground is to set the altimeter setting to the elevation of the airfield.

An altimeter is primarily an aneroid barometer calibrated to indicate altitude in feet instead of units of pressure. An altimeter reads accurately only in a standard atmosphere and when the correct altimeter setting is used. Since standard atmospheric conditions seldom exist, The altimeter reading usually requires corrections. It will indicate 10,000 feet when the atmospheric pressure is 697 hectopascals, whether or not the altitude is actually 10,000 feet.

The atmospheric pressure frequently differs at the point of landing from that of takeoff; therefore, an altimeter correctly set at takeoff maybe considerable y in error at the time of landing. Altimeter settings are obtained in flight by radio from navigational aids with voice facilities. Otherwise, the expected altimeter setting for landing should be obtained by the pilot before takeoff.

To illustrate this point, figure 10-1 shows an example of altimeter errors due to change in surface pressure. The figure shows the pattern of isobars in a cross section of the atmosphere from New Orleans to Miami. The atmospheric pressure at Miami is 1019

Figure 10-1.-Altmeter errors due to change in surface pressure.

hectopascals and the atmospheric pressure at New Orleans is 1009 hectopascals, a difference of 10 hectopascals. Assume that an aircraft takes off from Miami on a flight to New Orleans at an altitude of 500 feet. A decrease in the mean sea level pressure of 10 hectopascals from Miami to New Orleans would cause the aircraft to gradually lose altitude, and although the altimeter indicates 500 feet, the aircraft would be actually flying at approximately 200 feet over New Orleans. The correct altitude can be determined by obtaining the correct altimeter from New Orleans and resetting the altimeter to agree with the destination adjustment.

NOTE: The following relationships generally hold true up to approximately 15,000 feet:

34 hectopascals = 1 in. (Hg) = 1,000 feet of elevation, Since 1 hectopascal is equal to about 30 feet (below 10,000 feet altitude), a change of 10 hectopascals would result in an approximate error of 300 feet.

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