From: Subject: Thursday Weather Topic in Greater Depth Date: Thu, 22 Oct 2009 10:28:34 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/online/archive/course/09_fall/f09w05r_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Thursday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Wednesday, 7 = October=20 2009

ALTIMETERS and ALTIMETER ERRORS

Pressure altimeters are barometers that indicate altitude. These = instruments=20 are based upon the assumption that air pressure decreases with increased = altitude from some standard sea level air pressure at a fixed rate of = decrease.=20 Specifically, sea level pressure - taken at the reference altitude - is = assumed=20 equal to the standard 1013 .25 millibar (mb) or (29.92 inches of mercury = at 59=20 degrees Fahrenheit) and to have a vertical decrease in pressure = equivalent to=20 that specified by a model, usually the Standard Atmosphere. For = practical=20 purposes, the exponential decrease in pressure over the lowest 10,000 ft = of the=20 atmosphere can be approximated by a vertical pressure decrease of 0.30 = inches of=20 mercury for every 300 ft rise in altitude (or 10 mb for every 100 meter = change).=20 While this assumption is satisfactory for most purposes, several factors = must be=20 considered in order to obtain an accurate determination of altitude. =

SOURCES OF ALTIMETER ERROR AND CORRECTIONS

The altitude indicated by the pressure altimeter may not be identical = to the=20 true altitude because the actual conditions encountered typically will = differ=20 from the assumptions. Hence, an aircraft trying to maintain a constant = altitude=20 using a pressure altimeter is actually flying on an undulating constant = pressure=20 surface rather than on a flat constant altitude surface. This variation = may=20 become a serious problem when flying in mountainous areas or at low = levels in=20 poor visibility. Certain corrections can be made by the pilot to reduce = the=20 altimeter error.

1. Errors due to non-standard temperature profiles aloft

The vertical temperature profile and average temperature of the layer = at the=20 moment may not be equivalent to that specified by the Standard = Atmosphere. The=20 indicated pressure altitude depends upon the average temperature (hence, = the=20 density) of the intervening air column between the altimeter and sea = level.

If the average temperature of the intervening atmospheric layer were = colder=20 than the standard, the altimeter will read too high, and the pilot would = be at a=20 lower actual altitude than indicated on the pressure altimeter. For = example, on=20 a cold day, the indicated altitude may read 10,000 ft, but the actual = altitude=20 is 9500 ft because pressure decreases more rapidly in a cold air column. = Conversely, if the mean temperature of the air layer were warmer than = standard,=20 the altimeter will read too low, and the pilot would be at an altitude = higher=20 than indicated. The indicated altitude of 10,000 ft may be actually = 10,400 ft.=20

By measuring the free air temperature at flight level, the pilot can = use an=20 on-board flight computer to make an appropriate adjustment to the = altimeter=20 setting, based upon the departure of the existing air temperature from = the=20 hypothetical temperature of the Standard Atmosphere.

2. Errors due to non-standard sea level pressure

The mean sea level corrected pressure below the aircraft at the = moment may=20 not be equivalent to the standardized pressure of 1013.25 mb. When the = surface=20 air pressure is lower than the standard, the indicated altitude will be = higher=20 than the actual altitude.

ALTIMETER SETTINGS

A correction for this latter error can be applied by checking the = "altimeter=20 setting" reported at the airport before take- off. The altimeter setting = represents the value of atmospheric pressure to which the scale of a = pressure=20 altimeter is set to indicate the true altitude of an aircraft at the = airport=20 elevation. The pilot would use this setting to adjust the movable scale = on the=20 aircraft's altimeter accordingly. This local altimeter setting = incorporates the=20 large-scale changes in the observed air pressure at the airport at any = given=20 time. For flights below 18,000 ft, the pilot must routinely contact the = local=20 flight center to update altimeter settings along the flight path. Above = 18,000=20 ft, a pressure altimeter would be set at the standard 29.92-inch = altimeter=20 setting.

Altimeter settings are available with other pilot information from the = NWS=20 Aviation Weather Center.

Return to the Thursday=20 Daily Weather Summary

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu =
=A9=20 Copyright, 2009 The American Meteorological Society. =