evaporation duct is zero. When the N-unit gradient is
zero or negative and the atmosphere is stable (positive
bulk-Richardson number), the evaporation duct height
is a linear function of the N-unit gradient and the
atmospheric stability; when the atmosphere is unstable,
the evaporation duct height is a power function of the
N-unit gradient and the atmospheric stability. When the
computed value of the evaporation duct height is AO m,
it is set to 40 m.
2. Radiosonde Data Set Selection - Using this
option, an M-unit profile is entered by operator selection
of a radiosonde data set from the atmosphere
environmental file (AEF). M-unit versus height pairs
are extracted for the first 30 levels of the sounding or for
levels between 0 and 10,000 m heights. When the
lowest sounding level is not at the 0 m height, a
sea-surface M-unit value is extrapolated using the
lowest M-unit value/height pair in the profile, assuming
a standard atmospheric gradient.
The surface wind speed and evaporation height
complete the information required to generate an RDS.
The evaporation duct height is computed in the same
manner as when an M-unit profile is entered. The
location and the date and time of the RDS are specified
by the operator on the Evaporation Duct-Height
Parameters Input form.
3. Historical Refractivity Data Set - Using this
option, a historical RDS is generated for an
operator-specified location, month, and profile type
(standard atmosphere, surfiace-hosed duct, elevated
duct, or combined surface-based and elevated duct).
The upper air data used to specify the M-unit profile
with respect to height are retrieved from the Radiosonde
Data file. The mean surface wind speed and evaporation
duct height are retrieved from the Surface Observation
Data tile. Note that the closest long-term mean
radiosonde observation location and Marsden square
containing the information desired are retrieved from
the PDB. In some data-void regions, this may result in
an inappropriate RDS being created. Use the
climatological electromagnetic propagation conditions
summary function to evaluate the general climatologic
electromagnetic propagation conditions before using
this option. The use of climatological refractivity data
sets should be limited to planning functions.
4. Analysis of an AMR tape - This option allows
the operator to create an RDS by analyzing a tape
generated by the AN/AMH-3 electronic refractometer
set. These devices are routinely flown on E-2C aircraft.
Refer to the functional description for additional
information.
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.
BASIC CONSIDERATIONS
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.
Altimeter Errors (Pressure)
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
10-4
