level and the altitude of the level, temperature and dew-
point depression, and winds. The three groups are
repeated for each of the mandatory levels.
NOTE: In table 1-8. the subscripts used with each
of the symbolic letters are the international symbolic
format. The subscripts identify the level for which the
data is being reported. such as the subscript "0" for
surface, "1" for first level, "n" for any other level, "t" for
tropopause level, and "m" for max-wind level.
Although these subscripts are necessary when “looking
up” the appropriate definition for a symbolic code letter
in the International Codes manual, the subscripts make
the code seem more complicated than it really is. We
will ignore the subscripts used to identify the level in the
remainder of this discussion on the TEMP code. Only
significant subscripts used to define terms for other
purposes are included.
SURFACE PRESSURE.—In the first of the three
groups for surface information. the 99 is the indicator
for "surface information" while the PPP is the
hundreds, tens. and units of the surface pressure in
hectopascals. In the example. 030 represents 1,030 hPa.
PRESSURE LEVEL ALTITUDE.—The first of
the three groups for the remaining “mandatory levels”
contains PP. the hundreds and tens digits of the reported
pressure level, and hhh, the altitude in meters or
decameters of the reported pressure level. For levels up
to and including 700 hPa, the altitude is reported in three
digits to the nearest meter with the thousands value, if
any, deleted. For all levels above 700 hPa, the altitude is
reported to the nearest decameter (tens of meters) with
the ten-thousands value deleted. Refer to table 1-6 in
module I, (Standard Pressure Surfaces) to determine
the standard altitudes of the mandatory levels from the
1,000- to 10-hPa levels. For example. an 850-hPa level
altitude of 1,457 meters is encoded 85457. To decode a
reported altitude in Part A of 10711, the first two digits.
10, indicate the 100-hPa level. The 711 is the altitude in
decameters, or "something-7,110 meters." Since the
standard altitude of the 100-hPa level is approximately
16,180 meters (with the ten-thousands value of 1), one
could correctly assume that the reported altitude is
actually 17,110 meters.
T E M P E R A T U R E / D E W - P O I N T
DEPRESSION. —Following
the
surface-pressure
group and the pressure-level/altitude groups, the next
group contains the coded temperature and dew-point
depression. The temperature is reported by TTTa. The
TT is the tens and units value of the temperature, in
degrees Celsius. at the surface or the pressure level. The
tenths value of the temperature is also used to indicate
whether the reported temperature is positive or negative
in the coded Ta. Zero, and all even “tenths values” in
this position indicate a positive temperature. while an
odd value indicates a negative temperature.
When
encoding, the tenths value is dropped to the next lower
tenths value, if necessary, to indicate the proper
temperature sign. For example, a temperature of -
23.8°C is encoded 237, while a temperature of +23.9°C
is encoded 238.
The radiosonde instrument measures temperature
and relative humidity, and the MRS system (or
observer) calculates the difference between the two
instrument-reported readings when determining dew-
point temperature. Only the dew-point depression, or
the absolute difference between the air temperature and
the dew-point temperature (with respect to liquid
water), is reported in the TEMP code by DD, a coded
figure. Dew-point depression (always an unsigned
number), is normally calculated to the nearest tenth of a
degree Celsius, and encoded using WMO code table
0777. Code figures 00 through 50 report dew-point
depressions from 0.1°C through 5.0°C, respectively.
Code figures 56 through 99 represent dew-point
depressions rounded off to the nearest whole degree
from 06°C through 49°C (subtracting 50 from the coded
figure yields the dew-point depression in whole
degrees).
WINDS.—The group ddfff is used to report wind
direction and wind speed. The dd is the true direction in
tens of degrees from which the wind is blowing.
Observed wind directions are rounded off and reported
to the nearest 5 degrees, as specified by WMO
regulations. The fff is the wind speed in hundreds, tens,
and units. The units of speed are specified in the
Identification Data section. For example, a wind from
275° true at 159 knots is encoded 27659; winds of 275°
at 25 knots are encoded 27525, and winds of 270° at 25
knots are encoded 27025.
Tropopause Data
In table 1-8, tropopause data is contained in section
3 of message Part A, and may also be contained in Part C
in the identical format. Tropopause data is only reported
in the part of the message (A or C) that pertains to the
level of the atmosphere in which the tropopause is
located. The tropopause level is selected by the MRS
system as the base of the layer in which the temperature
stops decreasing with height or decreases very slowly
with height, normally between the 250 hPa and 200 hPa
level. Criteria on which the MRS system makes the
selection is contained in the Federal Meteorological
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