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 TTT_a. 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 T_a. 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 1-25