B5-30 PREPARATION OF CHARTS 
humidity pertaining to the first (lowest) crossing and the number of 
crossings will be determined. The method for determining the altitude is 
described inJB5-15.9.The relative humidity will be determined by noting 
on the radiosonde recorder record, the humidity ordinate opposite the 
point where the temperature trace reached 0°C; allappropriate correc- 
tions should be applied. Freezing-level data will be transmitted in ac- 
cordance with instructions in the Federal Meteorological Handbook No. 1 
(Surface Observations). 
15.8 Icing-Level Data. Icing of the raob balloon will be assumed under 
the conditions described in §B4-6.6(h), and the altitude pertaining to the 
beginning of such conditions will be determined in accordance with in- 
structions inJB5-15.9.Icing-level data willbe transmitted in accordance 
with instructions in Federal Meteorological Handbook No. 1 (Surface 
Observations). 
15.9 Determination of the Altitude of Freezing and Icing Levels. Table 
16 will be used to determine the altitude above sea level of freezing and 
icing levels. The table is based upon the ICAO Standard Atmosphere, and 
altitudes of levels determined by means of it will, therefore, usually dif- 
fer from altitudes of the same levels determined by means of the adia- 
batic chart. Since the altimeters used in mostaircraft are calibrated to 
the standard atmosphere, altitudes indicated by analtimeter will gener- 
ally be in much closer agreement with altitudes determined by the table 
than with those determined by an adiabatic chart. Determine from an 
inspection of the recorder record the contact corresponding to the icing 
or freezing level. In the former case, it will be the first lengthened con- 
tact; and in the latter, the first contact above the surface corresponding 
to a temperature of O°C. 
a. Find the pressure corresponding to this contact from the pres- 
sure calibration chart furnished with each radiosonde, and 
from Table 16, find the altitude to the nearest hundred feet 
corresponding to this pressure. 
b. Use Table 16 in the same manner to find the altitude in hun- 
dreds of feet corresponding to the station pressure at the time 
of release of the radiosonde. 
c. Subtract algebraically the altitude in (b) from the altitude 
found in (a). Add the station altitude in feet to the remainder. 
The resultant value will be the required altitude in feet, m.s.l. 
Note that this subtraction must be made algebraically, as 
shown in the following example. 
1-1-69 FMH#3 
