RELEASE AND RECORDER RECORD B4-15 
close to freezing or below, and the relative humidity high. 
The selection of a level at the base of a stratum within which 
icing occurs should be correlated with the selection of RAICG 
and RAFRZ data. 
i. Ata pressure corresponding to any mandatory level (manda- 
tory levels are as specified in the Federal Meteorological 
Handbook No. 4 (Radiosonde Code)) unless for some other 
reason a level has already been selected within +100 mb. of 
the pressure at the mandatory level. 
6.7 Selecting Significant Relative Humidity Levels. Inselecting levels 
for relative humidity, any points where the relative humidity deviates 10 
percent or more in linearity from the relative humidity curve is signifi- 
cant and a level is placed at this point. Departure from linearity is es- 
tablished by placing a straight-edge between relative humidity ordinate 
values of two consecutive levels previously selected. The horizontal 
distance of any trace on the curve from the straight-edge is the depar- 
ture from linearity. The level selected in accordance with this procedure 
should be placed at the point of maximum departure from linearity, ex- 
cept that if an accompanying change in temperature lapse rate or a re- 
quirement for an additional level should make it desirable, the level may 
be placed within one contact above or below this point. The maximum 
humidity departure may be extended (vertically on the recorder 
record) to this level provided it is not 10 percent less or greater than 
the actual recorded relative humidity and that by doing so, no relative 
humidity trace, between the extended trace and the previous significant 
level, departs from linearity by 10 percent or more. When relative hu- 
midity level selection is continued above this level, the extended relative 
humidity trace will be considered rather than the actual trace. 
Relative humidity will be evaluated on the recorder record only for 
levels with temperatures of -40°C or warmer and within the portion of 
the recorder record where relative humidity data are normally received. 
A significant level, based on either temperature or humidity must be 
placed within the -37°C to -40°C portion of the recorder record. This 
level for the final measurable humidity, however, must be placed at the 
highest point necessary to provide the maximum amount of relative hu- 
midity data for the CPD blocks. 
Example: Pressure at -37°C is 312 mb. Pressure at -40°C is 289 mb. 
In this case a level would be selected at or between 289 and 300 mb, to 
provide relative humidity data for the 300 mb CPD block. 
IL 1=@g) FMH#3 
