TM 5-822-5/AFM 88-7, Chap. 1
should he based on actual air temperatures obtained
If the temperature record of the station closest to
from the meteorological station closest to the
the construction site is not long enough to
construction site. This is desirable because dif-
determine the mean or design freezing index values,
ferences in elevation or topographical position, or
the available data should be related, for the same
nearness to bodies of water, cities, or other sources
period, to that of the nearest station or stations of
of heat may cause considerable variation in air
adequate record. Site air freezing index values can
freezing indexes over short distances. These vari-
then be computed based on this established relation
ations are of greater relative importance in areas of
and the indexes for the more distant station or
design freezing index of less than 1,000 degree
stations.
b. Design freezing index. The design freezing
Fahrenheit days (i.e. mean air freezing index of less
than about 500 degree Fahrenheit days) than they
index should he used in determining the combined
are in colder climates. The daily maximum and
thickness of pavement, base, and subbase required
minimum and mean monthly air temperature records
to limit subgrade frost penetration. As with any
for all stations that report to the U.S. National
natural climatic phenomenon, winters that are
Weather Service are available from Weather Service
colder than average occur with a frequency that
Centers. One of these centers is generally located in
decreases as the degree of departure from average
each state. The mean air freezing index may be
becomes greater. A mean freezing index cannot be
based on mean monthly air temperatures, but
computed where temperatures in some of the win-
computation of values for the design freezing index
ters do not fall below freezing. A design method has
may he limited to only the coldest years in the
been adopted that uses the average air freezing
desired cycle. These years may be selected from the
index for the three coldest years in a 30-year period
tabulation of average monthly temperatures for the
(or for the coldest winter in 10 years of record) as
nearest first-order weather station. (A local
the design freezing index to determine the thickness
climatological data summary containing this
of protection that will he provided. A distribution of
tabulation for the period of record is published
design freezing index for North America is shown
annually by the National Weather Service for each
in figure 18-2 and is to be used as a guide only.
of the approximately 350 U.S. first-order stations.)
18-7
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