information for "A World Atlas of Atmospheric Radio Refractivity"

(Bean et al., 1966) are also included as Appendix D.

3. DATA PRESENTATION
Scale. Each graph has a dual scale on the ordinate to show both the
refractivity gradient in N-units/km and the equivalent effective earth
radius factor, "k", which is used to compensate for ray bending in
terrain profile analysis of radio links. The factor "k" is related to

the refractivity gradient AN/Ah in N-units/km by

-1
~ AN a
ke 1 + 6370 77 x 10

6

where 6370 km represents the actual earth radius (Dougherty, 1968) .

Time. Prior to 1957 the standard time of RAOBs was 0300Z and 1500z;
since then it has been 0000Z and 1200Z. In this report, all times of
observation have been converted to an arbitrary "Local Standard Time"
(LST) based on the assumption of zones exactly 15 degrees wide and
centered on longitudes of 0°, 15°, 45°, etc. Figure 1 has a scale for
comparing the local time of observation with the standard 0000zZ and

1200Z observations based on the local time on the Greenwich meridian.

Refractivity Distributions. Unless otherwise indicated, the graphs show

the cumulative distribution of the radio refractivity gradients in the
ground-based 100-m layer, as calculated from radiosonde observations
during the period specified under "Data". The analyses are by ITS,
except where other agencies or individuals are referenced below the,
graph.

Each curve shown is based on all observations in a particular
month for the period indicated, and includes data obtained from observa-
tions at more than one time of day, if available. For Aden and Nicosia,
however, the data were analyzed separately for the different times of
observation, and additional graphs are presented which indicate the
characteristic differences between observations at different times of
day at these stations. For shorter periods, diurnal characteristics

are also shown for Calcutta, Clark Field, Tampa, and Tan An.


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radio path of most terrestrial line-of-sight microwave links is rela-
tively close to the surface, and the gradients in the lowest 100-m layer
are therefore more suitable for propagation estimates than the gradients
over the lowest 1 km. Microwaves may be affected by atmospheric layers
of rather limited vertical extent, and 1-km layer statistics tend to
smooth out many of the extreme gradients.

Information on the refractivity gradients to be expected in
any part of the world is available in the "World Atlas of Atmospheric
Radio Refractivity" (Bean et al., 1966). This Atlas contains maps
showing the 100-m gradients exceeded for selected percentages of time.
For application to the design of radio links, many engineers prefer a
complete cumulative time probability distribution of the gradients at
specific locations. The Atlas contains such distributions of the 50-m
gradients at 22 stations worldwide; the present report supplements the
Atlas by providing distributions of 100-m gradients at 87 stations in
the Northern Hemisphere (see map, figure 1, and station index, Appen-
dix C). Data for four months of the year are shown on the same graph to
facilitate seasonal comparisons. Accompanying each graph is information
on the length of record analyzed, the hours at which observations were
taken, and general climatic and topographic details in the vicinity of

the station.
2. SOURCE OF DATA

Refractivity gradients can be calculated from the radiosonde
observations (RAOBs) made by national meteorological services in the
various countries (see Appendix A). Although these observations of the
vertical changes in temperature, pressure, and humidity do not provide
as much detail and accuracy as is desirable for studies of radio refrac-
tivity, they are the only available source of worldwide, long-term,
upper-air data.

Climatological RAOB data for the U.S. and many foreign loca-
tions are on file at the National Climatic Center in Asheville, N.c. The
analysis of these data to obtain refractivity statistics requires that

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each sounding; an interpolation must be made to obtain the refractivity
value at the desired height (for example, 100 meters above ground); and
the gradient over the interval (ground surface to 100 m) can then be
calculated. To avoid possible misleading indications related to the
year-to-year variations in weather conditions, data for a number of years
must be considered if a good climatological average is to be obtained.

Most of the gradient distributions in this report were calcu-
lated at the Institute for Telecommunication Sciences (ITS) in Boulder
as a part of a project sponsored by the U.S. Navy Weather Research
Facility to prepare worldwide refractivity information in map format
(Bean et al., 1966). To the extent possible at the time, climatic data
for a 5-year period, including two observations per day, were used in
this analysis. Only one month in each season was processed, and the
observations at different times of day were not treated separately, with
the exceptions of Aden and Nicosia. Stations were selected to provide
wide geographical coverage, rather than to give a dense coverage in a
few areas (i.e., funding limitations prevented any attempt to analyze
all available RAOB data). In the U.S., for example, figure 1 shows that
data from only 22 stations have been analyzed; figure 2 indicates a
total of 85 stations in the domestic RAOB network of the National
Weather Service as of October 1973 (NOAA, 1973). Thus, greatly increased
density of coverage of refractivity information is possible in the U.S.,
but it should be noted that figure 2 indicates only locations where the
basic data (temperature, pressure, humidity) would be available; refrac-
tivity is not routinely calculated at most RAOB stations.

This report includes additional refractivity data based upon
work done in other projects or by institutes in other countries. Not all
analyses cover periods of several years--a few are based on data for
only one month. While these shorter periods do not provide good clima-
tological statistics, the analyses have been included to illustrate the
type of diurnal variations to be expected, to indicate a later verifica-
tion of an earlier analysis, or to provide information on areas where

no other refractivity data were available. Errata and supplementary