REFRACTIVITY GRADIENTS IN THE NORTHERN HEMISPHERE
C.A. Samson*

The continued expansion of microwave radio links and the
resulting congestion have increased the need for better
performance estimates. In the evaluation of refractivity
effects, the designer may wish to consider the average
gradients at specific locations for different seasons,

as well as diurnal changes. This report presents graphs
showing the cumulative probability distributions of the
atmospheric radio refractivity gradients in the ground-
based 100-m layer for 87 stations in the Northern Hemi-
sphere. These are based on climatological data from
radiosonde observations, and show the average conditions
in one month of each season. A limited number of diurnal
comparisons are included, as well as information on tke
climate of each site.

Key words: Refractivity gradients, radiosonde data

1. INTRODUCTION

The increasing demand for radio services, particularly those
requiring wide bandwidths, is a universal problem. In the United States,
for example, there has been rapid development in recent years of the
specialized common carrier industry and increased utilization of CARS**
microwave links. Because of frequency congestion, many new systems are
using frequencies above 10 GHz, which are more susceptible to various
atmospheric effects than the lower microwave frequencies.

One of the most important influences on system performance is
the bending of the radio beam caused by variations in the vertical
refractivity gradient. This is a factor in the determination of the
maximum feasible path length, the probability of multipath fading or
ducting, and the ultimate reliability of a given link and system. The

*The author is with the Institute for Telecommunication Sciences, Office
of Telecommunications, U.S. Dept. of Commerce, Boulder, Colorado 80302.

**Community Antenna Relay Service


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 (RAOBsS) 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

the refractive index be calculated for the individual data points on