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Estimation of Tropospheric Radio Refractivity and Its Variation with Meteorological Parameters over Ikeja, Nigeria

Authors:
_____________________________________________________________________________________________________
*Corresponding author: E-mail: profdon03@yahoo.com;
Journal of Geography, Environment and
Earth Science International
10(1): 1-12, 2017; Article no.JGEESI.32534
ISSN: 2454-7352
SCIENCEDOMAIN international
www.sciencedomain.org
Estimation of Tropospheric Radio Refractivity and
Its Variation with Meteorological Parameters over
Ikeja, Nigeria
D. O. Akpootu
1*
and M. I. Iliyasu
2
1
Department of Physics, Usmanu Danfodiyo University, Sokoto, Nigeria.
2
Physics Unit, The Polytechnic of Sokoto State, Sokoto, Nigeria.
Authors’ contributions
This work was carried out in collaboration between both authors. The data for the work was sourced
and analyzed by author DOA. Author DOA also designed the study, drafted and edited the
manuscript. Author MII assisted in literature searches. Both authors read and approved the final
manuscript.
Article Information
DOI: 10.9734/JGEESI/2017/32534
Editor(s):
(1) Iovine Giulio, CNR-IRPI (National Research Council-Institute of Research for the Geo-hydrologic Protection) of
Cosenza, Italy.
(2)
Teresa Lopez-Lara,Autonomous University of Queretaro, Qro, Mexico.
Reviewers:
(1) Purushottam Bhawre, Barkatullah University, Bhopal, India.
(2)
Oluwaseun Ajileye, Obafemi Awolowo University, Ile-Ife, Nigeria.
Complete Peer review History:
http://www.sciencedomain.org/review-history/18869
Received 1
st
March 2017
Accepted 18
th
April 2017
Published 1
st
May 2017
ABSTRACT
Estimation of radio refractivity is critical in the planning and design of radio links/systems for the
purpose of achieving optimal performance. This present work investigates the tropospheric radio
refractivity over Ikeja, Lagos State, South Western, Nigeria (Latitude 6.58°N, Longitude 3.33°E and
altitude 40 m above sea level) and the sensitivity of radio refractivity due to meteorological
parameters of monthly average daily atmospheric pressure, relative humidity and temperature for a
period of 12-years. The statistical estimation of tropospheric radio refractivity has been evaluated
using the method recommended by the International Telecommunication Union (ITU). The result
indicated that the radio refractivity during the rainy season is greater than the dry season. It
was observed that the maximum average value of radio refractivity of 389.45 N-units and
minimum average value of 373.04 N-units occurred during the rainy and dry seasons in the
months of April and January respectively. The dry term contributes 67.98% to the total value of the
Original Research Article
Akpootu and Iliyasu; JGEESI, 10(1): 1-12, 2017; Article no.JGEESI.32534
2
radio refractivity while the wet term contributes to the major variation. The average refractivity
gradient computed for the study area under investigation was
−44.32
N-units/km and the
average effective earth radius (k factor) was 1.39 which corresponds to the conditions of super-
refraction.
Keywords: Radio refractivity; dry term; wet term; meteorological parameters; refractivity gradient.
1. INTRODUCTION
The part of the atmosphere most closely related
to human life is the troposphere. It is the lowest
layer of earth’s atmosphere and region of all
weather on earth. The troposphere extends from
the earth’s surface to an altitude of about 10 km
at the earth’s poles and 17 km at the equator [1].
Since temperature decreases with altitude in the
troposphere [2], warm air near the surface of the
earth can rapidly rise replacing the cold dense air
at the upper part of the atmosphere. This will set
up convection current in the air molecules of the
troposphere. Such vertical movement or
convection current creates clouds and ultimately
rain from the moisture within the air, and
gives rise to the weather condition we
experience. At the lower part of the earth
called the troposphere, the tropospheric
refraction is due to the fluctuations of weather
parameters like temperature, pressure and
relative humidity [3].
Radio signals can be reflected, refracted,
scattered, and absorbed by different atmospheric
constituents [4]. However, the degree of
atmospheric effects on radio signals depends
mainly upon the frequency, power of the signal
and on the state of the troposphere through
which the radio wave propagates. The
characterization of tropospheric variability has
great significance to radio communications,
aerospace, environmental monitoring, disaster
forecasting etc. The quality of propagation of
radio waves transmitting to a receiving antenna
mostly depends on performance and reliability of
the links [5]. Generally for radio link design, the
measured data for signal strength at a particular
location under study is required by radio-
planning-engineers [6]. Consequently, a radio
propagation model is required to be used for the
evaluation of signal level variations that occur
at various locations of interest over different
times of the year. An important element of such
type of radio propagation model is the variation
of radio refractivity in the troposphere [7].
According to [5], radio wave systems could
become unavailable due to seasonal variation of
refractive index.
The structure of the radio refractive index, n, at
the lower part of the atmosphere is a very
important parameter in the planning of the
communication links. It is defined as a ratio of the
radio wave propagation velocity in free space to
its velocity in a specified medium [8]. Radio–
wave propagation is determined by changes in
the refractive index of air in the troposphere [9].
Changes in the value of the radio refractive index
in the troposphere can curve the path of the
propagating radio wave. The atmosphere radio
refractive index depends on air temperature,
humidity, atmospheric pressure and water
vapour pressure. Even small changes in any of
these variables can make a significant influence
on radio-wave propagation, because radio
signals can be refracted over the whole signal
path [10]. Refractive index is not constant in
the atmosphere and its space-time
distribution results in scattering, sub-refraction,
super-refraction, ducting and absorption
phenomena [11].
The variation of refractive index is due to various
phenomena affecting the propagation of radio
signal, which for instance include refraction,
bending, ducting and scintillation, range and
elevation errors in radar acquisition and radio-
station interference [8,12–15]. The variation of
refractive index as well as specific attenuation of
micro/radio wave may be estimated indirectly
with the measurement of temperature, pressure
and relative humidity. The effect of temperature
and relative humidity on specific attenuation of
microwave was studied by different researchers
[16,17].
The establishment of a radio refractive index
database is necessary because the knowledge of
radio refractive index is always required when
measurements are made in air [18,19]. Several
research work on radio refractivity for different
regions and climates using measured local
meteorological data have been investigated in
Nigeria and other parts of the world. This include
e.g., [9,20–30] to mention but a few. The results
of their works show that the local climate has an
appreciable influence on the radio refractivity and
hence on the transmitted radio signals.
The purpose of this study is to estimate the
tropospheric radio refractivity and its variation
with meteorological parameters of atmospheric
pressure, relative humidity and temperature for
Ikeja, Lago
s state, Nigeria during a period of 12
years.
2. STUDY AREA
Fig. 1 shows the study region for Ikeja, a coastal
area, and the capital of Lagos State, Nigeria. The
state is located in the south-
western part of
Nigeria. The state has common boundary with
O
gun State, Republic of Benin and terminates in
Atlantic Ocean in the south [31
]. The state has
twenty local government areas out of which
sixteen are within the metropolitan Lagos; the
land coverage is about 3,475 km
2
. However, the
size of this land coverage is reduced by
Lagoons, rivers, creeks and swamps. The Lagos
city is the commercial centre of Nigeria where
several businesses are found. For example, the
Murtala International Airport in Ikeja and head
offices of m
any airlines are within and around the
Fig. 1.
Map of (a) Africa showing the location of Nigeria (b) Nigeria showing the location of
Ikeja
in Southwest Nigeria and (c) Ikeja showing the location of the meteorological station in
murtala
Akpootu and Iliyasu; JGEESI, 10(1): 1-12, 2017
; Article no.JGEESI.32534
3
The purpose of this study is to estimate the
tropospheric radio refractivity and its variation
with meteorological parameters of atmospheric
pressure, relative humidity and temperature for
s state, Nigeria during a period of 12
-
Fig. 1 shows the study region for Ikeja, a coastal
area, and the capital of Lagos State, Nigeria. The
western part of
Nigeria. The state has common boundary with
gun State, Republic of Benin and terminates in
]. The state has
twenty local government areas out of which
sixteen are within the metropolitan Lagos; the
. However, the
size of this land coverage is reduced by
Lagoons, rivers, creeks and swamps. The Lagos
city is the commercial centre of Nigeria where
several businesses are found. For example, the
Murtala International Airport in Ikeja and head
any airlines are within and around the
airport, the city and the state also accommodates
headquarters of many companies. As a coastal
city, rising temperature and increase in sea level
could lead to disappearance of the beaches
under erosion and flooding,
the area mi
be damaged by storm [31
]. The state is
essentially a Yoruba-
speaking environment. The
seasons in the area is broadly divided into dry
and wet under the influence of Intertropical
Convergence Zone, ITCZ (where easterly trade
winds origin
ating from northern and southern
hemispheres converge) that migrates along with
the position of strong rainfall [32]. Nigeria being a
tropical region has two seasons
the dry. The wet season is characterized by
heavy rainfall. The season falls
months of April and October. The dry season, on
the other hand, is characterized by scanty or no
rainfall and dry dust laden atmosphere. The
season falls between th
e month of November
and March [3
]. It must be noted that some areas
in Lagos St
ate, which is very close to the Atlantic
Ocean, experience rainfall almost throughout the
months of the year [33].
Map of (a) Africa showing the location of Nigeria (b) Nigeria showing the location of
in Southwest Nigeria and (c) Ikeja showing the location of the meteorological station in
murtala
muhammed international airport
; Article no.JGEESI.32534
airport, the city and the state also accommodates
headquarters of many companies. As a coastal
city, rising temperature and increase in sea level
could lead to disappearance of the beaches
the area mi
ght also
]. The state is
speaking environment. The
seasons in the area is broadly divided into dry
and wet under the influence of Intertropical
Convergence Zone, ITCZ (where easterly trade
ating from northern and southern
hemispheres converge) that migrates along with
the position of strong rainfall [32]. Nigeria being a
tropical region has two seasons
– the wet and
the dry. The wet season is characterized by
heavy rainfall. The season falls
between the
months of April and October. The dry season, on
the other hand, is characterized by scanty or no
rainfall and dry dust laden atmosphere. The
e month of November
]. It must be noted that some areas
ate, which is very close to the Atlantic
Ocean, experience rainfall almost throughout the
Map of (a) Africa showing the location of Nigeria (b) Nigeria showing the location of
in Southwest Nigeria and (c) Ikeja showing the location of the meteorological station in
Akpootu and Iliyasu; JGEESI, 10(1): 1-12, 2017; Article no.JGEESI.32534
4
3. METHODOLOGY
The measured daily climatic data of atmospheric
pressure, relative humidity and temperature
utilized in this present work were obtained
online from Tutiempo Network, S.L
(en.tutiempo.net/climate/ws-652010.html). The
daily data were averaged into monthly data. The
study area under investigation is Ikeja (Latitude
6.58 °N, Longitude 3.33 °E and altitude 40 m
above sea level) with weather station number
652010 (DNMM). To avoid possible misleading
indications related to yearly variation in weather
condition, the period under focus is twelve years
(2001, 2005 – 2007, 2009 – 2016) in order to
obtain a good climatological average. The quality
assurance of the meteorological measurements
was determined by checking the overall
consistency of the monthly average of the
climatic parameters used in the study
area.
The refractive index, , of the atmosphere is
dependent upon three factors, the atmospheric
pressure, temperature and humidity (water
vapour content). The value of the refractive
index, , is very close to unity (varying between
1.000250 and 1.000400) at or near the earth’s
surface and changes in this value is very small in
time and in space. With the aim of making them
more noticeable, the refractive index, , of air is
measured by a quantity called the radio
refractivity, , which is related to the refractive
index, , as discussed in [8,34]:
=1+×10

(1)
Although, as the radio refractivity, , is a
dimensionless quantity, it is expressed in N-units.
Therefore from equation (1) it is easy to deduce
that typically ranges between 250 and 400 N-
units. In terms of meteorological parameters, the
International Telecommunication Union (ITU) has
recommended the radio refractivity, , to be
expressed as [35]:
=
.
+4810
=

+

(2)
with the “dry term” of radio refractivity given by:

=77.6
(3)
and the “wet term” of radio refractivity given by:

=3.73×10
(4)
Where, is the atmospheric pressure (ℎ), is
the absolute temperature () and is the water
vapour pressure (ℎ). The dry term is due to
non-polar nitrogen and oxygen molecules. It is
proportional to pressure, P, and therefore related
to the air density. The wet term is proportional to
vapour pressure and dominated by polar water
contents in the troposphere.
It was mentioned in [34] and [8] that the
expression (2) may be used for all radio
frequencies; for frequencies up to 100 , the
error is less than 0.5%. At sea level, the average
value of 315 [34] is used.
The relationship between water vapour pressure,
, and relative humidity is given by the
expression as [34]:
=


(5)
Where,
=exp


(6)
Where is the relative humidity(%), is the
Celsius temperature () and
is the saturation
vapour pressure (ℎ) at temperature (). The
values of the coefficients ,  and (for water and
ice) are presented in [34]. In this study, that for
water were adopted and are given as =6.1121
=17.502 and  = 240.97 and are valid
between −20° to +50° with an accuracy
of ±0.20% . The radio refractivity, , also
decrease exponentially in the troposphere with
height [34].
=


(7)
Where is the refractivity at the height ()
above the level where the refractivity is
while
is the applicable scale height. [34] Suggested
that at average mid-latitude,
and are 315
and 7.35 km respectively. Hence, as a function
of height () is given by the equation.
(8)
However, the results of the work of [36] showed
that the model using the scale height of 7.35 km
and 7.00 km, as recommended for global
environment [34] and tropical environment [2]
respectively, gave reasonably accurate results
for the refractivity at the altitude of 50 m and 200
m for seven out of the twelve months of the year.
Akpootu and Iliyasu; JGEESI, 10(1): 1-12, 2017; Article no.JGEESI.32534
5
Although the scale height of 7.00 km gave a
better result at 50 m altitude while 7.35 km scale
height gave a better performance at 200 m.
The refractivity gradient is obtained by
differentiating equation (7) with respect to ,
thus, we’ve the refractivity gradient as


=



(9)
For a standard atmosphere, the refractivity
gradient is −39 N-units/km. According to [2],
when h < 1 km, refractivity gradient is well
approximated by its value in a standard
atmosphere. In this work we used the typical
values for a standard atmosphere [2], and
obtained the refractivity of a standard
atmosphere [2] as
= 312 − .
The vertical gradient of refractivity in the lower
layer of the atmosphere is an important
parameter in estimating path clearance and
propagation effects such as sub-refraction, super
refraction, or ducting according to the following
criteria [9]:
- Sub-refraction:


>−40
Refractivity N increases with height and in this
case (sub-refraction), the radio wave moves
away from the earth’s surface and the line of
sight range and the range of propagation
decrease accordingly.
- Super-refraction:


<−40
During super-refractive conditions,
electromagnetic waves are bent downward
towards the earth. The degree of bending
depends upon the strength of the super-
refractive condition. The radius of curvature of
the ray path is smaller than the earth’s radius
and the rays leaving the transmitting aerial at
small angles of elevation will undergo total
internal reflection in the troposphere and it will
return to the earth at some distance from the
transmitter. On reaching the earth’s surface and
being reflected from it, the waves can skip large
distances, thereby giving abnormally large
ranges beyond the line of sight due to multiple
reflections.
- Ducting:


<−157
During ducting phenomena, the waves bend
downwards with a curvature greater than that of
the earth. Radio energy bent downwards can
become trapped between a boundary or layer in
the troposphere and the surface of the earth or
sea (surface duct) or between two boundaries in
the troposphere (elevated duct). In this wave
guide-like propagation, very high signal strengths
can be obtained at very long range (far beyond
line-of-sight) and the signal strength may exceed
its free-space value.
The effective earth radius factor can be used to
characterise refractive conditions as normal
refraction or standard atmosphere, sub-
refraction, super-refraction and ducting
respectively. Thus, may be expressed in terms
of refractivity gradient,  ℎ
as [8,14] and
[37,38].
 ≈ 1+




(10)
Near the earth’s surface,


is about −39 N-
units/km which gives an effective earth radius
factor =
. This is referred to as normal
refraction or standard atmosphere. Here, radio
signals travel on a straight line path along the
earth’s surface and go out to space
unobstructed.
If
> >0 Sub-refraction occurs, meaning that
radio waves propagate abnormally away from the
earth’s surface.
When > >
In this case, Super-refraction
occurs and radio waves propagate abnormally
towards the earth’s surface thus extending the
radio horizon.
If < <0 ducting occurs and the waves
bend downwards with a curvature greater than
that of the earth.
4. RESULTS AND DISCUSSION
4.1 Radio Refractivity
Fig. 2 depicts the seasonal variation of radio
refractivity for the study area and period under
investigation. The radio refractivity at Ikeja
showed gradual increase from a minimum value
of 373.04 N-units in the month of January until it
climaxed at 389.45 N-units in the month of April
and decreases gradually until it gets to 380.60 N-
units in August which suddenly increases until it
Akpootu and Iliyasu; JGEESI, 10(1): 1-12, 2017; Article no.JGEESI.32534
6
reaches another peak value of 387. 41 N-units in
the month of November and drops to 380.81 N-
units in December. The maximum average value
of radio refractivity observed for the study area is
389.45 N-units in the month of April and the
minimum value of 373.04 N-units in January. The
pattern of variation can be attributed to rain
pattern in Ikeja over the period under study. The
results showed that high values of refractivity are
observed during the rainy season, with an
average value of 385.99 N-units, and low values
during the dry season, with an average value of
382.94 N-units. The noticeable drop in the value
of refractivity in the month of August may be
associated to August break, that is, a short
period of dryness. The high values observed
during the rainy season (April - October) are due
to high air humidity (very close to 100%)
observed in this part of Nigeria, when the city of
Ikeja is under the influence of a large quantity of
moisture-laden tropical maritime air resulting
from continuous migration of inter-tropical
discontinuity (ITD) with the sun. Generally, when
the dry and dust laden north-east winds
become dominant in December, the dry
harmattan season sets in, resulting in lower
values of refractivity. The high values of radio
refractivity and its variation is in agreement with
the result reported by [23] for Lagos. However,
the slight difference may be due to the erratic
nature because of the influence of the Atlantic
Ocean for the coastal region.
Fig. 2. Monthly radio refractivity variation
over Ikeja, Nigeria
Fig. 3 shows the monthly variation of radio
refractivity with atmospheric pressure. The
atmospheric pressure decreases gradually from
January to its minimum value in March and
suddenly increases from March and reaches its
peak value of 1014.16 hPa in the month of
August. It was observed that as the atmospheric
pressure increases from April to August the
refractivity decreases from April to August. As a
dip downward was observed for the refractivity a
maximum peak value with dip upward was
observed for atmospheric pressure; this implies
that during August break the region experiences
the highest value of atmospheric pressure. It was
observed also that as the atmospheric pressure
decreases from August to December, the
refractivity increases from August to November
and drop to December. High values of
atmospheric pressure were observed during the
rainy season with an average value of 1012.76
hPa and low values during the dry season with
an average value of 1010.94 hPa, similarly, the
average values of refractivity are 385.99 N-units
and 382.94 N-units during the rainy and dry
seasons respectively. It is important to note here
that the variation (increase and decrease) of both
terms does not occur exactly on same months
and considering their values during the rainy and
dry season’s shows that the dry term of radio
refractivity is proportional to atmospheric
pressure and therefore related to air density. The
maximum average value of radio refractivity
observed for the study area is 1014.162 hPa in
the month of August and the minimum value of
1010.603 hPa in March.
Fig. 3. Monthly variation of radio refractivity
with atmospheric pressure over Ikeja, Nigeria
Fig. 4 shows the monthly variation of radio
refractivity and relative humidity. The relative
humidity increases with increase in radio
refractivity from January to April, however, the
relative humidity increases continuously until it
gets to June and maintain almost a constant
value from June to July. A little dip is noticed in
August both in relative humidity and radio
refractivity, though, that of the radio refractivity is
more conspicuous, and this observation is line
with the study reported by [25] for coastal
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov
Dec
372
374
376
378
380
382
384
386
388
390
Radio Refractivity (N-units)
Months of the year
Jan Feb Mar A pr May Jun Jul Aug Sep Oct Nov Dec
372
374
376
378
380
382
384
386
388
390
Radio Refractivity
Atmospheric Pressure
Months of the year
Radio Refractivity (N-units)
1010
1012
1014
Atmospheric Pressure (hPa)
Akpootu and Iliyasu; JGEESI, 10(1): 1-12, 2017; Article no.JGEESI.32534
7
regions. The relative humidity increases with the
radio refractivity from August to September,
however, the radio refractivity increases to
November while that of the relative humidity
decreases from September to December; the
sharp fall in the values of the relative humidity
and radio refractivity exhibited similar pattern in
December, this could be as a result of high solar
insolation observed in the month of December
that reduced humid content in the atmosphere,
thereby reducing the radio refractivity. The
values of relative humidity were high during the
rainy season with an average value of 85.12 %
and low during the dry season with an average
value of 78.25%, likewise the values of the radio
refractivity with an average value of 385.99 N-
units and 382.94 N-units during the rainy and dry
seasons respectively for the study area under
investigation. The maximum relative humidity
was observed during the rainy season in the
month of June with an average value of 87.23 %
and the minimum relative humidity during the dry
season in the month of January with an average
value of 73.15%.
Fig. 4. Monthly variation of radio refractivity
with relative humidity over Ikeja, Nigeria
Fig. 5 shows the monthly variation of radio
refractivity with temperature (Kelvin) for the study
area under investigation. The values of
temperature increases with radio refractivity from
January to March, the temperature started
descending from March to August. A little dip is
noticed in August both in temperature and radio
refractivity as in the case of the variation
between radio refractivity with relative humidity.
The values of the temperature gradually increase
given almost a straight line pattern from August
to December. However, those of the radio
refractivity increase from August to November
and drop in December. High values of
temperature were observed during the dry
season with an average value of 301.09 K and
low values during the rainy season with an
average value of 299.68 K. The high values of
refractivity observed during the rainy season are
as a result of high moisture or humidity content in
the atmosphere and low temperature. The
maximum temperature was observed during the
dry season in the month of March with an
average value of 302.13 K, this is the transition
period from dry to rainy season, the minimum
temperature was observed in August with an
average value of 298.38 K, this clearly indicated
that the lowest temperature values are observed
when the sky is partly cloudy and partly clear for
the study area under investigation.
Fig. 5. Monthly variation of radio refractivity
with temperature over Ikeja, Nigeria
Fig. 6 shows the variation of radio refractivity with
saturation vapour pressure. Observations and
trend of the pattern similar to that of variation
between radio refractivity and temperature (Fig.
5) was observed. However, the values of
temperature are higher than that of saturation
vapour pressure, this similarity in pattern shows
that the saturation vapour pressure is strongly
dependent on the temperature.
Fig. 7 shows the variation of N
dry
and N
wet
.
Considering the values obtained from this study,
it can be seen that the dry term of radio
refractivity is a major contributor to the total value
of the radio refractivity. The pattern of variation
exhibited by the dry term is almost similar to that
of atmospheric pressure (Fig. 3), this implies that
the dry term of radio refractivity is proportional to
the atmospheric pressure and related to air
density. On the other hand, we observed the
similarity in the trend exhibited by the wet term
and radio refractivity (Fig. 2), therefore, we can
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov D ec
372
374
376
378
380
382
384
386
388
390
Radio Refractivity
Relative Humidity
Months of the year
Radio Refractivity (N-units)
72
74
76
78
80
82
84
86
88
Relative Humidity (%)
Jan Feb Mar Apr May Jun Jul Aug Sep O ct Nov Dec
372
374
376
378
380
382
384
386
388
390
Radio Refractivity
Absolute Temperature
Months of the year
Radio Refractivity (N-units)
298
300
302
Absolute Temperature (K)
Akpootu and Iliyasu; JGEESI, 10(1): 1-12, 2017; Article no.JGEESI.32534
8
safely conclude that the wet term of radio
refractivity contributes to the major variation of
the radio refractivity.
Fig. 8 shows the monthly variation of radio
refractivity with refractive index. It was observed
that the values of radio refractive index and
consequently the radio refractivity showed
seasonal variation with high value during the
rainy season and low values during the dry
season. The average value of radio refractive
index during the rainy season is 1.000386 and
during the dry season is 1.000383. The value of
radio refractivity during the rainy season is
385.99 N-units and during the dry season is
382.94 N-units. The maximum values of radio
refractive index and radio refractivity were
observed during the rainy season in the month of
April as 1.000389 and 389.45 N-units
respectively and the minimum values during the
dry seasons in the month of January as
1.000373 and 373.04 N-units respectively.
Fig. 6. Monthly variation of radio refractivity
with saturation vapour pressure over Ikeja,
Nigeria
Fig. 7. Monthly variation of dry and wet terms
radio refractivity over Ikeja, Nigeria
Fig. 8. Monthly variation of radio refractivity
with refractive index over Ikeja, Nigeria
Fig. 9 shows the monthly variation of percentage
difference between the N
dry
and N
wet
(N
dry
-
N
wet
).The result indicated that the maximum
percentage differences occurred in the month of
January (about 8.97%) and the minimum in the
months of March and April (about 7.85%).
Fig. 9. Monthly variation of percentage
difference between the dry and wet term radio
refractivity over Ikeja, Nigeria
Table 1 shows the monthly variation of the
percentage contribution of both the dry term and
wet term radio refractivity to the total radio
refractivity. It was observed that the dry term
radio refractivity is a major contributor to the total
radio refractivity with 67.98% and the wet term
radio refractivity with 32.02% for Ikeja, Lagos
state, South Western, Nigeria during the period
under investigation. It was also observed that the
monthly maximum contribution of the dry term is
in the months of July and August with 5.71% and
minimum in the month of March with 5.62%,
similarly, the monthly maximum contribution for
the wet term is in the month of April with 2.81%
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
372
374
376
378
380
382
384
386
388
390
Radio Refractivity
Saturation Vapour Pressure
Months of the year
Radio Refractivity (N-units)
32
34
36
38
40
42
Saturation Vapour Pressure (hPa)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
260
262
264
N
dry
N
wet
Months of the year
N
dry
(N-units)
110
112
114
116
118
120
122
124
126
128
130
N
wet
(N-units)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
372
374
376
378
380
382
384
386
388
390
Radio Refractivity
Radio Refractive Index
Months of the year
Radio Refractivity (N-units)
1.000372
1.000374
1.000376
1.000378
1.000380
1.000382
1.000384
1.000386
1.000388
1.000390
1.000392
Radio Refractive Index
8.5%
8.17%
8.32%
8.48%
8.85%
8.59%
8.3%
8.01%
7.85%
7.85% 8.12%
8.97%
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Akpootu and Iliyasu; JGEESI, 10(1): 1-12, 2017; Article no.JGEESI.32534
9
and minimum in the month of January with
2.43%.
Table 1. Monthly variation of the percentage
contribution of N
dry
and N
wet
Month
N
dry
-N
wet
%CN
dry
%CN
wet
Jan 148.94 5.65 2.43
Feb 134.90 5.63 2.71
Mar 130.26 5.62 2.80
Apr 130.31 5.63 2.81
May 132.97 5.65 2.77
Jun 137.77 5.69 2.71
Jul 142.71 5.71 2.61
Aug 146.91 5.71 2.53
Sep 140.83 5.70 2.64
Oct 138.09 5.68 2.69
Nov 135.68 5.67 2.73
Dec 141.08 5.65 2.60
Tot=67.98 Tot=32.02
Fig. 10 shows the monthly variation of dry and
wet term radio refractivity. It was observed that
the dry term radio refractivity are the significant
contributors to the total radio refractivity, the
highest value of N
dry
is recorded in the month of
August as 263.75 N-units and the lowest in the
month of March as 259.45 N-units. On the other
hand, the highest value of N
wet
is recorded in the
month of April as 129.57 N-units and the lowest
in the month of January as 112.05 N-units.
Fig. 10. Monthly variation of dry and wet term
radio refractivity over Ikeja, Nigeria
4.2 Refractivity Gradient
The refractivity gradient obtained for the study
area under investigation using equations (9) is
−44.32 N-units/km. The implication of the result
is that propagation in this geographic region is
mostly super-refractive, which implies that the
electromagnetic waves are bent downward
towards the earth. The degree of bending is a
function of the strength of the super-refractive
condition. As the refractivity gradient continues to
decrease, the wave path’s curve will approach
the radius of curvature of the earth. Super-
refraction occurs when the bending of the
trajectory of propagating radio wave bends
towards the ground surface is greater than its
bending in case of normal positive refraction.
4.3 Effective Earth Radius
The effective earth radius, k – factor obtained for
the study area under investigation using equation
(10) is 1.39. The implication of the result is that
propagation in this geographic region is mostly
super-refractive. Super refraction results from
such meteorological conditions as a rise in
temperature with increasing height (temperature
inversion), or a marked decrease in total
moisture content in the air, either of which will
cause a reduction in the dielectric constant
gradient with height. Under these situations the
K-factor increases resulting in an effective
flattening of the equivalent earth’s curvature. One
of the conditions which may cause this type of
abnormal refraction is the passage of warm air
over a cool body of water and water evaporation
can cause an increase in moisture content and a
decrease in temperature near the surface,
therefore producing a temperature inversion.
However, it is not only the temperature inversion
itself which causes the abnormal bending of the
microwave beam. The large increase in water
vapour content and hence, the dielectric constant
near the surface further increases this effect.
5. CONCLUSION
The issue of estimating tropospheric radio
refractivity under varying meteorological
conditions has been addressed. The method
recommended by the International
Telecommunication Union (ITU) has been
adopted in the evaluation of tropospheric radio
refractivity for Ikeja during the period under
investigation. The variation of radio refractivity
with meteorological parameters of atmospheric
pressure, relative humidity and temperature for a
period of 12-years has been investigated. The
results indicated that an average value of 385.99
N-units and an average value of 382.94 N-units
were observed during the rainy and dry seasons
respectively. This shows obviously that radio
refractivity during the rainy season is greater
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0
50
100
150
200
250
N
dry
& N
wet
(N-units)
Months of the year
Ndry
Nwet
Akpootu and Iliyasu; JGEESI, 10(1): 1-12, 2017; Article no.JGEESI.32534
10
than the dry season for the study area. The
maximum and minimum values of radio
refractivity were observed in the months of April
and January. The variation of radio refractivity
with atmospheric pressure indicates that both
radio refractivity and atmospheric pressure are
greater during the rainy season than in the dry
season. Similar observations were observed for
relative humidity. However, the variation of radio
refractivity with temperature indicated that high
values of radio refractivity and temperature were
observed during the rainy and dry seasons
respectively and low values of radio refractivity
and temperature were observed during the dry
and rainy seasons respectively. The dry term
contributes 67.98% to the total value of the radio
refractivity while the wet term contributes to the
major variation. The average refractivity gradient
and k – factor obtained from this study area
under investigation are −44.32 N-units/km and
1.39 respectively. The implication of the result is
that propagation in this geographic region is
mostly super-refractive. The results obtained
from this study are highly critical for optimal
planning and design of radio links/systems for
the study area under investigation and regions
with similar climatic information.
ACKNOWLEDGEMENTS
The authors are grateful to the Tutiempo
Network, S.L for providing all the necessary
meteorological data used in this present study.
The contribution and suggestions of the
anonymous reviewers that led to improving the
paper is well appreciated.
COMPETING INTERESTS
Authors have declared that no competing
interests exist.
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________________________________________________________________________________
© 2017 Akpootu and Iliyasu; This is an Open Access article distributed under the terms of the Creative Commons Attribution
License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any
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Peer-review history:
The peer review history for this paper can be accessed here:
http://sciencedomain.org/review-history/18869
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This work investigates the impact of radio refractivity over Enugu, South Eastern Nigeria using atmospheric parameters of temperature, pressure and relative humidity collected from August 2017 to July 2018 respectively, we downloaded our data from Automated Weather Station and Signal Strength Meter in the and Technology using Davis weather station vantage pro 2 positioned close to the ground surface. The data were logged at 30 minutes' interval continuously for each day during the period. Hourly, daily and monthly averages of radio refractivity during dry and wet seasons were calculated from the data obtained. The result indicated that the radio refractivity during the wet season is greater than the dry season. This is as a result of variation in atmospheric parameters such as relative humidity, pressure and temperature, which cause the radio refractivity to vary at different times of the day; while the pressure variation seems to be insignificant. However, results of the refractivity show that the propagation conditions have varying degree of occurrence with super-refractivity conditions observed to be prevalent throughout the one-year period. The months of December and January have the lowest value of refractivity and June and July have the lowest vale of refractivity.
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Temporal variations in surface refractivity, N, in the lower part of the atmosphere is an important parameter in the study of micro-meteorology and in planning of terrestrial communication links. Previous studies across West Africa on surface refractivity were based on extrapolated data from radiosonde measurements which lacked the required resolution and coverage necessary for the investigations of spatial and temporal variations of surface refractivity across West Africa particularly in the lower atmosphere. In this study, West Africa continental areas have been partitioned into four climatic zones: Mangrove Rain Forest (below latitude 5 o N); Tropical Rain Forest (within 5 o N and 10 o N); Guinea Savannah (within 10 o N and 15 o N); and Sudan Sahel (within 15 o N and 20 o N). The Surface Meteorology and Solar Energy (SSE) dataset used for this study are satellite and model-based products covering thirty-six meteorological stations in four climatic zones across West Africa within Latitude 3 o and 20 o N. The data contained three variables including atmospheric pressure at 2 m and 10 m, temperature at 2 m and 10 m, and relative humidity at 2 m and 10 m within a period of 22 years (1983-2005). The results showed that the range of deviation was determined by seasonal moisture distribution. Seasonal variation of surface refractivity in climatic zones 1 and 2 showed similar trends of bimodal pattern and a deep in August due to short break in rainfall. Surface refractivity values were generally low during the dry months and high during the wet months. Surface refractivity was observed to have higher values at 2 m than 10 m.
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The variability in the temperature and Relative Humidity (RH) observed within Lagos State, a coastal region in Nigeria, is investigated using data from Nigeria Meteorological Agency (NIMET) for 1980 to 2010. The results reveal an annual mean value of 27.20°C and 83.01% for the temperature and RH respectively and an increasing trend in RH over the study period while such rising trend in the mean temperature is reversed from 2005 to 2010. The findings show an inverse relationship between the temperature and RH while it further indicates that low temperature is associated with increased rainfall under the accompanying cloudy condition and vice versa. These observations are supported by the strong correlation coefficients between the RH and the rainfall (0.72) and that between the RH and the temperature (-0.95) while-0.59 is obtained between the rainfall and the corresponding temperature. The correlations show that the impacts of the precipitation on RH are stronger than the effects it has on the temperature while both temperature and RH strongly depends on each other. Hence, under a future global warming, extremely warm atmospheric condition characterized by high RH in the coastal region could cause heat stroke, discomfort and health problems among the inhabitants. However, the area becomes conducive and attractive to tourists under moderate RH and good temperature.
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This work studied the hourly and seasonal variations in the tropospheric surface refractivity (N) over Makurdi (7.74 o N, 8.51 o E), Nigeria. The results showed among other things that the tropospheric surface refractivity was prevalent between February and October. The rainy season refractivity occurred between the hours of 0800h LT and 0400h LT, while in the dry season, it occurred between 00and 1200h LT. The dry term contributions to N were 68.3% and 77.2% in rainy and dry seasons respectively. Thus, the tropospheric surface refractivity over Makurdi, Nigeria is driven by the dry term which is a function of pressure.
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This study focuses on establishing whether climate change has been occurring in Nigeria. Four States in the South Western part of the country were considered in the study. Climatic data like the rainfall, raindays, surface air temperature for a period of 32 years were collected and analysed. Also, the yield, the static water level and some other data of the boreholes sunk in the region were collected to examine the effect of climate change on groundwater. The climatic data were collected from the Nigerian Meteorological Agency Lagos, while the hydrogeological data for the boreholes were collected from Rural Water Sanitation Agency (RUWATSAN) in each State. The results show that the percent change in the average cumulative rainfall for Osogbo, Osun State, Ibadan, Oyo State, Ikeja, Lagos State and Abeokuta, Ogun State range from 1.2-4.4, 3.1-15.4, 7-16.3 and (-)14- 18, respectively. Also, the percent change in the average surface air temperature for Osogbo, Osun State, Ibadan, Oyo State, Ikeja, Lagos State and Abeokuta, Ogun State range from 0.64-1.61, - 0.94-1.27, -1.92-0.97 and -2.74- (-0.27), respectively. The percent change in the rainfall indicates there has been increase in the amount of precipitation over the decades considered. Most of the data for the yield of the boreholes for the period (1980-2012) considered in this study were not available. However, the data available indicates that yield has not changed significantly. The results show there is no conclusive evidence to establish presence of climate change.
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The characteristics of radio refractive index in the troposphere as they relate to meteorological parameters-is of fundamental importance in planning and advancing the understanding of radio wave propagation and wireless communication systems within the troposphere. The vertical profiles of radio refractivity gradient (G) within 1km above the earth's surface are important for the estimation of anomalous propagation (AP) of microwave radiation. Furthermore the effect of AP in weather radar measurements may be important as spurious echoes from distant ground targets may appear as precipitation leading to incorrect rainfall estimations. AP may also affect dramatically the quality of clear air radar observations. In this paper,we present the vertical and temporal variations of refractive index represented by refractivity (N-units) and refractivity gradient (G) (N-units km-1) in the troposphere to evaluate the occurrence of AP over Julius Nyerere International Airport (JNIA), Dar es Salaam, Tanzania. Meteorological data of air temperature, relative humidity, and atmospheric pressure collected from radiosonde station at JNIA during January, February, August, 2012 and September, October, and November, 2013, were used to compute refractivity, refractive index and refractivity gradient. The percentage of occurrences of super-refraction, sub-refraction, normal-refraction and ducting conditions are presented. Results reveal that, the tropospheric radio wave propagation conditions over JNIA have varying degree of occurrence with normal-refraction conditions prevalent at all the levels except in February, 2012. During this month, super-refraction and normal-refraction conditions have prevailed at the altitude of 1km. The presented results in this paper indicate how the variation in meteorological parameters such as humidity and temperature in the lower troposphere can cause AP. These results can be used by air traffic controllers at:
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Data from satellite application facility on climate monitoring (CM SAF) have been used to estimate and study seasonal variation of columnar refractivity in relation with relative humidity and temperature for twenty six stations categorized into four climatic regions (coastal, Guinea savannah, midland and sub Sahel regions) over Nigeria. Mean, standard error, maximum, minimum values and ranges of these variables were also determined for different regions at each atmospheric level. The results have revealed that the monthly variation of refractivity along side with meteorological factors (relative humidity and temperature) from one region to another over Nigeria are influenced by seasonal variation, geographical location and environmental situation of each region. It is also noticed that the range of relative humidity and temperature are wider in the northern part (56.19 and 72.51% and 5.64 and 5.90 K respectively) compared to the southern part (17.85 and 25.27% and 3.19 and 5.44 K respectively) of the country. This effect was observed in the variation of refractivity also. Effect of inter tropical discontinuity (ITD) was recorded in the southern variation of relative humidity and refractivity.
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M. Tamosiunaite, S. Tamosiunas, V. Dauksas, M. Tamosiuniene, M. Zilinskas. Prediction of Electromagnetic Waves Attenuation due to Rain in the Localities of Lithuania // Electronics and Electrical Engineering. - Kaunas: Technologija, 2010. - No. 9(105). - P. 9-12. The rain attenuation must be taken into account when new Telecommunication systems are projected. Attenuation due to rain depends on the rain rate. The rain rate values exceeded for 0.01% of the time obtained from the data of rainfall amount measured with one - minute intervals of the time must be used in calculation of rain attenuation. One - minute rain rate data are not always readily available from local weather agencies. Only the average annual, daily, hourly or ten - minutes precipitation amounts are available in many cases. The relations between the one - minute rain rate and the value obtained by using the rainfall amount data of longer duration may be used. The one - minute rain rate values in the localities of Lithuania were determined by using the rainfall amount values measured in the Lithuanian weather stations with ten - minutes integration time. They were compared with rain rate -value determined by using model presented in the previous paper and ITU - value. By using the values mentioned above and the known method the one minute rain rate values were determined. The lowest values of rain rates were obtained in Klaipeda. Using the rain rate value obtained here and the ITU - value, the values of the specific rain attenuation have been determined. The specific rain attenuation values obtained using the local hydro meteorological data measured in Kaunas is by about 2 times higher than ones determined by using the ITU - rain rate - value. Ill. 1, bibl. 15, tabl. 2 (in English; abstracts in English and Lithuanian).
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As a result of examination based on a newly available data set of millimeter-wave rain attenuation measured in the UK, it is found that the ITU-R specific rain attenuation model tends to appreciably underestimate millimeter-wave rain attenuation at frequencies above about 60 GHz for the UK rain climate. This tendency is very similar to that previously reported for the Japanese experimental data at frequencies up to 245 GHz. Furthermore, an alternative specific rain attenuation model based on the Japanese experimental data is found to be in fairly good agreement with the experimental data in the UK at frequencies up to 137 GHz.
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Yearly, seasonal and daily variations of radio refractivity have been analyzed. The method proposed in the recommendation of International Telecommunication Union ITU has been used. The local meteorological data have been used in calculation of radio refractivity. The highest values of the radio refractivity have been observed in Klaipeda (in Seacoast) in the year 2009. In July, the values of the radio refractivity were highest in all localities investigated here and over all the time of the day in the year 2009. In the continental part of Lithuania (in Vilnius and Kaunas), analysis of radio refractivity has been made by using the meteorological data of longer period (starting from April 2005 up to July 2010). Five-year meteorological data collected in February, April, July and October have been used. It was obtained that the values of refractivity in the year 2010 are much higher than ones obtained in all the years of the period investigated here.