PROJECT REPORT

ON

DIURNAL VARIATION OF SURFACE RADIO REFRACTIVITY

OVER NIGERIA

BY

ASHAYE, ADEMOLA ADEWALE

(PHY/01/9935)

SUBMITTED TO

THE DEPARTMENT OF PHYSICS,

FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE, ONDO STATE.

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD

OF BACHELOR OF TECHNOLOGY DEGREE IN PHYSICS ELECTRONICS

SEPTEMBER, 2007

ii

CERTIFICATION

This is to certify that this project report was written by Ashaye, Ademola Adewale

(PHY/01/9935) and submitted to the Department of Physics, Federal University of

Technology, Akure, Ondo state, Nigeria.

Dr. Falodun, S.E.

PROJECT SUPERVISOR

iii

DEDICATION

This report is dedicated to the Incomparable God for his favour and protection throughout my

study years.

iv

ACKNOWLEDGEMENT

I am grateful to God who preserved my life throughout the period of my study in the Federal

University of Technology, Akure.

A lot of thanks go to my supervisor, Dr. Falodun, S.E. for the guidance and assistance offered

during the project. I appreciate very much his understanding and encouragement to see the

project trough.

I also thank my parents Mr. & Mrs. B.O Ashaye for their unflinching support through out the

period of my study. You are the best parents in the world.

v

ABSTRACT

A good knowledge of the spatial behaviour of the refractivity in the lower troposphere

is essential information for the design of UHF, VHF and SHF radio links, as well as for the

evaluation of interference between terrestrial communication systems. This report presents

the diurnal variation of surface radio refractivity over five meteorological stations spread

across Nigeria. The stations include Port Harcourt (04o 51' N, 07

o 01' E), Ibadan (07

o 20' N,

03o 54' E), Abuja (09

o 15' N, 07

o 00' E), Jos (09

o 52' N, 08

o 54' E) and Maiduguri (11

o 51' N,

13o 05' E). Monthly mean values of temperature, pressure and relative humidity values were

used to calculate the mean refractivity for all the stations. It was observed that surface

refractivity has considerable latitudinal and seasonal variations.

vi

TABLE OF CONTENT

Title Page No

Title Page i

Certification ii

Dedication iii

Acknowledgement iv

Abstract v

Table of content vi

List of Figures vii

List of Tables ix

CHAPTER ONE

1.0 Introduction 1

CHAPTER TWO

2.0 Literature Review 5

2.1 Electromagnetic Radiation and Propagation 5

2.2 Refractive Index and Refractivity 5

2.3 Causes and Effects of Refractivity in the Troposphere 6

2.3.1 Absorption, Refraction and Anomalous Propagation 6

vii

2.4 The Refractivity Equation 11

CHAPTER THREE

3.0 Results 13

3.1 Source and Scope of Data 13

CHAPTER FOUR

4.0 Discussion 39

4.1 Diurnal Variations of Surface Refractivity Values 39

4.2 Seasonal Variations and Range 42

CHAPTER FIVE

5.0 Summary and Conclusion 47

References 48

viii

LIST OF FIGURES

FIGURE 1.1 Division of the Atmosphere 2

FIGURE 2.1 A simple refraction 7

FIGURE 2.2 Four Classifications of Refraction 9

FIGURE 3.1 Map of Nigeria showing location of the meteorological stations 13

FIGURE 3.2 Graphs showing Refractivity Variations for January 2000 – 2005 16

FIGURE 3.3 Graphs showing Refractivity Variations for February 2000 – 2005 18

FIGURE 3.4 Graphs showing Refractivity Variations for March 2000 – 2005 20

FIGURE 3.5 Graphs showing Refractivity Variations for April 2000 – 2005 22

FIGURE 3.6 Graphs showing Refractivity Variations for May 2000 – 2005 24

FIGURE 3.7 Graphs showing Refractivity Variations for June 2000 – 2005 26

FIGURE 3.8 Graphs showing Refractivity Variations for July 2000 – 2005 28

FIGURE 3.9 Graphs showing Refractivity Variations for August 2000 – 2005 30

FIGURE 3.10 Graphs showing Refractivity Variations for September 2000 – 2005 32

FIGURE 3.11 Graphs showing Refractivity Variations for October 2000 – 2005 34

FIGURE 3.12 Graphs showing Refractivity Variations for November 2000 – 2005 36

FIGURE 3.13 Graphs showing Refractivity Variations for December 2000 – 2005 38

FIGURE 4.1 Diurnal Variation of Surface Radio Refractivity for March 2002 39

FIGURE 4.2 Diurnal Variation of Surface Refractivity for Ibadan, March 2002 40

FIGURE 4.3 Diurnal Variation of Water Vapour Pressure for Ibadan, March 2002 40

FIGURE 4.4 Diurnal Variation of Temperature for Ibadan, March 2002 41

FIGURE 4.5 Diurnal Variation of Pressure for Ibadan, March 2002 41

FIGURE 4.6 Table and graph showing seasonal variation

of Ns values for Port-Harcourt, 2002 44

ix

FIGURE 4.7 Table and graph showing seasonal variation

of Ns values for Ibadan, 2002 44

FIGURE 4.8 Table and graph showing seasonal variation

of Ns values for Abuja, 2002 45

FIGURE 4.9 Table and graph showing seasonal variation

of Ns values for Jos, 2002 45

FIGURE 4.10 Table and graph showing seasonal variation

of Ns values for Maiduguri, 2002 46

x

LIST OF TABLES

TABLE 3.1 Typical Refractivity values calculated for January 2000 14

TABLE 3.2 Computed Refractivity data for January 2000 – 2005 15

TABLE 3.3 Computed Refractivity data for February 2000 – 2005 17

TABLE 3.4 Computed Refractivity data for March 2000 – 2005 19

TABLE 3.5 Computed Refractivity data for April 2000 – 2005 21

TABLE 3.6 Computed Refractivity data for May 2000 – 2005 23

TABLE 3.7 Computed Refractivity data for June 2000 – 2005 25

TABLE 3.8 Computed Refractivity data for July 2000 – 2005 27

TABLE 3.9 Computed Refractivity data for August 2000 – 2005 29

TABLE 3.10 Computed Refractivity data for September 2000 – 2005 31

TABLE 3.11 Computed Refractivity data for October 2000 – 2005 33

TABLE 3.12 Computed Refractivity data for November 2000 – 2005 35

TABLE 3.13 Computed Refractivity data for December 2000 – 2005 37

TABLE 4.1 Surface Refractivity values for Port-Harcourt, Ibadan, Abuja,

Maiduguri, and Jos, March 2002 42

TABLE 4.2 Table showing seasonal variation and range of Surface refractivity 39

1

CHAPTER ONE

1.0 INTRODUCTION

The Atmosphere

The Earth's atmosphere is a layer of gases surrounding the planet Earth and retained

by its gravity. It contains roughly (by molar content/volume) 78% nitrogen, 20.95% oxygen,

0.93% argon, 0.038% carbon dioxide, and the remaining 0.07 percent is a mixture of

hydrogen, water, ozone, neon, helium, krypton, xenon, and other trace components. This

mixture of gases is commonly known as air. The atmosphere protects life on Earth by

absorbing ultraviolet solar radiation and reducing temperature extremes between day and

night.

The layers of the atmosphere are the troposphere, the stratosphere, the mesosphere,

the thermosphere, and the exosphere. The troposphere is the layer in which weather occurs

and extends from the surface to about 16 km above sea level at the equator. Above the

troposphere is the stratosphere, which has an upper boundary of about 50 km above sea level.

The layer from 50 to 90 km is called the mesosphere. At an altitude of about 90 km,

temperatures begin to rise. The layer that begins at this altitude is called the thermosphere

because of the high temperatures that can be reached in this layer (about 1200°C). The region

beyond the thermosphere is called the exosphere. The thermosphere and the exosphere

overlap with another region of the atmosphere known as the ionosphere, a layer of ionized air

extending from almost 60 km above Earth’s surface to altitudes of over 1,000 km.

2

Fig 1.1: Division of the Atmosphere

The troposphere

The troposphere is the lowest layer of the earth's atmosphere and the site of all

weather on the earth. The troposphere is bounded on the top by a layer of air called the

tropopause, which separates the troposphere from the stratosphere and at the bottom by the

surface of the earth. The troposphere is wider at the equator (16 km) than at the poles (8 km).

The temperature of the troposphere is warmest in the tropical (latitude 0º to about 30º

north and south) and subtropical (latitude about 30º to about 40º north and south) climatic

zones and coldest at the polar climatic zones (latitude about 70º to 90º north and south).

Observations from weather balloons have shown that temperature decreases with height at an

average of 6.5º C per km, reaching about -80º C above the tropical regions and about -50º C

above the polar regions.

Refraction in the troposphere is caused by the variations in space of the refractive

index n. Typically, the refractive index decreases with increasing height. The refractive

index governs the speed of propagation of electromagnetic waves in a medium. This can

observed from equation 1.1.

3

𝐸 𝑟, 𝑡 = 𝐸𝑜 𝑒 𝑖[𝑛 𝒓 𝑘𝑜 .𝒓− 𝜔𝑡 ] (1.1)

where:

𝜔 = 2𝜋𝑓,

and

𝑘𝑜 = 2𝜋/𝜆

𝑘𝑜 𝑖𝑠 𝑎 𝑣𝑒𝑐𝑡𝑜𝑟 𝑛𝑜𝑟𝑚𝑎𝑙 𝑡𝑜 𝑡ℎ𝑒 𝑤𝑎𝑣𝑒𝑓𝑟𝑜𝑛𝑡

The value of n(r) represents the refractive index at the point r (x,y,z). If the refractive index

falls with height, the wavefront will be progressively refracted downwards. This downwards

bending can compensate to more or lesser effect for the curvature of the earth - i.e. the

radiowaves can propagate beyond the visible horizon.

Typically, the refractive index in the troposphere falls slowly with height and the

resulting refraction causes the radio horizon to appear to be 1.33 times further away than the

geometric horizon. This effect is known as anomalous Propagation (AP). AP takes place

when an unusual, other-than-normal vertical distribution of temperature, moisture, and

pressure exists within the atmosphere. AP conditions include sub-refraction, super-refraction

and trapping.

Nigeria’s telecommunication network depends largely on VHF, UHF and microwave

line of sight terrestrial paths, many of which are relay links. Fading resulting from

atmospheric refraction has been observed on these paths, particularly during the dry

harmattan season between December and March (Adeyemi and Adedayo, 2004).

This report describes the analysis of radio refractivity over five radiosonde stations in

Nigeria. The aim is to study the refractivity variations over different periods of the day. An

4

attempt is made to draw conclusions on the various factors responsible for these observed

variations.

5

CHAPTER TWO

2.0 LITERATURE REVIEW

This section considers a few concepts that are necessary in the discussion of radio

refractivity.

2.1 ELECTROMAGNETIC RADIATION AND PROPAGATION

Electromagnetic radiation is energy propagating in the form of an advancing

disturbance, or wave, in the electrical and magnetic fields. In a vacuum, the wave propagates

at the speed of light which is 299,792,458 m/s. However the speed of light varies when it

moves through different materials. For example, light moves slower in water, glass, and

through the atmosphere than in a vacuum. The ratio of the speed in a vacuum to the speed in

a medium is the absolute refractive index for the medium. Furthermore, the wave propagates

with a slower speed in the atmosphere than the speed of light. The wave’s speed, direction of

propagation, and amplitude are dependent upon several atmospheric variables including

temperature, moisture, and pressure.

2.2 REFRACTIVE INDEX AND REFRACTIVITY

In free space, an electromagnetic wave will travel in a straight line because the

velocity of the wave is the same everywhere. The ratio of the distance a wave would travel in

free space to the distance it actually travels in the earth’s atmosphere is called the refractive

index. The refractive index is symbolized by "n" and a typical value at the earth’s surface

would be 1.000300. Thus, "n" would gradually decrease to 1.000000 as we move upward

toward the theoretical interface between the atmosphere and free space. For example, in the

time it takes for electromagnetic energy to travel a distance of one wavelength in air at 1000

hPa, 15°C temperature, and 40 percent relative humidity, it could have travelled 1.0003

6

wavelengths in free space, which makes 1.0003 the refractive index. The normal value of n

for the atmosphere near the earth’s surface varies between 1.000250 and 1.000400. Since the

refractive index produces a somewhat unwieldy number, we use a scaled refractive index

called refractivity symbolized by "N".

𝑁 = 𝑛 − 1 × 106 (2.1)

Refractivity is a function of pressure, temperature, and vapour pressure (moisture).

The refractive index of air and hence N depends mainly on the atmospheric pressure P (hPa),

the temperature T(K) and the partial pressure of water vapour e (hPa).

Experimental results have shown that atmospheric refractivity near the earth’s surface

normally varies between 250 and 400 N units in the tropical regions, (i.e. the smaller the N-

value, the faster the propagation speed). Refractivity values become smaller with decreasing

pressure and decreasing moisture, but larger with decreasing temperature. All of these

variables usually decrease with increasing altitude. However, the increase in N due to

decreasing temperature is not sufficient to offset the decrease in N due to a decrease in

moisture and pressure. As a result, refractivity values will normally decrease with increasing

height.

2.3 CAUSES AND EFFECTS OF REFRACTIVITY IN THE TROPOSPHERE

2.3.1 Absorption, Refraction and Anomalous Propagation

As electromagnetic energy propagates through the atmosphere, it is attenuated (i.e.,

undergoes a loss in overall energy) by absorption and scattering. The major gaseous

absorbers in the atmosphere are water vapour, carbon dioxide, ozone, and oxygen. Each is

selective about the wavelength it absorbs, (e.g., oxygen absorbs UV energy). However, for

most radar, absorption is fairly negligible in terms of its effect on electromagnetic

7

propagation. Electromagnetic energy is also scattered by liquids and solids in the atmosphere.

This effect is greatly dependent on the size of the particle in relation to the wavelength, but as

with absorption, scattering represents a small factor in electromagnetic propagation.

Changes in temperature, moisture, and pressure in the atmospheric column cause a

change in atmospheric density, which in turn causes variations in the speed of

electromagnetic waves in both the vertical and horizontal directions. These changes in speed

lead to changes in the propagation direction, or bending, of the waves. The bending of

electromagnetic waves as they pass through the atmosphere is an example of refraction (see

Figure 2.1). Refraction is always such that the waves turn toward the medium in which they

travel more slowly, as they pass from a faster speed medium into a slower speed medium.

This is the case shown in Figure 2.1, where medium a is the faster speed medium. Refraction

causes waves to turn back toward the slower speed medium as they pass from the slower into

the faster medium.

This bending toward the slow speed medium can lead to trapping in which waves are

unable to propagate out of the slow speed medium.

Fig 2.1: A simple refraction

8

Events of refraction are always present in our atmosphere, and are quite normal.

However, when the structure of the atmosphere causes abnormal bending of the

electromagnetic, anomalous propagation (AP) occurs. Anomalous propagation takes place

when an unusual, other-than-normal vertical distribution of temperature, moisture, and

pressure exists within the atmosphere.

Anomalous propagation occurs in many forms: One type of refractive condition can

extend the normal detection range of electromagnetic waves and, if conditions intensify,

produce false echoes or ghosting. In RADAR detection, ghosting can cause returning echoes

to fool the radar equipment into displaying faraway echoes as though they are much closer

than they actually are.

With another type of refractive condition, anomalous propagation may produce a

shadow zone (commonly referred to as a radar hole), sometimes allowing an aircraft or ship

to approach within visual range but to remain undetected by radar.

Another naturally occurring anomalous propagation feature is known as a duct.

Simply put, a duct is a region of the atmosphere that traps electromagnetic waves (prevents

them from spreading out), and thus allows them and their energy to propagate over long

ranges. Ducts provide significant opportunities to exploit the atmosphere.

Refractive conditions are categorized into four basic classifications: Normal, Super-

refractive, Trapping, and Sub-refractive (Figure 2.2).

9

Figure 2.2: Classifications of Refraction

Normal Refraction

In free space, an electromagnetic wave will travel in a straight line because conditions

are uniform and the index of refraction is the same throughout the column. Within Earth’s

atmosphere, however, the velocity of the wave is less than that of free space. So the

propagating wave will be bent downward from a straight line. This is described as normal

refraction.

Normal refractivity exists in most areas for about 50% of the time. Anomalous

propagation is not present under normal refractive conditions. Normal refraction occurs under

normal (standard) atmospheric conditions in which moisture, temperature, and pressure all

decrease with altitude.

Super-Refraction

In this situation, the vertical distributions of temperature, moisture, and pressure cause

the radar waves to bend more toward the surface of Earth than under normal conditions.

As the refractivity gradient continues to decrease, the wave path’s curve will approach

the radius of curvature of the earth.

10

Super-refractive conditions can extend radar coverage up to 50% above normal.

Temperature increasing with height and/or rapid moisture decreases with height can cause

electromagnetic waves to bend downward more than normal.

Sub-refraction

Occasionally, motions in the atmosphere produce a situation where the temperature

and humidity distributions create an increasing value of N with height. This occurs when

density contrast in the atmosphere is weak, such as when water vapour content increases

and/or temperature decreases rapidly with height. The beam bends less than normal and

climbs excessively skyward. This phenomenon is known as subrefraction. Subrefraction

causes the radar to overshoot targets that are normally detected. Subrefractive conditions are

generally rare, and usually occur in desert regions.

Trapping

If the radius of curvature for the wave becomes smaller than Earth’s, waves may

become trapped between two areas: Earth’s surface, and the negative gradient causing the

downward refraction.

Trapping produces the greatest extremes in radar performance and can significantly

extend radar ranges. Radar waves refracting sharply downwards, and then reflecting off of

Earth’s surface, may travel distances well beyond normal. Trapping can occur between the

surface and an overlying region of the atmosphere with faster speed characteristics. It can

also occur between two layers of the atmosphere that have different characteristics. This is

known as an elevated duct.

11

2.4 THE REFRACTIVITY EQUATION

The refractivity equation is given by:

77.6

𝑇+ ( 𝑃 +

4810 𝑒

𝑇 ) (2.2)

In other to facilitate the description of dry and wet components of the radio

refractivity, equation 2.1 is usually written as:

𝑁 =77.6

𝑇 𝑃 +

3.732 ×105 𝑒

T2 (N − 𝑢𝑛𝑖𝑡𝑠) (2.3)

With the dry term, N dry, of radio refractivity given by:

N dry = 77.6

𝑇 𝑃 (2.4)

and the wet term, N wet, by:

N wet = 3.732 ×105 𝑒

T2 (2.5)

where:

P = Atmospheric Pressure (hPa)

T = Temperature (K)

e = water vapour pressure (hPa)

The expressions in equation 2.2 to equation 2.5 are valid for frequencies ranging from

1MHz to at least 30GHz (Bean and Dutton).

12

The relationship between water vapour pressure e and relative humidity RH is given

by:

𝑒 =𝑅𝐻 × 𝑒𝑠

100 (hPa) (2.6)

with:

𝑒𝑠 = 5854

𝑇5 10(20 − 2950

𝑇 ) (2.7)

where:

RH = relative Humidity (%)

T = Temperature (K)

es = saturation vapour pressure (hPa) at the temperature T (K)

13

CHAPTER THREE

3.0 RESULTS

3.1 Source and Scope of Data

The data used for this study was obtained from Nigeria Meteorological Centre, Lagos.

Pressure, temperature and relative humidity data for year 2000 to 2005 were collected for five

meteorological stations in Nigeria. These stations include Port Harcourt (04o 51' N, 07

o 01'

E), Ibadan (07o

20' N, 03o 54' E), Abuja (09

o 15' N, 07

o 00' E), Jos (09

o 52' N, 08

o 54' E) and

Maiduguri (11o 51' N, 13

o 05' E).

Fig 3.1: Map of Nigeria showing location of the meteorological stations.

14

The monthly mean values of pressure, temperature and water vapour pressure were

used to compute the monthly mean values of Ns, from 00hr to 23hr LT. The temperature

values were converted from Celsius (oC) to Kelvin (K) while equations 2.7, 2.6 and 2.3 were

used to compute saturated vapour pressure, vapour pressure and refractivity respectively. A

typical table of values is shown below in Table 3.1.

Table 3.1: Typical refractivity values calculated for January 2000

Typical results of the computed values of Ns are shown in Tables 3.2 to 3.13. The

resultant graphs are shown in Figures 3.2 to 3.13.

15

Table 3.2: Computed Refractivity data for January 2000 - 2005

16

Figure 3.2: Graphs showing Refractivity Variations for January 2000 - 2005

17

Table 3.3: Computed Refractivity data for February 2000 - 2005

18

Figure 3.3: Graphs showing Refractivity Variations for February 2000 - 2005

19

Table 3.4: Computed Refractivity data for March 2000 - 2005

Table 3.4: Computed Refractivity data for March 2000 - 2005

20

Figure 3.4: Graphs showing Refractivity Variations for March 2000 - 2005

21

Table 3.5: Computed Refractivity data for April 2000 - 2005

22

Figure 3.5: Graphs showing Refractivity Variations for April 2000 - 2005

23

Table 3.6: Computed Refractivity data for May 2000 - 2005

24

`

Figure 3.6: Graphs showing Refractivity Variations for May 2000 - 2005

25

Table 3.7: Computed Refractivity data for June 2000 - 2005

26

Figure 3.7: Graphs showing Refractivity Variations for June 2000 - 2005

27

Table 3.8: Computed Refractivity data for July 2000 - 2005

28

Figure 3.8: Graphs showing Refractivity Variations for July 2000 - 2005

29

Table 3.9: Computed Refractivity data for August 2000 - 2005

30

Figure 3.9: Graphs showing Refractivity Variations for August 2000 - 2005

31

Table 3.10: Computed Refractivity data for September 2000 - 2005

32

Figure 3.10: Graphs showing Refractivity Variations for September 2000 - 2005

33

Table 3.11: Computed Refractivity data for October 2000 - 2005

34

Figure 3.11: Graphs showing Refractivity Variations for October 2000 - 2005

35

Table 3.12: Computed Refractivity data for November 2000 - 2005

36

Figure 3.12: Graphs showing Refractivity Variations for November 2000 - 2005

37

Table 3.13: Computed Refractivity data for December 2000 - 2005

38

Figure 3.13: Graphs showing Refractivity Variations for December 2000 - 2005

39

CHAPTER FOUR

4.0 DISCUSSION

4.1 Diurnal Variations of Surface Refractivity Values

The results of the calculated Surface Refractivity show that Ns vary from 00hr to 23hr

LT daily (See Table 3.1). Fig 4.1 shows a typical variation of Ns for the month of February, a

typical dry season month.

Ns value reaches a first maximum between 06hr and 08hr from where it starts to

decrease slowly till around 15hr where it reaches its lowest values. However, N again

increases steadily from 15hr to reach another peak at around 21hr LT.

The diurnal variations are influenced by three atmospheric variables: temperature,

water vapour and pressure. Of the three atmospheric variables that influence refraction, water

vapour has the greatest effect on refractivity variations. This is reflected in the wet-term

component of the refractivity equation as shown in equation 2.5.

Fig 4.1:

40

The high values of the vapour pressure in the morning and evening periods (see

Figure 4.3) are responsible for the corresponding higher values of the refractivity at these

periods compared with the afternoon hours.

Although pressure is one of the meteorological elements that influence refraction, its

effects are small. Pressure variations alone provide no significant change in refraction. This is

evident in the figures below.

41

These different variables can work together to produce different refraction conditions

resulting in either normal propagation or anomalous propagation (AP).

It is also observed that Ns has latitudinal variations (see Fig 4.1). These latitudinal

variations occur because Ns values are generally higher in the south (Port-Harcourt (04o 51' N,

07o 01' E), Ibadan (07o 20' N, 03o 54' E)) than at the middle belt (Abuja (09o 15' N, 07o 00' E), Jos (09o 52' N,

42

08o 54' E)) and at the extreme north (Maiduguri (11o 51' N, 13o 05' E)). Typical Ns values showing

latitudinal variations are expressed in the Table 4.1. These variations occur because water

vapour pressure values, which is the most important parameter in the determination of surface

refractivity value, reduces with increasing distance from the coastal regions.

4.2 Seasonal Variations and Range

The values of the surface refractivity in all the five stations were observed to be

generally high during the wet season (April – September). It varies between 386.20 N-units

and 381.76 N-units for Port-Harcourt, 375.46 N-units and 369.29 N-units for Ibadan, 373.84

N-units and 365.53 N-units for Abuja, 361.94 N-units and 340.03 N-units for Maiduguri, and

350.27 N-units and 342.17 N-units for Jos. These values yields a range of 4.43 for Port-

Table 4.1: Surface Refractivity values for Port-Harcourt, Ibadan, Abuja, Maiduguri and Jos, March 2002

43

Harcourt, 6.17 for Ibadan, 8.31 for Abuja, 21.91 for Maiduguri and 8.10 for Port-Harcourt.

However, it is observed that the range of Ns values is higher in the dry season (November –

February) than in the wet season. This shows that there is more significant variation of Ns

during the dry season.

Table 4.2 compares Ns values for dry and wet seasons in the five stations. It also

shows seasonal variation range (using July and February to represent typical wet and dry

season respectively) for the five stations being considered.

The fact that there is more significant variation of Ns values during the dry season is

also evident in Figures 4.6 to 4.10.

Table 4.2: Table showing seasonal variation and range of surface refractivity

44

Fig 4.6: Table and graph showing seasonal variations of Ns values for Port-Harcourt, 2002

Fig 4.7: Table and graph showing seasonal variations of Ns values for Ibadan, 2002

45

Fig 4.8: Table and graph showing seasonal variations of Ns values for Abuja, 2002

Fig 4.9: Table and graph showing seasonal variations of Ns values for Jos, 2002

46

Fig 4.10: Table and graph showing seasonal variations of Ns values for Maiduguri, 2002

It can be observed from figures 4.6 – 4.10 that when the dry harmattan season sets in

by November, N values falls and gradually reduces reaching its lowest values around January

(for Port-Harcourt and Ibadan) and February/March (for Maiduguri and Jos).

This regional variation is due to the fact that the dry season is more pronounced in the

northern part of the country than at the southern part where Port-Harcourt and Ibadan are

located.

47

CHAPTER FIVE

5.0 SUMMARY AND CONCLUSION

The main results of this study are:

1. Surface radio refractivity varies with the time of the day as well as the seasons of the

year in all the five stations.

2. The values of Ns in all the five stations are generally higher during the rainy season

than during the dry season.

3. There is more significant variation of Ns in the dry season than in the wet season for

all the stations. This is evident from the fact that the range of Ns values is higher in

the dry season.

4. The diurnal and seasonal variations of Ns follow more closely the variation of water

vapour than the variation of any other parameters.

5. There is also significant latitudinal variation. Surface refractivity reduces with

increasing distance from the equator. Typical values are 385.57 N-units (Port

Harcourt (04o 51' N, 07o 01' E), 371.63 N-units (Ibadan (07o 20' N, 03o 54' E)), 347.57 N-units

(Abuja (09o 15' N, 07o 00' E), 303.00 N-units (Jos (09o 52' N, 08o 54' E)) and 291.27 N-units

(Maiduguri (11o 51' N, 13o 05' E)).

48

REFERENCES

1. Adeyemi, B., Adedayo, K.D., (2004). Atmospheric radio refractivity and water

vapour density at Oshodi and Kano, Nigeria. Nigeria Journal of Pure &

Applied Physics, Vol. 3 No. 1.

2. Bean, B.R, and Dutton, E.J., (1968). Radio Meteorology. Dover Publication Co. New

York. 259-273 pp.

3. Falodun, S.E., (2006). Experimental Studies of surface Radio Refractivity in Akure,

South-Western Nigeria. Science Research Annals, Vol. 2 No. 2, 15-22 pp.

4. Microsoft Corporation Encarter Encyclopedia (2007). Layers of the atmosphere.

5. Rec. ITU-R P.453-9, (2004). The radio refractivity index; its formula and refractivity

data. 453-459 pp.