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Topic  4:   WEATHER

Meaning of weather and Climate. 


Weather: The conditions of the atmosphere which occur at a place at specific time period from hour to hour or day to day. 


Weather; Is the atmospheric condition of a place recorded over a short period of time that can be hourly daily, weekly.

Therefore, when we say it is hot, or wet, or cloudy we are saying something about weather, which is the condition of the atmosphere at a place at a specific time period (from hour to hour or day to day). It is never being static, and thus cannot be generalized. In the same country, even over a small area weather can vary tremendously. It may be sunny in one part of district but raining heavily in few kilometers. 


Climate: Is the average weather condition of an area recorded over a long period of time (over 30 or 35 years). 

This involves systematic observation, recording and processing of the various elements of climate such as rainfall, temperature, humidity, air pressure, winds, clouds and sunshine before any standardization of the climatic means or average can be arrived at. For example, when we say climate of Norway is hot wet equatorial climate that is the summing up of the average everyday weather conditions of the country throughout the year.


 Importance of weather:

a)  Good weather improves people's lives by facilitating different human activities 

b)  Weather determines the kind of clothing to be worn by people in an area.

c)  Knowledge of the weather of a place enables people to carry out economic activities which can be sustained by the weather in that place. e.g. dairy cattle do well in a cool and wet place.

d)  By studying the weather of a place over a long time, we can establish its climate.


Weather and Human activities 

 Most human activities affected by weather that is experienced at a place. For example, When the rains are heavy, flooding occurs and causes damage to crops, animals and infrastructure like roads. This means that few economic activities can take place. When there is no rain, plants and animals die, rivers and streams dry up, and irrigation and other economic activities like fishing cannot be carried out. Therefore weather affects the social economic activities of a place.


Elements of Weather

are the basic atmospheric condition of a place at a given time. There are 7 element of weather

i) Sunshine 

ii) temperature

 iii) Humidity

 iv) Precipitation 

  v) wind 

vi) Cloud cover

 vii) atmospheric pressure



i.             Sunshine refers to the sun's rays that reach the surface of the earth. 

Importance of Sunshine: The sun's energy enables plants to manufacture food through the process of photosynthesis People need sunshine to dry their crsava, millet, maize and fish They also use sunshine to warm themselves Dries clothes after washing them


ii.            Temperature Refers to how hot or cold an object or place is or is the degree of hotness or coldness of a body. The heat in the atmosphere is supplied by the sun.

 Importance of Temperature: Warm is essential for plant and animal survival Plants need warm in order to

manufacture their food, Facilitates formation of clouds


iii.           Humidity Is the amount of water vapor(moisture) in the atmosphere. There are two types of humidity:

a)  Absolute humidity: Is the actual amount of water vapor or moisture in a given volume of air at a particular temperature.

b) Relative humidity: Is the ratio of the actual amount of water vapor or moisture in a given volume of air (i.e.

absolute humidity) to the maximum amount of water vapor that the same volume of air can hold at the same temperature. 

Importance of humidity; help to facilitate rain formation by adding the moisture on the atmosphere. 

  iv. Cloud cover .Clouds are masses of tiny droplets of water or ice particles or both which are suspended in the

atmosphere. They are formed when water vapor or moisture in the atmosphere cools and condenses. Importance of cloud cover: 

Are important because they condense to form rain and other forms of precipitation.


v.    Precipitation This refers as the fall or deposition of moisture, water vapor or frozen water from the atmosphere onto

the earth‘s surface. 

Importance of Precipitation or rain is that, rain provides water for plants to grow (i.e. water for irrigation, growth of grass and pasture depend on rain), Also animals too need water to drink and human needs for domestic and industrial uses. Note: Excess rain and rainstorms result in disasters due to flooding


vi.   Wind is air in motion. It is made up of variety of gases, such as oxygen and carbon dioxide.

    Wind move horizontally from areas of high pressure to areas of low pressure. Importance of wind: They are important in many aspects of weather for instance they cause flow of heat and moisture and are also responsible for the movement of clouds. Wind direction and wind speed or velocity is two important aspects in geography. Wind direction is important because it help us to understand characteristics of the particular winds in terms of temperature and moisture content. Wind speed or velocity determines wind strength or force and therefore determines weather conditions of a place. Example, When strong winds blow over a place, little or no rainfall will be experienced.


vii.  Atmospheric Pressure; This is the force at a point on the earth's surface due to the weight of air above that point. 

The atmosphere is the air surrounding earth's surface and it has weight, which is force with which it presses down on a unit area. -Areas under very low pressure may experience very strong winds periodically as air flows into such areas from high pressure regions.



                    WEATHER STATION

Weather station Is a place where the elements of weather are measured and recorded. Or

Is  a station or place for taking meteorological observations, making weather forecasts, and disseminating such information. Examples of the elements are temperature, humidity, pressure, rainfall, wind direction and speed, cloud cover, and sunshine.



Stevenson screen  is the white wooden box, which is mounted on four legs with louver sides in order to allow free air to enter inside. 

In order to prevent sun‘s heat to reach inside, the roof is double bodied. The screen is painted white to improve insulation. The screen is then placed on a stand which is about 121 centimeters above the ground. 

Instruments used to measure elements of weather. Instruments used for measuring elements of weather include rain gauge, wind vane, anemometer, barometer, hygrometer, and the camp-bell stokes sunshine recorder. 



            1. Temperature:

Temperature is the degree of hotness or coldness of a body, 

An instrument used to measure temperature is thermometer and expressed in degrees of centigrade or Fahrenheit scales. 

There are three main types of thermometer inside the Stevenson screen, these includes minimum

thermometer, the maximum thermometer, and six‘s thermometer.


a)     Maximum thermometer

Is the thermometer made of glass and uses mercury to measure and record the highest temperature reached in a day. 

The maximum temperature is read from the side of the metal index nearest to the mercury. When the temperature falls the mercury falls, leaving behind the metal index still indicating the maximum temperature reached. A magnet is used to bring back the metal index into contact with the mercury.


b)    Minimum thermometer 

Is the thermometer made of glass and uses alcohol to measure and record the lowest temperature reached in a day. 

Alcohol is used because it has lower freezing point than mercury. Any fall in temperature will cause the alcohol column to contract and the meniscus (the curved upper surface of the alcohol column) will pull the index back along the tube whenever temperature rises, the alcohol will expand and flow freely past the metal index without pushing it up. Therefore the metal index is always left as a record of the lowest temperature reached between the readings. The part of the metal index away from the bulb will indicate the lowest temperature reached.


c)     Six‘s thermometer 

This thermometer is ―U‖ shaped glass tube  used for measuring and recording maximum and minimum temperature reached in a day. 

When temperature rises, the alcohol on the left hand side expands, pushing the mercury underneath downward and up the right hand side. While the mercury on the right hand side rises, it pushes the metal index upwards, until the highest temperature is reached for the day. This temperature is read from the scale on the right hand side, when temperature fall the alcohol on the left hand side contracts. This pushes the mercury downward and up by the mercury. Reading and Recording Temperature are taken every day in the morning at regular fixed times but not later than 9:00 a.m. In well-equipped meteorological station, temperature are measured and recorded continuously by self-recording instrument called thermograph.


What does the following mean?

 (a)The mean dairy temperature Is the average of the maximum and minimum temperature. The monthly range of temperature is the difference between the highest mean daily temperature and the lowest mean daily temperature in a month.

(b)           The dairy range of temperature Daily range of temperature is the different between the maximum and the minimum temperatures.

(c)           The annual range of temperature The annual range of temperature in a year which is the difference between the highest mean monthly temperature and the lowest mean monthly temperature. The temperature are represented by using a graph. Lines are drawn to connect places with the same mean monthly temperature. These lines are known as isotherms.

Range temperature = maximum temperature – minimum temperature


Mean Temperature = sum of temperature                                          Number of temperature


            Factors affecting temperature 

Factors that affect temperature include: altitude, ocean currents, and distance from the sea, latitude and prevailing winds.


i)    Altitude 

Temperature decreases with an increase in altitude at the rate of 0.6 0 c for every 100 meters. That is why the summit of Mount Kilimanjaro with 5895 meters above sea level permanently covered with ice.


ii)   Ocean currents 

Ocean currents are either warm or cold. Warm currents raise the temperature of wind blowing across them while cold currents lower temperature of such winds. Some winds blowing on shore will influence the temperature of such winds in some of coastal areas with the temperature of the ocean currents.


iii)  Distance from the sea 

Distance from the sea affects temperature. This is because large areas of land masses lying at great distance from the sea do not get the moderating influence of the oceans. Lack of these moderating effects makes the land masses experience very low temperature in winter and very high temperature in summer. This extremely results in high annual range of temperature. The interior of the continent of Asia is a typical example.


iv)  Latitude 

The amount of heat received at any place depends on the angle at which the sun‘s rays strike the surface of the earth and the duration of sunshine. At the equator the sun‘s rays reached the earth‘s surface at almost right angles. Throughout the year but the angle decreases as one move towards the pole. Therefore temperature decreases with increase in latitude because the sun rays spread over a larger area and its heating effect decreases.


v)   Prevailing wind.

Winds blowing from the land towards the sea(off shore) are carrying moist air from the land and yield no rain except in the sea. These winds usually lead to dryness of the land with very low rains in the coast. Effects of these winds are development of deserts. Eg. The sahara desert


 2. Humidity

The sun‘s heat causes water to evaporate from the surface of the ocean and other water bodies. to form water vapor in the atmosphere. Humidity is the amount of water vapor in air. Or Humidity is the state of the atmosphere in relation to the amount of water vapour it contains. Humidity indicates the degree of dampness of the air and it is one of the main influences on weather. It is expressed in either absolute or relative terms. Absolute humidity, expressed in grams per cubic meter, is the actual amount of water vapour present in a certain volume of air at a given temperature. Relative humidity is the amount of water vapour present in a mass of air expressed as a percentage of the total amount of water vapour that would be present when that air is saturated at the temperature. Air is saturated when the atmosphere cannot hold any more water vapour. This condition depends on the temperature and pressure of the air. Humidity is measured

by a hygrometer which consists of wet and dry bulb thermometers. The wet bulb thermometer is kept moist (wet) by wrapping it in a Muslin which is then dipped in a container of distilled water. So when the air is not saturated the two

thermometers show different readings, when the air is saturated the two thermometers show the same readings. Therefore when there is a big difference in reading between the two thermometers humidity is low and when there is small difference humidity is high.


3. Precipitation:

Precipitation refers to the deposition of moisture from the atmosphere on the earth‘s surface.  It includes dew, frost, snow, mist and hail, sleet and rain.


v  Dew: Is the water vapor that condenses on solid objects when the dew is reached. The droplets formed after condensation of water vapor which is then deposited on the cool surface such as building, leaves, grass and stones.

v  Frost: Is a tiny ice crystals deposited on objects on the ground; frost is formed when temperature falls below freezing point.

v  Snow: Is the frozen droplets of water; snow formed when water vapor in the atmosphere turns into crystals of ice and reaches the ground before melting.

v  Mist: Refers to tiny water droplets suspended immediately above the ground. Frost is similar to mist but it is denser with less visibility.

v  Hail: Is the form of precipitation falling with small ice blocks Sleet: is a mixture of snow and rain. It forms when the temperature of the ground is lower than the temperature above.

v  Rain: Is the droplets of water falling from the atmosphere after condensation. When water vapor rises, It cools at high altitude until dew point is reached. Dew point is the temperature rate at which the atmosphere is saturated with water vapors. Condensation takes place after dew point has been reached to form water droplets. These droplets combine to form larger drops which fall as rain.



a)  Convectional rainfall When rain is formed through vertical rising of moist air currents it is called convectional rain Convectional currents arise due to differential heating of the earth‘s surface. Convectional rains are common in the tropical areas.




b) Relief rain Sometimes moist winds are forced by a high mountain to rise and when they reach high altitude the moisture in them condenses to form droplets, which fall as rain. 

Rain formed in this way is called relief or orthographical rain. 

             The sides of the mountain facing the direction of the winds is known as the wind ward side while  That which faces away from the direction of the wind is known as the lee wards side or the rain shadow,  the lee ward side gets very little rain . A typical example of rain shadow in Tanzania is found in the western side of mountain Kilimanjaro, winds blow from the Indian Ocean in the east and are forced by this mountain to rise and drop most of their moisture on the eastern and south eastern slopes. When these winds blow to the western side of the mountain they already relatively dry.





c)  Cyclonic rain may occur when large masses of air with different characteristics of temperature and moisture meet. 

As the warm and moist air is forced up over the cool and dry air, it expands. At higher altitude the warm air cools and water vapor condenses to form clouds and rain. On the other hand tropical cyclones are formed over oceans is the tropical between latitude 8 0 N and 8 0 S.

 They usually bring very heavy rainfall and are associated with thunderstorms and very fast moving wind, which often causes destruction along coastal settlements. In the Caribbean and U.S.A tropical cyclones are called hurricanes in Africa they are known as cyclones and in China and Japan they are called typhoons. 




Rainfall is measured by using a rain gauge. The rain gauge consist of an outer case , a copper-receiving vessel, a

funnel whose diameter is normally 13 centimeters, a clear glass jar or bottle and a graduated measuring cylinder.


                 Rainfall Variation 

Rainfall variation is a normal phenomenon on the earth which is caused by a number of factors. Some of them include Ocean currents, distance from the equator, prevailing winds, water bodies, nature and shape of the coast, distance from the sea, altitude and human activities.

v      Ocean Currents. There are two types of Ocean Currents. The warms and the cold currents. The warm Ocean Currents yield rainfall over the adjustment land. This is because the winds cross over them do carry large amounts of moisture for example Mozambique current. Much rainfall is experienced along the East Coast of Africa. Cold ocean currents are crossed by wind which have no moisture, hence brings very little or no rain in the adjacent land. For example Banguera currents in south-west Africa coast and the canary current in the north-west coast of Africa.

v      Distance from the equator. Areas along the equator receive more rain than areas away from it. This is because of high amount of solar radiation, which result into evaporation and raising of air moisture hence condensation takes place resulting into convectional rainfall. Congo basin and Brazil are few examples which receive much rainfall due to their position. 

v      Prevailing winds. Winds blowing from the land towards the sea (off-shore) are carrying moist air from the land and yield no rainfall except in the sea. These winds usually lead to dryness of the land with very low rains in the coast. Effects of these winds are development of deserts. Eg, the Sahara Dessert in the North of Africa. 

v      Lack of water bodies. Areas with large water bodies such as areas around lake Victoria receive much rain than areas without water bodies like central Tanzania and North Eastern Kenya. Water bodies increase much rain to adjacent areas. 

v      Nature and shape of the coast. The coastal areas, which align parallel to the prevailing wind such as the N.E trade winds move parallel with the North Eastern Coast of Kenya, yield no rainfall. As result a dry climate is experienced along the coast.

v      Distance from the sea. The areas near the sea or ocean experience high rainfall due to winds blowing moisture from the sea which would cause rainfall to the coastal areas. Areas that are very far from the sea receive very little or no rainfall for example, Dar es Salaam in Tanzania and Mombasa in Kenya receive heavier rains than Dodoma, Singida and Tabora, which have long distance from the Indian Ocean.

v      Altitude. Altitude is also a factor for uneven distribution of rainfall, highlands force the warm air to rise over them. When they condense, they cause rain to fall on the wind ward side. The other side of the highland (leeward) may receive very little or no rainfall at all. 

v      Human activity. Besides other environmental benefit plants or trees intercept precipitation and return moisture to the air through transpiration and evaporation. This process becomes balanced when there is no destruction of the vegetation. Human activity such as settlement, animal rearing, farming and transportation however can cause rainfall variations on the earth‘s surface through land degradation.. The destruction of vegetation disturbs evaporation, condensation and precipitation process, In turn the amount of precipitation in the air is interrupted either by causing floods or droughts. The California desert in U.S.A and the Sahel in Africa are the results of droughts caused by human activities, which have led to the expansion of the deserts in the 1980s.


              RAINFALL EFFECTS

Too much rainfall for example, the case of Elnino (1998) results into negative impacts on life and properties. 

 Loss of life and property

 Heavy rainfall may cause destruction of houses, roads and bridges, crop   Loss of life for both human and animals.

  Floods, resulted by heavy rainfall results into eruption of diseases such as malaria, cholera


On the other hand, too little rainfall 

§  Leads to little yield of food and cash crops. 

§  Famine and hunger in the parts of the world is a result of lack of rainfall. 

§  Prolonged dry seasons resulted into the loss of lives of both animals and the people. North Eastern Kenya, Somalia, Sudan, and Ethiopia are some of the areas where people and animals have lost their lives because of long droughts. 


Rainfall reliability leads to continuous crops production and animal keeping hence improves standard of living and the industrial development of a nation may be realized. Sustainable agriculture is made possible and people are assured of enough food and cash crops.


4, Pressure

The air surrounding the earth‘s surface from the atmosphere. The atmosphere has weight. The force with which it presses down on a unit area is called atmospheric pressure. The pressure is exerted equally in all directions. To demonstrate that atmospheric pressure is exerted equally in all directions one can do the following experiment. Take a glass with full of water. Cover the top of the glass with a piece of thin paper Hold the glass upside down.  Atmospheric pressure is measured by an instrument called a barometer.


              There are two types of barometer, mercury barometer and aneroid barometer.. Lines joining places with the same pressure are called isobars  


Pressure decreases at the rate of 10 millibars for every 100 meters increase in height because the thickness of the atmosphere decreases.


The summary of  Pressure 

1.            What is pressure? Pressure is the air surrounding the earth‘s surface forms the atmosphere.

2.            Define atmospheric pressure – Is the force with which atmosphere presses down on a unit area 3. Atmospheric pressure is measured by a barometer

4. Two types of barometers are; a) A mercury barometer. b) Aneroid barometer.

5. Pressure is expressed in millimeter.

6. A bar is the standard unit of pressure measurement

7. It is divided into one thousand units called millibars.

8. Lines joining places with the same pressure is called isobars.

9. Mention three roles of pressure in human life.

a)  Partial filling of tires hinders proper movement of the vehicle.

b) Pressure is also helpful to the patients whose parts of body fail and pump blood to other parts of the body.

10.Pressure decreases at the rate of 10 millibars for every 100 meters


5. Winds

Wind is air in motion from high pressure to low pressure area.



 Is when winds blow from sea to land during a day.

During the day the land is usually warmer than the sea, and the air pressure on the land is lower than that over the sea. Therefore winds blow from sea to land. These winds are known as sea breezes. 




This occurs during night when winds blow from land to sea.

             During the night the land is cooler than the sea and there is low pressure on the sea. Therefore winds blow from land to sea, these are called land breezes.

A depression is an area of low pressure in which winds blow a circular motion. This motion is anti-clockwise in the Northern hemisphere but clockwise in the southern hemisphere. A depression develops when cold heavy air comes into contact with warm moisture. A depression is usually associated with cyclonic rains. 

Anticyclones are areas of high pressure in which winds blow in a clockwise circular motion in the Northern hemisphere. They normally follow a depression and are associated with cool fine weather with no rain.

        Instruments used to measure wind      

Wind direction is measured by a wind-vane.

A wind-vane consists of a freely rotating arrow fitted to a central rod. The arrow of the wind-vane always points to the direction from which winds blows, and the wind is named after this direction. Four arms marking the directions of the cardinal points are fixed to the stationary central rod.



             Wind speed is measured by an instrument called an anemometer. This instrument consists of three or four horizontal arms that when there is wind the arms rotate. This movement operates a meter which records the speed of the wind in kilometers per hour. 





            Weather forecasting 

Weather forecasting is defined as a prediction of weather condition of a place at a given time approximately 24 to 48hrs.  

Weather forecasting is obtained through two methods namely, traditional and modern methods. 

The traditional method predicts weather through observation of types of birds voices, crocking of frogs, rainbow, migration of birds, changes of wind direction, position of the moon and general body feeling. The method is still widely used in predicting weather by the indigenous communities. 

Modern method involves the use of modern equipment such as satellites, electronic computers, radar, and aircraft, balloons, ships, submarines, rockets and daily records from weather stations. This method of predicting weather is more reliable and accurate.



1.        Radiosonde This is an instrument used to register pressure and temperature from the ground to roughly 12km above the ground. The instruments are fixed to balloons filled with helium gas and are released into the atmosphere every day at a particular time from different weather stations. The radiosonde transmits radio signals to computers on the ground station where they are analyzed. The signal transmitted give pressure, temperature and humidity readings at different altitude. These balloons are made in such a way that they burst when they reach a certain altitude.

2.        Satellite The satellite used for predicting weather are human made space. They move around the earth so that they can provide various types of information. They are also used in radio, television and telephone communication. Satellite transmits photographs on weather conditions on a daily basis. The movement of clouds can be predicted from an analysis of the movement of winds force period of 24hours.

3.        Persistence method This is simple way of making a weather forecast. It assumes that the atmosphere conditions of a place at the time of forecast will not change. For example, if it is 28°C in Dodoma today the persistence method predicts that it will still be 28°C in Dodoma tomorrow. If two inches of rain fell in Arusha yesterday, this method predicts that two inches of rain will fall in Arusha today.

4.        Statistical forecasting In this method, meteorologists ask themselves how is the weather usually like at this time of the year?. They then collect records of average temperatures and rainfall over the last five years. This give forecasters an idea of what the weather is "supposed to be like" at the certain time of year.


Weather forecasting is useful to people in the following ways;

v  Help farmers in planning their work for a year. This is because weather broad casting helps farmers to adjust their farming activities to suit the expected weather conditions.

v  Weather broadcasting influence transport since it helps sailors and air travelers to know condition of the winds. v Helps in planning sport activities.

v  Helps in selecting suitable clothes.

v  Helps fishermen to plan their fishing activities.

v  Helps contractors to plan for suitable houses.

v  Helps in geographical expeditions like tours and military.

v  Military personnel benefit from weather broadcasting as they can plan their military activities depending on

weather conditions.



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