WEATHER
What is weather?
Weather: is the condition of the atmosphere which occurs in a short period of time in a particular place. It is usually essential from place to place and time to time. However weather is the term used to describe the condition of the place based on the specific time and place. Example weather are such as the condition of being hot, windy, cold, foggy, or cloudy.
METEOROLOGY
Meteorology: is the scientific study of weather. It is usually deal with study of all physical processes which take place in the atmosphere and their dynamism such as atmospheric pressure, water vapours, wind, temperature, precipitation and clouds.
Meteorologist: is a person who deal with study of weather. Is usually deal with study of all physical processes which take place in the atmosphere and their dynamism such as atmospheric pressure, water vapours, wind, temperature, precipitation and clouds.
IMPORTANCE OF WEATHER TO MAN
1. Good weather improves people's lives.
2. Weather determines the kind of clothing to be worn by people in a given place.
3. It enables people to carry out economic activities. e.g. dairy cattle do well in a cool and wet place.
4. It help to determine the kind of climate of the place.
5. It help to determine the success and failure of human activities.
6. Reliable rainfall improves crops production.
7. It helps in climate classification.
THE INFLUENCE OF WEATHER TO HUMAN ACTIVITIES
1. Enough rains support agriculture activities
2. Heavy rains cause floods and destroy human activities e.g transportation.
3. Reliable rains support the growth of plant and animals: When there is no rain, plants and animals die.
4. Rainfall support availability of rivers and streams: this support irrigation and other economic activities like fishing.
5. Weather affect water supply to the societies.
6. Good weather condition support transportation activities like air transport and water transport.
ELEMENTS OF WEATHER AND THEIR IMPORTANCE
Element of weather: Are the basic atmospheric condition of a given place and time which usually tend to be differ between one another. However there are seven (7) basic element of weather which are:-
i. Sunshine.
ii. Temperature.
iii. Humidity.
iv. Cloud cover.
v. Precipitation.
vi. Wind.
vii. Atmospheric pressure.
1. TEMPERATURE
Temperature: iRefers to the degree of hotness or coldness of the place. It is normally measured by the instrument called Thermometer and expressed in degrees of a centigrade or Fahrenheit scales. There are several types of thermometer which shows the highest temperature reached during a day and the minimum thermometer which shows the lowest temperature reached. The line drawn on the map with the same amount of temperature I'd called Isotherms.
1. Maximum thermometer: it used to measure the maximum temperature reched during the 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.
2. Minimum thermometer: It used to measure the minimum temperature reached in the day. minimum thermometer contains alcohol instead of mercury. 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.
3. Six’s thermometer: It used for measuring both maximum and minimum temperature. The thermometer consists of a “U” shaped glass tube. The metal index nearest to the bulb indicates the minimum temperature and the other metal records the maximum temperature. When the mercury in the tube maximum temperature falls, the alcohol in the bulb contracts. This causes the mercury to push index B towards the bulb. The minimum temperature is read from the part of index B from the mercury.
THE FORMULA FOR CALCULATING TEMPERATURE
i. Average (mean) Temperature= MAX TEMP + MIN TEMP / 2 (Max temp + min temp divided by 2).
ii. Annual (Daily) diurnal range temperature = MAX TEMP –MIN TEMP.
iii. Montly temperature= DAILY MAX TEMP + DAILY MIN TEMP/ 2 ( Max temp + min temp divided by 2)
FOR Fahrenheit (◦F) to Celsius (◦C)◦ C = 5/9 (◦F -32) Where by
◦ C = degree Celsius and◦F = degree FahrenheitFOR
Celsius (◦C) to Fahrenheit (◦F)◦F = 9/5 (◦ C +32)Where by ◦ C = degree Celsius and◦F = degree Fahrenheit
FACTORS AFFECTING TEMPERATURE DISTRIBUTIONS
Factors that affect temperature include: altitude, ocean currents, distance from the sea, latitude and prevailing winds.
1. Altitude: Temperature decreases with an increase in altitude at the rate of 0.60c for every 100 meters. That is why the summit of Mount Kilimanjaro with 5895 meters above sea level permanently covered with ice.
2. 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.
3. 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 this 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.
4. 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 poles (fig 3.8) Therefore temperature decreases with increase in latitude because the sun rays spread over a larger area and its heating effect decreases.
5. Vegetations: Areas with dense firests has low temperature because trees regulate temoerature while areas with no vegetation receives high temperature.
6. Water bodies: This include kakes, rivers, weels, oceans swamps etc. Areas with large water bodies such as ocean receive high temoerature than areas with no water bodies.
7. Preveiling winds: both on-shore and off-shore wind has impact to the temperature received of the place. The temperature of the place can be lowered if the place receive the cold air masses.
IMPORTANCE OF TEMPERATURE
i. Warm is essential for plant and animal survival.
ii. Plants need warm in order to manufacture their food.
iii. It facilitates the formation of rainfall through evaporation.
iv. It enhance human activities like trade, tourism, agriculture and so many human activities.
v. It influence the rate of physical weathering in the formation of soil.
vi. It helps in weather fore casting.
vii. It helps in climate classification system.
2. PRECIPITATION
precipitation: refers to the deposition of moisture in the earth surface from the atmophere. It is formed when the amount of water vapour in the atmosphere condensed in to water droups hence fall to the earth surface as the precipitation. It is meadured by the instrument called Rain Gauge which is ecpressed by Mllimiters (MM).
It includes dew, frost, snow, mist and hail, sleet and rain. The line drawn on the map with the same amount of rainfall is called Isohytes
FORMS OF PRECIPITATION
i. Dew: Is the water vapor that condense on solid objects when the dew is reached.The droplets formed after condensation of water vapour which is then deposited on the cool surface such as building, leaves, grass and stones.
ii. Frost:: Frost is a tiny ice crystals deposited on objects on the ground, frost is formed when temperature falls below freezing point.
iii. Snow: snow is the frozen droplets of water, snow formed when water vapour in the atmosphere turns into crystals of ice and reach the ground before melting.
iv. Mist: 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: 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.
vi. Rain: Rain are the droplets of water falling from the atmosphere after condensation. When water vapour 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 vapour. Condensation takes place after dew point has been reached to form water droplets. These droplets combine to form larger drops which fall as rain.
TYPES OF RAINFAL
1. Convectional rainfall: it 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.
2. Relief rainfall/orographic rainfall: It is formed when the moist air forced by mountain air to rise and expand then when reach at high altitude 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.
3. Cyclonic rain: When large masses of air with different characteristics of temperature and moisture, cyclonic rain may occur. 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 vapour condenses to form clouds and rain. On the other hand tropical cyclones are formed over oceans is the tropical between latitude 80N and 80S. 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 DISTRIBUTIONS
Rainfall variation: is a normal phenomenon in which the amout of rainfall cmrecived by the place on the earth surface tend to be differs from one place to another place. Normally throughout the earth surface and worldwide the rainfall distribution in unequal since some places receive high rainfall while in some areas receive low amount of rainfall.
FACTORS INFLUENCING RAIN DISTRIBUTION ON THE EARTH SURFACE
The rain variation on the earth surface is the outcome of several factors such as:-
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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. People of such areas includes the eastern part of South Africa receives heavy rain (orographic) while the Western part experiences prolonged droughts due to effects of Drankensburg mountains. The eastern part of East Africa also receives heavier rainfall than the western side due to warm moist winds blowing from the Indian Ocean.
8. 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. When people cut trees or clear the bushes, land is naturally destroyed.
The destruction of vegetation disturbs evaporation, condensation and precipitation process, which are necessary elements of rainfall. 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.
EFFECTS OF RAINFALL
Negative impacts
1. It leads to occurance of floods.
2. Destruction of properties: such as houses, roads and bridges, crops.
3. loss of life for both human and animals.
4. Eruption of diseases such as malaria, cholera, and dysentery.
5. Too little rainfall leads to Famine and hunger due to poor production.
6. Heavy rain leads to soil erosion.
Positive effects
i. Moderate rain leads to high crops production and animal keeping.
ii. It improve soil fertility.
iii. It help in formation of water bodies which are very potential in human activities.
iv. It provide water for practical experiments.
v. It enhance plant growth.
vi. It help in climate classification.
3. THE ATMOSPHERIC PRESSURE
Atmospheric pressure: refers to the forces which exerted by the air on the earth surface. Normally the earth's atmospheric pressure is made up of number of gases (air) nad those gase has weight and therefore they exert the pressure on the earth surface. However atmospheric pressure varies with the temperature and altitude throughout the earth world. It is measured by using instrument which is called Barometer. However there are two types of mercury
(i). barometer
(ii). aneroid barometer.
ii. Aneroid barometer: It consists of an air fight box of thin metal containing very little air. The top of this box moves inwards and outwards with changes in atmospheric pressure. This movement is transmitted by a system of levers to a circular seal with a pointer which shows the reading of atmospheric pressure.
Pressure is expressed in millimeters with reference to the height of mercury column or in millibars. A bar is the standard unit of pressure measurement. It is divided into one thousand units called millibars. At sea level pressure is one thousand millimeters of mercury or 1.5 kilograms of force per square centimeters.
This is equivalent to 1013.2 millibars approximately one bar. The Lines drwan on the map joining places with the same amount of pressure are called Isobars and the pressure is greater at sea level where the whole thickness of the atmosphere exert its weight. Pressure decreases at the rate of 10 millibars for every 100 meters increase in height because the thickness of the atmosphere decreases.
NB: The lines which join all places with equal pressure are called Isobars.Thus pressure varies from one place to another it is not the same in one or all the regions.
FACTOR INFLUENCING VARIATION IN ATMOSPHERIC PRESSURE
1. Temperature: As temperature decrease pressure rises at the same height due to low temperature cold air tend to sink, thus inducing high pressure to develop But as temperature increases hot air tends to rise making it possible for low pressure to develop over the area.
2. Altitude: Usually pressure of the atmosphere decrease with increasing height. Pressure is therefore lower at the mountain top than at the sea level. This is because at the sea level air has to support a greater height than it does on the top of mountain. Thus there is less force or weight of air at the mountain top than at the sea level.
3. Latitude: normally the atmospheric pressure increase with increase in latitude because latitude affect temperature of the place Therefore the place near the equator has low pressure due to high heat while at the polar regions has high pressure because of low heat.
4. Earth rotation: as a ball causes formation of subtropical high-pressure belt around 35º north and south of the equator where air sinks down and gets compressed .Due to this compression the sinking of air is heated up and rises again. The same situation takes place around circumpolar or temperature belt around 60º north and south of the equator.These are the areas where the warm subtropical air rises over the cold polar air expands and gets cooler.This situation result into Depression cyclonic or frontal.
5. Revoluation of the Earth: The earth revolution causes seasons where some seasons are warm while others are cold .Revolution of the earth affects the position of the low pressure belt i.e doldrum. Normally doldrum moves northwards and southwards of the equator depending on the seasons.
THE WORLD PRESSURE BELTS
These can be analyzed according to temperatures such as follows:-
1. The Equatorial low pressure belt / equatorinally trough: This is called doldrum it is located around 5º north and south of the equator.The air here rises due to high temperature it is a wind a convergence zone. This means winds meet there and it is called intertropical convergence zone (ITCZ).
2. Subtropic high pressure belt (Horse latitudes): This is located at 30º north and south of the equator with descending air current due to heat of temperature. High pressure is attributed by accumulation of cold air which moves from the equator towards poles.This belt is also referred to as the "horse belt".
3. Temperature low pressure belt (sub polar low temperature belts): This is found at 60º north and south of the equator. It is also known as cyclonic activities zone or zone of convergence. Existance of low pressure is mainly dynamic rather than a result of temperature change.
4. Polar high-pressure belt: This is confined at 90º north and south of the equator. Pressure is permanently high due to low temperature attributed by a dense descending air from the temperature low pressure belt.
THE WORLD PRESSURE DISTRIBUTION SYSTEM
4. WINDS
Wind: Refers to the air in motion from the region of high pressure to the region of low pressure. Normally wind is made by variety of gases, such as oxygen and carbon dioxide. However wind move horizontally from areas of high pressure to areas of low pressure. Wind have both speed and direction. The wind direction is measuredure by Wind Vane While the wind speed is measured by Anemometer. 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. 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. The lines drawn on the map joining all place with same amount wind is called Isokinetics
TYPES OF WINDS
There are three main types of winds which are:-
a. Local winds
b. Planets winds.
c. Wind systems/ prevailing winds.
A. LOCAL WINDS
Local systems: are winds which cover and affect small geographical area. Normally local winds are controlled by the surrounded”terrain”or environment they are not controlled by planetary system of the winds they are rather diurnal than seasonal. There winds do affect animals, plants and movement of industrial pollutants.
TYPES OF LOCAL WINDS
1. Anabatic and Katabatic
i. Anabatic winds: These are cold local winds which blow from, the valley button up to the hilltops. They blow in the afternoon especially in summer. During the day the slopes exruince. Low pressure due to direct sun rays which make them hot and winds move from button upwards. They further condense and come down causing the become foggy.
ii. Katabatic winds: These are cold local winds which blow from the mountain to the valley. They usually occur during the night where air is rapidly cooled by terrestrial radiation on the ground surface. During the night the valley slopes become very cold as they lose the heat rapidly, hence high pressure on them. Due to convectional currents the valley buttom becomes warm creating low pressure. Winds sink down through the valley slopes and rise up from the valley buttom . These winds are called Katabolic winds.
2. Sea breeze and land breeze
Sea breeze (Day breeze): is the air in motion from the sea to the land during the day. It occurs during the day when there is low pressure on the earth surface due to high heat.
ii. Land breeze (Night breeze): refers to the air in motion from the land to the sea during the night. It occurs during the night due to high pressure from the aerth surface hence at tge sea tgere is low pressure due to high temperature. Normally during the night the land cools more rapidly than sea. Thus wind tends to move from land to the sea this action is called land breeze
3. Descending and ascending wind
i. Ascending winds: These are movement of warm winds from the low land to the windward side of the mountain. Are mostly warm winds and they are called Fohn. The name Fohn is common in Alps Mountains of Switzerland.Where during winter the Fohn winds descend from the Swiss Alps into Switzerland.
ii. Descending winds: are movement of cold down slope from the leeward side of the mountain. Normally these cold winds blow from France south ward torwards Mediterranean Sea. These winds are called "Mistral" but in USA the same winds are called "chinook". This situation occurs mostly in the leeward side of the mountain. Some local winds blow from the high pressure over the Sahara during spring. These winds are warm, dusty and dry because they originate from the desert which is dry.
B. PLANETARY WINDS
Planetary winds: are winds which blow and affect large geographical area. Types of planetary winds
1. MONSOON WINDS
Monsoon winds: These are winds which blowing periodically from one season to another season from east to west or east to west. Are seasonal winds if the pressure of a large continent is high the wind will blow out of it to the sea where pressure is low These are called ''offshore winds". These winds are common during winter in Asia as inland temperature is very low while sea temperature is still warm and less dense.If the pressure over a large continent is low than winds blow into it from the sea where pressure is higher.These winds are called "onshare winds" and they are common in Asia during summer.
2. JET STREAMS
Jet stream: refers to the strong current of blowing wind through atmosphere at very high altitude. The wind blow from west to east at very high speed. Evidences of strong winds in the upper troposphere first came when first world war, several interwar ballons blown off- course were observed travelling at speed in eccess of 200 km per hour.
Pilots in the second world war, flying at height above 8km found east ward flights much faster and their return westward journey much slower than expected. The explanation was to and to be a belt of upper air westernizes the Rossby wave which form a compute pattern around the globe.
Types of polar jet strems
There are five recognizable jet streams, two are particularly significant with a third having seasonal importance
i. Polar front jet stream (PEJS): This occurs in middle latitudes between 40 and 60 degrees and a height of 9000 – 12000 meter more or less at the tropopause in the hemisphere. The winds form the division between the ferrell's and polar cells which is the boundary between warm tropical and cold polar air these winds are responsible for giving fine or wet weather on the earth's surface.
ii Sub tropical set stream (STJS): This occurs at about 12000 meters and 25 to 30 degrees from the equator and form the boundary between the Hadley and Ferrell cell in effect vortex associated with the mid-latitude cell. They have lower velocity compared to PFJS but follow the west-east pattern.
iii. Equatorial Easterlies set stream (EEJS): This is more seasonal being associated with the summer monsoon of the Indian sub continent.
iv. Arctic set streams (AJS): This has been traced across Alaska and Canada at about 7600 meters.
v) Polar night set stream (PNJS): This is recently discovered above the Aretic circle in the lower Stratosphere.
3. DESERT/HARMATTAN WINDS
The desert or Harmattan wind: These are NE winds which blow from the Sahara across West Africa between November and March. These winds are dry and dusty because they originate from the desert. The winds blow during winter towards the Gulf of Guinea where the land is cold while over Mediterranean sea pressure is low because the sea is much warmer than the land.
Berg winds: These are the winds which originate from the plateau of South Africa during winter. At this time the high pressure lies over the plateau and it is a region of descending air the winds blow out toward the South SE and SW.Those winds are warmer.
4. AIR MASSES
Air masses: Air mass is a large volume of air approximately to have uniform characteristics of temperature and humidity. Normally the air masses can be cold airmasses or warm air masses.
CHARACTERISTICS OF AIR MASSES
Characteristics of air mass normally come from the region where the air masses were formed hence this including:-
1. They usually have the same direction.
2. They are large volume of air approximately to have uniform characteristics of temperature and humidity.
3. They originate from the area with uniform build and shape like desert.
4. They affect the temperature of the place where they blow.
5. The air mass may be warm, dry or moist.
6. The air masses are influenced by prevailing or moving winds.
C. WIND SYSTEMS/PREVAILING WINDS
Wind systems: are winds related to pressure belts. Usually these winds are called prevailing winds and they are deflected according to ferrell's law prevailing winds are those which blow more frequently than other wind in a particular region. Among these winds are trade winds westerly and polar.
i. Trade winds: These winds meet at the doldrum they are noted for their constancy of force and direction.They are terrible tropical storms which occur at certain season. At the equatorial belt the air is heated and rises to be replaced by air moving in (winds) from north and south.These form north east trade winds and south east trade winds which obey ferrell's law.
ii. Westerly winds: These blow across latitude 35º and 60º to the polar front these winds are characterized by constant strength and direction in the southern hemisphere while in the northern hemisphere and masses disrupt much of their strength .These winds include north west and south west westerly.
iii. Polar winds: These winds are of solid air they are more pronounced in the southern hemisphere and irregular in the northern hemisphere due to interruption by mountains.
iv. Monsoon winds: These are seasonal winds if the pressure of a large continent is high the wind will blow out of it to the sea where pressure is low These are called ''offshore winds". These winds are common during winter in Asia as inland temperature is very low while sea temperature is still warm and less dense.If the pressure over a large continent is low than winds blow into it from the sea where pressure is higher.These winds are called "onshare winds" and they are common in Asia during summer.
v. The upper air movement: Prevailing winds are normally surface winds not very far from the earth's surface. These systems of wind are of high speed and high altitude approximately 1000 to 12000 meters above the sea level within these winds are narrow bands of extremely fast moving air known as "jet streams"
IMPORTANCE OF WINDS
i. They support the gormation of rainfall due to movement of water vapour.
ii. The change the temperature of the place to influence heat transfers.
iii. It help to determine the climate of the place.
iv. It help to determine the climate of the place.
v. It help in climate classification.
5. HUMIDITY
Humidity: This refers to the atmospheric moister or amount of water vapour in the atmosphere.The moister is obtained from various sources such as oceans, lakes, seas, dams, ponds and rivers. It is of high importance as far as weather as climate is concerned. Air absorbs water through the process of evaparation which result in water changing from its liquid state to gaseous state, the gaseous state is called water vapour. The amount of water vapour in the air is called humidity and it is measured by an instrument called hydrometer placed in the Stevenson screen.
FACTORS INFLUENCING THE ATMOSPHERIC OF WATER VAPOUR
The amount of water vapour in the atmosphre is unevenly distributed through out the earth surface due to various factors like:-
1. Altitude: Normally at low altitude where pressure is high air absorb more moister (water vapour) but at high altitude where pressure is lower, air expands and cools and its capacity to absorb moister is reduced due to high content of materials like dust particles. Therefore at high altitude there is high humidity (water vapour) while at low altitude there is low water vapour due to high degree of impurities.
2. Latitude: Humidity higher in lower latitudes than in higher latitude because there is greater rate of evaporation at lower latitudes due to high temperature. Likewise the mount of moister in the air is higher in summer .
3. Temperature/the rate of evaporation: Arise in temperature lowers relative humidity if the amount of moister remains constant because when air is heated it expands. The volume increase and therefore distribution of water vapour per unit. The air will have a greater capacity of holding more water vapour if air is cooled it contracts and decrease in volume making the space which moister can occupy become less. That is why if air continues to cool it will reach saturation and it gets rid of excess water vapour through condensation.
4. Vegetation: Areas with dense forests like Amzon are more humid because plants contribute the rate of evaporation through leaves and roots (transpiration) but areas with no vegetation like in desert areas are less humid or has low water vapour.
5. Large water bodies: All places which are near large water bodies tend to be humid especially if the temperature is high causing a lot of evaporation. Areas that are far away from large water bodies such as deserts center has little water vapour the air has high capacity of holding moisture.
6. Prevailing winds: on-shore wind cause high humidity than the place which receive off- shore wind.
7. Ocean currents: areas which receive the warm ocean currents are humid because the warm ocean currents ehence high moisture supply but if the place receive the cold ocean currents are less humid because of low moisture supply.
MEASUREMENTS OF HUMIDITY
1. Absolute humidity: This is the actual amount of water vapour in a specific volume of atmosphere i.e the mass of water vapour per cubic unit of air and it is normally expressed in g/m3.
2. Relative humidity: This is the proportion of water vapour present in the air compared to the maximum amount of water vapour, possible at the same temperature and it is usually expressed in percentage.Normally saturated air has relative humidity 100%.
3. Specific humidity: Is the ratio of amount of water vapour presence in the atmosphere to that presence in the air which is expressed in gram per cubic meters (g/m³).
3. Dew points: is the highest level of saturation formed when the air mass in the atmosphere is completely saturated by water.
Sources of water vapour in the atmosphere
i. Evaporation from water bodies.
ii. Evaporation from springs.
iii. Evaporation from plants.
iv. Evapo-transpiratation.
v. Evaporation from the land surface
vi. Volcanic eruptions.
IMPORTANCE OF WATER VAPOUR IN THE ATMOSPHERE
i. Humidity is very important to weather forecasting.
ii. Water vapour brings about precipitation in form of rain, hail, snow and sleet.
iii. Water vapour helps in absorbing heat which penetrates into the atmosphere in the form of radiant energy from the sun to the earth.
iv. It is also act as a blanket which prevents the rapid escape of heat from the earth’s surface and therefore maintain heat budget.
v. It enhence plant growth.
vi. It affect the rate of microbes in soil decomposition.
6. CLOUDS
clouds: Refers to the condensed water vapour in the atmosphere which formed when the temperature of atmoshere fall to below 0.c to form water. The lines drawn on the map with the same amount of clouds is called Isonephs
FACTORS AFFECTING THE AMOUNT OF CLOUDS COVER IN THE ATMOSPHERE
Clouds like other the elements of weather tend to be differ from one place to place and time to time, but this is the outcome of the following factors:-
i. The rate of evaporation.
ii. Amount of water vapour in the atmosphere.
iii. Large water bodies.
iv. Prevailing winds.
v. Vegetation.
vi. Ocean currents.
TYPES/CLASSIFICATION OF CLOUDS
There are many different types of clouds, but they are often difficult to distinguish as they form constantly changes. The general classification of clouds was proposed by LUKE HOWARD in 1803 this classification is based on form appearance and height and
he used four Latin words cirrus, cumulus, stratus and nimbus.
i. High clouds (6,000 to 12,000m)
ii. Middle clouds (2,100 to 6,000m).
iii. Low clouds (below 2,100m).
1. High clouds 6,000 – 1,200m: consists of cirrus which start with cirro.
i. Cirrus: is composed of small ice crystals, wispy, fibrous or feather. Its a appearance is min bands or patches.
ii. Cirrocumulus: is composed of ice crystals:” globular or ripple like in appearance looks like ripples or wavy structures in the sand on a sea shore) Forming a thin cloud.
iii Cirrostratus: it looks like a thin white, almost transparent sheet that gives the sun and moon haloes.
2. Middle clouds below 2,100 - 6,000m: consists of clouds whose names start wish acto.
i. Altocumulus: is composed of water droplets in layers or patches globular or bumpy. Looking with Flattened based arranged in line or waves. This indicates fair weather.
ii. Altostratus: is composed of water droplets forming sheets of grey or water looking clouds, partly or totally covering the sky.
3. Low clouds below 2100m.
i. Stratocumulus: is a large globular mass: bumpy – looking soft and grey in appearance forming pronounces regular patten. It is in fact darker, lower and a heavier type of actocumulus cloud. It always allows the penetration of some our rays called crepuscular rays.
ii. Stratus: is fog- like low cloud forming a uniform layer: brings dull weather and often carom paled by drizzle.
iii. Nimbostratus: Dark grey and rainy – looking dense and shape less often gives continuous rain clouds of great vertical extent.
iv. Cumulus: round topped and flat based forming a whitish grey globular mass consisting of individual cloud units.
v. Cumulonimbus: is a lowering cloud which is usually white or black appearing as globular mass whose rounded tops often spread out forming an anvil structure. It is associated with convection rain lighting and thunder. This cloud is sometimes called "thunder head".
NB: Cirrus and curriform clouds occupy high level and form high clouds; clouds that start with acto are found in the middle level constituting middle clouds. Nimbus is the term that applies to rain bearing clouds; stratus applies to clouds that form layers or uniform cover and cumulus is the term used for clouds that occur in a massive nature.
IMPORTANCE OF CLOUDS COVER
i. They influence the formation of rainfall and other forms of precipitation.
ii. They prevent the incoming sun radiant energy in the earth surface.
iii. They help in weather forecasting.
iv. The regulate the temperature of the place.
7. SUNSHINE
Sunshine: Refers to the amount sun's rays or sun lights which received by the earth surface. It is measured by the instrument called Sun shine recorder
The line drawn on the map with the same amount of sunshine is Isohels: for sunshine.
FACTORS AFFECTING THE AMOUNT OF SUNSHINE
The amount of sun rays received by the earth surface is not the same through out the world wide it tend to be differ from one time to time and place to place due to the following factors:-
i. Cloud cover.
ii. Time.
iii. Season.
iv. Latitudes.
v. Aspects.
vi. Prevailing winds.
IMPORTANCE OF SUNSHINE
i. It support plant growth through photosysnthesis.
ii. It used in to drying food like cassava, millet, maize and fish
iii. People use sun shine to warm themselves.
iv. It helps people to dries clothes after washing them.
NB: Isopleths map: Are the statistical maps which are designed to show the distribution of different geographical phenomena in quantitative manner by means of lines in a given place. Normally The lines are established on a map face to join point with equal amount of distribution with reflection to an actual area on the earth surface represented. However they at differently named depending on a nature of phenomena shown on the map. Hence these are:-
i. Isohyets: for rainfall
ii. Isotherms: for temperature
iii. Isobars: for atmospheric pressure
iv. Isonepth: for cloudiness
v. Isohaline: for salinity
vi. Isobaths: for ocean depth
vii. Isokinetics: for wind.
viii. Isohels: for sunshine.
MEASURING AND RECODING ELEMENTS OF WEATHER
weather.
INSTRUMENTS OF MEASURING AND RECORDING ELEMENTS OF WEATHER
1. Thermometer: is the instruments which used for measuring the amount of temperate reached on the day. It is usually expressed by degree centigrade (°c) and Farhei height (°F).
Types of thermometers
i. Minimum thermometer: it is used to measured the minimum/lowest among of temperature reached on the day.
ii. Maximum thermometer: is the type of thermometer which uses to measured the highest/maximum among of temperature reached on the day.
iii. Six thermometer: Is the type of thermometer which used to measure the amount both minimum and maximum temperature reached on the day.
2. Rain gauge: used for measuring rainfall.
3. Hygrometer: used for measuring the amount of water vapour in the atmosphere.
4. Sunshine recorder: used for measuring the amount of sun rays.
5. Barometer: used for measuring the amount of atmospheric pressure.
6. Anemometer: Used for measuring wind speed.
7. Pyranometer: for measuring solar radiation.
8. Rain gauge: for measuring precipitation.
9. Wind vane/weather vane: for measuring wind direction.
WEATHER STATION
weather station: Is a place where the elements of weather are measured and recorded. Examples of the elements are temperature, humidity, pressure, rainfall, wind direction and speed, cloud cover, and sunshine.
STEVENSON SCREEN
A Stevenson screen: is the wooden box which consisted of differed instruments used to measure and recording elements of weather. The instruments which can be kept in the Stevenson screen are such are maximum thermometers, minimum thermometer, and six thermometers. Normally the stevenson screen is made to allow air temperature to be measured. It consists of a wooden box with louver sides in order to allow free air to enter inside. White colour in order to prevent sun rays.
Stevenson screens: is a meteorological screen which is used to shield or store the weather measuring instruments against the precipitation and direct heat radiation from outside sources while still allowing the air to circulate freely around them. It was firstly designed by Thomas
Stevenson. It contains dry bulb thermometer, and wet bulb thermometer, minimum and maximum thermometers, hygrograph, and thermograph.
Types of Stevenson screens
1. Small Stevenson screen: This consists of instruments like wet bulb thermometer and dry bulb thermometer.
2. Large Stevenson screen: This consist instruments like maximum thermometer and
,minimum thermometer, wet bulb and dry bulb thermometer, hygrometer, barometer, , Dew cell and rain gauge.
CONDITION NECESSARY TO CONSIDER DURING THE ESTABLISHMENT OF WEATHER STATION
i. The station should be kept in a open space. So as to influence free flow of air.
ii. The area should be free from taller structures like trees, buildings, etc.
iii. The grasses in the ground should be short.
iv. The ground surface should be free from flood and should not retain water during and after rain.
v. The station should be fanced with gate so as to avoid interference.
vi. The station should have prismatic compass with all four cardinal points.
vii. The ground surface should be level with all gentle slopes.
WEATHER FORECASTING
Weather forecasting: is the process of predicting about weather condition of a given place and time. It is normally done approximately 24 + 48hrs.
Methods of weather forecasting
Weather forecasting is obtained through two methods namely, traditional and modern methods
1. TRADITIONAL METHOD
Traditional methods: It is done through observation of some phenomenas such types of birds voices, crocking of frogs, rainbow, migration of birds, changes of wind direction, position of the moon and general body feeling.
METHODS USED IN TRADITIONAL WEATHER FORECASTING
i. Blossoming of the fruits trees like apricot, budding of acacia species indicate the onset of rains
ii. The development of young leaves, grasses emerging indicates the onset of rains.
iii. The dropping of leaves of fig tree indicates the onset of summer.
iv. Dropping of fruits before maturity indicates the very dry season or drought is expected.
v. Dropping or drying of immature fruits indicates the onset of drought.
vi. The appearance of dark clouds indicate the onset of rains
vii. The dark clouds preceding the strong winds indicates the thunderstorms.
viii. The rainbow colours: red dominating means more rains to come, if blue colour dominates means rain has passed.
ix. Well moisture soils tested by hand means humidity.
x. Appearance of red ants and increasing the size of anti-hills which are moist indicates rains
xi. Occurrence of army worms indicates the onset of drought
xii. Migration and immigration of birds is a good sign for rains.
xiii. Moon crescent facing upwards indicates upholding water and when facing downwards is releasing rains
xiv. The star pattern and movement of stars from west to east under clear skies indicates the onset of rains in 3 days.
xv. Grunting of pigs indicates the low humidity and increase in temperature.
xvi. Well-fed calves jumping around happily in the veld on their way home from grazing indicate good rains.
xvii. Increased libido in goats and sheep with frequent mating indicates good rains.
xviii. Snakes moving down in the mountains is a good sign for rains
xix. Frequent appearance of tortoise wandering around indicates good rains
xx. Mist covering hills and mountains indicates a good signs of rains
xxi. High temperature at night indicates a good rains
xxii. Low temperature at night indicates a late onset of rains
xxiii. Drying of wells, springs, rivers and wetlands indicates the onset of drought
xxiv. The presence of millipedes and frogs indicates the onset of rains season.
xxv. Singing of insects such as nyenze indicates the onset of high temperature summer season.
xxvi. Occurrence of more grasshopper indicates the less rains and hunger
xxvii. The appearance of pangolin indicates the rain prediction.
xxviii. The flowering of mango tree indicates the potential drought season.
ADVANTAGES OF TRADITIONAL WEATHER FORECASTING
1. They are easily available and accessible.
2. They are simple to understand by farmers.
3. They are easy to interpret.
DISADVANTAGES OF TRADITIONAL WEATHER FORECASTING
1. They only work when combined with scientific methods
2. They are cultural based and interpreted differently in different areas
3. They do not provide the predictions on the immediate future
4. They cannot predict mid-season dry spells or their probabilities.
5. They do not indicate rainfall distribution.
2. MODERN METHOD
Modern methods: it involve 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.
MODERN METHODS/ INSTRUMENTS USED FOR WEATHER FORECASTING
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 everyday 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. Weather Satellite; is an instrument that is used to take the pictures of clouds from space to help us see where and how fast clouds are moving. Apart from showing the locations of cloud cover, the satellites can also be used to measure temperature of the earth’s surface. The satellite used for predicting weather in the space. The satellite 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 simplest way of weather forecasting. 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. Weather Radar: these are the instruments that are use to show the location of areas of precipitation very clearly. The radar images are also able to show the intensity of precipitation.
5. Synoptic method; is the detailed analysis of current weather reports from a large area. The current weather data are related with the past analogous situations and forecasts are prepared from the relationship of both current and past data on weather.
6. Statistical method; the weather data are recorded/established between different weather elements and the resulting weather is forecast.
CHALLENGES OF MODERN WEATHER FORECASTING
1. Shortage of weather data due to few weather stations.
2. Poor telecommunications.
3. Insufficient understanding of atmospheric processes
4. Unstable atmospheric composition.
5. It need high cost to acquire weather forecast instruments.
IMPORTANCE OF WEATHER FORECASTING
Weather forecasting is useful to people in the following ways:-
1. It help farmers to plan for their farming activities: This is because weather broad casting helps farmers to adjust their farming activities to suit the expected weather conditions.
2. It influence transportation activities: This include both water transport and air transport since it helps sailors and air travelers to know condition of the winds.
3. Helps in planning for sport activities.
4. It helps in selecting suitable clothes.
5. It helps fishermen to plan for heir fishing activities.
6. It helps contractors to plan for a suitable houses.
7. It helps to plan for tourism activities.
8. It enables man to plan military activities: Military personnel benefit from weather broadcasting as they can plan their military activities depending on weather conditions.
9. It helps in predicting about natural calamities like floods/Tsunami, cyclones, etc.
10. It helps people to plan for the power generation
TYPES OF WEATHER FORECASTING
i. Short range: This takes a duration is 1-2 days
ii. Medium range: It akes a duration of 3 -4 days to 2 weeks
iii. Long range: It is done in the period more than 4 weeks
STAGES OF WEATHER FORECASTING
i. Proper recording of data
ii. Careful study of synoptic charts
iii. Search for similar situation from the historical data
iv. Preparation of the weather condition charts
v. Providing quickly correct conclusion abiut future weather phenomenon.
GUIDING QUESTIONS
1. list down any five instruments used for measuring and recording elements of
2. Outline five relation ship between weather and human activities.
3. Discuss six factors for variation of distribution of water vapour in the atmosphere.
4. List down any five conditions necessaried to be considered during the establishment of weather station.
