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TOPIC 4
WEATHER
The Meaning of Weather
- Weather is the condition of the atmosphere in one area at a particular time, for example if it is raining, hot, or windy.
- Weather, state of the atmosphere at a particular place during a short period of time. It involves such atmospheric phenomena as temperature, humidity, precipitation (type and amount), air pressure, wind, and cloud cover. Weather differs from climate in that the latter includes the synthesis of weather conditions that have prevailed over a given area during a long time period generally 30 years.
- Weather is defined as conditions of the atmosphere which occur at a place at specific time periods, that is, from hour to hour or day to day. It changes from time to time and from place to place. For example, it may be raining in the morning and sunny in the afternoon.
The scientific study of weather is called meteorology and a person who studies weather is called meteorologist.
Importance of Weather:
1. Carrying out the economic activities.
The knowledge of weather (and hence climate) enables people to carry out their economic activities depending on the weather and climatic conditions of their localities. For example, people living in cold areas which receive high rainfall can engage in dairy farming and the growing crops such as tea, coffee, banana, etc.
2. Shaping the earth.
Weather is one of the fundamental processes that shape the Earth. The process of weathering breaks down the rocks and soils into smaller fragments and then into their constituent substances. In this way, weather plays a major role in erosion of the surface soil, hence shaping the earth.
3. Supporting the lives of plants and animals.
The weather of any given region is important because it has a considerable impact on the water, sunlight and temperature of an ecosystem. Variation in long-term weather patterns and tendencies can result in certain regions getting more or less water or sunlight than other areas. These factors play an important role by influencing the type of plants and animals that can survive in the area.
4. Establishing the climate of a place.
Studying weather characteristics of a given place over along period of time (usually 30 to 40 years) enables the climatic conditions of that place to be established. Therefore, weather can be used as a basis for determining the climate of a given place.
5. Weather determines the kind of clothing to be worn by people in an area.
6. Certain weather patterns can also cause dangerous storms and natural disasters. We tend to be acutely aware of the weather when we are faced with exceptional or dangerous phenomena that could endanger our property, safety or even lives. Such phenomena are, for example, strong winds, hail, heavy rainfall, sleet, ice and frost.
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| Good weather encourages enjoyment |
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
1. Sunshine
2. Temperature
3. Humidity
4. Cloud cover
5. Precipitation
6. Wind
7. Atmospheric pressure
IMPORTANCE OF EACH ELEMENT OF WEATHER
1. Sunshine
This is the sun's rays that reach the surface of the earth.
Importance of Sunshine
a. The sun's energy enables plants to manufacture food through the process of photosynthesis
b) People need sunshine to dry their crops, food like cassava, millet, maize and fish They also use sunshine to warm themselves Dries clothes after washing them
2. Temperature
This is how hot or cold an object or place is. The heat in the atmosphere is supplied by the sun.
Importance of Temperature
a) Warm is essential for plant and animal survival
b) Plants need warm in order to manufacture their fool Facilitates formation of clouds
3. Humidity
Is the amount of water vapor(moisture) in the atmosphere
There are two types of humidity:
a) Absolute humidity and
b) Relative humidity
I. Absolute humidity: Is the actual amount of water vapor or moisture in a given volume of air at a particular temperature.
II. 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
4. 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.
5. Precipitation
This refers as the fall or deposition of moisture water vapor or frozen water from the atmosphere onto the earth’s surface. All life on the earth is purely dependent on moisture provided through precipitation.
Importance of Precipitation
a) rain provides water for plants to grow (i.e. water for irrigation, growth of grass and pasture depend on rain).
b) Animals also need water to drink
c) hhuman needs water for domestic and industrial uses.
Note: Excess rain and rainstorms result in disasters due to flooding
6. Wind
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
a) They cause flow of heat and moisture and their transfer from one point to another and are also responsible for the movement of clouds.
b) 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.
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.
7. 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. The weight of atmosphere on the earth's surface and at sea level at 1033.3g/cm2.
Areas of very low pressure cause feeling of weightlessness and that is the feeling one gets when he or she is on top of a very high mountain like Mt Kilimanjaro. Areas under very low pressure may experience very strong winds periodically as air flows into such areas from high pressure regions.
Instruments Used for Measuring Elements of Weather
1. 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.
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 used 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.
Is the thermometer used 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 used for measuring and recording maximum and minimum temperature reached in a day. The thermometer consists of a “U” shaped glass tube. 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 falls the alcohol on the left hand side contracts. This pushes the mercury downward and up by the mercury. Reading and Recording Temperature are taken everyday 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.
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| Six’s Thermometer |
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.
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.
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.
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. When the air is not saturated water evaporates from the muslin and cools the wet bulb. The cooling effect causes the mercury to contract. The dry bulb is not affected in the same way as wet bulb because it does have a Muslin wrapping.
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| Hygrometer |
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.
i. 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.
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 vapor in the atmosphere turns into crystals of ice and reaches 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
vi. Sleet
Is a mixture of snow and rain. It forms when the temperature of the ground is lower than the temperature above.
vii. Rain
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.
Types of Rainfall
1. 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.
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| Convectional Rainfall |
2. 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 .
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| Relief 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 vapor condenses to form clouds and rain. On the other hand tropical cyclones are formed over oceans is the tropical between latitude 8°N and 8° 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.
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| Cyclonic Rain |
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.
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| Rain gauge |
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.
i. Ocean Currents
i. 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.
ii. 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. 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.
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. 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.
iii. 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.
iv. 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.
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.
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.
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.
Rainfall Effects
Too much rainfall for example, the case of Elnino (1998) results into negative impacts on life and properties such as:
1. Heavy rainfall may cause destruction of houses, roads bridges and crops and
2. Loss of life for both human and animals.
3. Floods, which are the results of heavy rainfall may result into eruption of diseases such as malaria, cholera, and dysentery.
4. 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 in some parts of the world have 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
It is 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.
Types of Barometer
There are two types of barometer, mercury barometer and aneroid barometer.
i. Mercury Barometer
It is a simple apparatus consisting of a glass of about 9.2 centimeters long and bowl filled with mercury. The glass tube is filled with mercury and being put upside down in the bowl of mercury. At sea level the mercury will fall until the column is about 76 centimeters above the surface of the mercury in the bowl. A vacuum is left at the top of the tube. To read the pressure centimeter rule is placed besides the glass tube and the change in height of the mercury column gives the reading of the atmospheric pressure. Mercury barometers are not portable.
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. Lines joining places with the same pressure are called isobars Lines joining places with the same pressure are called 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.
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| Barometer |
Questions and Answers
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.
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.
c) Other pressured things include the decoration balloons and playing.
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.
SEA BREEZE (DAY) Is when winds blow from sea to land during a day.
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| Sea Breeze |
LAND BREEZE (NIGHT)
This occurs during night when winds blow from land to sea.
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| Land Breeze |
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. 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. There are great variations in the general patterns of wind systems in areas of the earth’s surface where there are large land masses adjacent to large water bodies. During summer, intense low pressure develops in central Asia in the Northern hemisphere and winds blow inform of high pressure over the Indian and pacific oceans. Wind belts of the world Occasionally in the westerly wind system depressions and anticyclones develop.
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.
1. 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.
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| Wind Vane |
2. 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.
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| Anemometer |
WEATHER STATION
Various definitions of weather station
- Weather station is a place where the elements of weather are measured and recorded.
- Weather station is a station for taking meteorological observations, making weather forecasts, and disseminating such information.
- Weather station is a place where the elements of weather are measured and recorded.
- Weather station is a station for taking meteorological observations, making weather forecasts, and disseminating such information.
- A weather station is a facility, either on land or sea, with instruments and equipment for measuring atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include temperature, barometric pressure, humidity, wind speed, wind direction, and precipitation amounts.
Wind measurements are taken with as few as other obstructions as possible, while temperature and humidity measurements are kept free from direct solar radiation, or insolation. Manual observations are taken at least once daily, while automated measurements are taken at least once an hour. Weather conditions out at sea are taken by ships and buoys, which measure slightly different meteorological quantities such as sea surface temperature, wave height, and wave period.
Weather station data can be used to gauge current weather conditions and to predict the future weather forecast, like temperature high/lows, cloud cover and probability of precipitation. Weather stations are used by meteorologists, weather buffs, gardeners, farmers, outdoor enthusiasts, students, pilots and anyone who enjoys weather data or relies on the weather to make decisions.
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| Weather Station |
How to Establish Elements of Weather
Selecting an appropriate site for the weather station is critical for obtaining accurate meteorological data. Typically, the site should represent the general area of interest, and be away from obstructions such as buildings and trees.
When establishing a weather station the following guidelines must be considered:
1. The station should be located on an open space with free circulation of air.
2. There should be a wide view of the surrounding landscape and the sky.
3. The site should be free from obstructions by trees, buildings, mountains, etc. The station should not be under the shadows of objects. The open areas should be covered by short grass, or where grass does not grow, the natural earth. Avoid large industrial heat sources, rooftops, steep slopes, sheltered hollows, high vegetation, shaded areas, swamps, areas where snow drifts occur or low places holding stagnant water after rains.
4. The ground should be plain or gently sloping at a gradient not more than 5°.
The station should be fenced to keep off intruders, trespassers and passers-by and should always be locked. Only authorized people should have access into the station.
5. The geographical location of the station should be established by placing a compass in the station. This will help in determining the direction of wind shown by a wind sock /vane put in the station.
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| Weather Station |
Stevenson Screen
It 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.
Its Functions
WEATHER FORECASTINGIts Functions
The Stevenson screen holds instruments that may include thermometers (ordinary, maximum/minimum), a hygrometer, a psychrometer, a dew cell, a barometer and a thermograph. Its purpose is to provide a standardized environment in which to measure temperature, humidity, dew point and atmospheric pressure.
The traditional Stevenson Screen is a box shape, constructed of wood, in a double louvered design. However, it is possible to construct a screen using other materials and shapes, such as a pyramid. The World Meteorological Organization (WMO) agreed standard for the height of the Stevenson Screen is between 1.25 m and 2 m above the ground.
The siting of the screen is very important to avoid data degradation by the effects of ground cover, buildings and trees. It is recommended that the screen be placed at least twice the distance of the height of the object, e.g., 20 m from any tree that is 10 m high. In the northern hemisphere, the door of the screen should always face north so as to prevent direct sunlight on the thermometers. In polar regions, with twenty-four hour sunlight, the observer must take care to shield the thermometers from the sun and at the same time avoiding a rise in temperature being caused by the observer's body heat.
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| Stevenson screen |
The general purposes of the Stevenson Screen are:
- to ensure the safety of the delicate instruments kept in it which could easily be damaged if kept in the open air;
- to ensure accurate measurements of the meteorological data; and
- to protect instruments against precipitation and direct sunlight and heat, while still allowing air to circulate freely around them.
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.
i.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.
ii. 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.
Modern Methods and Instruments Used for Weather Forecasting
1. Radiosonde.
This is an instrument used to register pressure and temperature from the ground to roughly 12 km 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.
This is an instrument used to register pressure and temperature from the ground to roughly 12 km 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.
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 simpler 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.
This is simpler 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.
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.
Importance of Weather Forcasting
1. Farmers 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.
2. Weather broadcasting greatly influence transport since it helps sailors and air travelers to know condition of the winds.
3. Helps in planning sport activities.
4. Helps in selecting suitable clothes.
5. Helps fishermen to plan their fishing activities.
6. Helps contractors to plan for suitable houses.
7. Helps in geographical expeditions like tours and military.
8. Military personnel benefit from weather broadcasting as they can plan their military activities depending on weather conditions
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This is great
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