10) Direction of Mountain ranges â Mountains act as barriers to the movement of wind hence affecting both temperature
ATMOSPHERE and ATMOSPHERIC PRESSURE The Atmosphere Definition 1: Atmosphere is defined as the blanket of air surrounding the Earth. Definition 2: Atmosphere can also be defined as the envelope of air that completely surrounds the Earth. It is the presence of the atmosphere with its abundant supply of oxygen that makes the Earth a unique planet in the universe. The oxygen in the atmosphere has been responsible for the origin and growth of life on Earth. Composition: The atmosphere is made up of a layer of colourless, odourless and tasteless gases and tiny solid particles. The composition of air is not constant. It varies from time to time and from place to place. However, the approximate proportion of gases in the atmosphere is as follows: Gas
Percentage
Nitrogen
78.08%
Oxygen
20.94%
Argon
0.93%
Carbon Dioxide
0.03%
Hydrogen, Neon, Helium etc
0.02%
Total :
100%
1
Activity 1: Using your geometrical skills, make a pie chart to represent the above data.
The above gases are called fixed components of the atmosphere, because their relative proportion is constant in the lower layers of the atmosphere. Besides the fixed components, there are a number of variable components of the atmosphere – dust, smoke, salt grains, pollen grains, emissions released from chimneys of factories and water vapour (one of the most important). On the basis of composition of gases, the atmosphere is divided into: Heterosphere (80 kms to around 480kms above the surface of Earth). Homosphere (from surface of the Earth to 80kms) Activity 2: Make a simple pictorial representation for illustrating division of the atmosphere on the basis of composition of gases.
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The composition of air varies from place to place and from time to time. The amount of oxygen and water vapour decreases as we go higher. Activity 2.1: (To be done by research) Can you list down the importance of water vapour and dust in the atmosphere???
Which areas do you think will have more dust particles? Urban or rural areas? Justify your answer.
Find below the composition of gases in planets Venus, Earth and Mars. What are your inferences? Write them down in the box provided below the picture.
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Significance of the atmosphere:1. The ozone acts as a filter and protects the Earth from harmful ultraviolet and infra-red rays of the sun. This way the atmosphere also prevents the Earth from getting too hot during the day. 2. The atmosphere protects us from millions of meteors and meteorite particles which fall towards the Earth every day. 3. The blue sky and the red or orange colour in sky during sunrise or sunset is because of the particles in atmosphere which scatter solar radiation. 4. There are changes in weather due to presence of atmosphere. 5. Sound waves can travel only due to air. 6. Carbon dioxide is needed by plants in process of photosynthesis. 7. Carbon dioxide and water vapour in the lower layers of the atmosphere absorb the heat radiated from the Earth‟s surface and keep the atmosphere warm even during the night. This is known as „the green house‟ effect. 8. The heating up of the Earth‟s atmosphere to different temperatures InIn nInferences:-
Wng
Which factors influence the fixed and variable components of the atmosphere?
produces circulation in the atmosphere leading to wind and precipitation (rain, snow, sleet etc.) and hence the circulation of fresh water. 4
9. Oxygen is crucial in respiration and combustion. 10. Argon has commercial and industrial uses, especially in manufacturing of bulbs, welding equipment and lasers. 11. Nitrogen combines with other elements to form nitrogen compounds which are basic for all organic processes. Nitrogen from air is also used for manufacture of nitrogenous fertilizers. Activity 3: (To be done by research) Can you think of some other significances of our atmosphere? _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ Layers of atmosphere:_______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________
layer – formation, significance and depletion :One of the most significant components of our atmosphere is the ozone layer which protects us from the harmful ultraviolet radiation of the Sun. Ozone is formed due to the chemical reaction in the stratosphere. An oxygen molecule O2 absorbs the ultraviolet energy and splits it into two oxygen atoms O + O. A free oxygen atom O combines with an O2 molecule and forms the ozone O3. In the troposphere, ozone is generated only in small amounts. But in the stratosphere it is present in significant amounts. The ozone layer acts as a filter and protects us from harmful ultraviolet rays of the Sun. But it is depleting and even an „ozone hole‟ is being detected through satellite images by NASA. The largest ozone hole was noticed in September 2000 near Antarctica. Extensive use of CFC‟s (Chloroflurocarbons) in refrigerators, aircrafts, airconditioners are responsible for this. Even volcano eruptions give out sulphur 5
dioxide which speeds up destructive chemical reactions. Many chemicals namely chlorine and bromine are banned which are thought to cause the depletion of the ozone layer. Solar Radiation Hydrogen and helium are the predominant gases that constitute the Sun. The proportion of hydrogen and helium is 3 : 1. This means that there is 3 times more hydrogen as compared to helium. The core of the Sun acts like a gigantic nuclear reactor and converts huge quantities of hydrogen into helium. In this process of nuclear fusion, the Sun releases tremendous energy in all directions. But the Earth because of its small size intercepts only a small part of the Sun‟s radiant energy. Sun is constantly radiating heat energy into the space in form of electromagnetic radiation. This is known as solar radiation. It travels at speed of about 3,00,000 km per second and covers a distance of about 150 million kms in about 8 min and 20 seconds to reach the surface of the Earth. Only 1 in two billion parts of Sun‟s tremendous energy reaches the Earth. The solar radiation is responsible for: the movement of air and ocean currents due to difference in heating across the Earth. driving the hydrological cycle through evaporation and precipitation. helping in cycling and recycling of nutrients and chemical elements in the biosphere. helping the plants to prepare food through the process of photosynthesis. Weathering of rocks Cultivation of crops
Activity 4: (To be done at home) Stick the picture of Sun. Also write some information about the Sun.
____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ ____________________________________________________________________________________ 6 _____________________________________________________________________________________ _____________________________________________________________________________________
Insolation and Terrestrial Radiation Definition 1: The amount of solar energy received by the Earth is called insolation (in +sol or sun + radiation). Definition 2: The incoming solar energy intercepted by the Earth is defined as insolation. The Sun‟s energy travels to the Earth as electromagnetic radiation which can be thought of as energy moving in waves. There are short waves and long waves. The incoming solar radiation is received in the form of short waves. Heat in the form of short waves is not as easily absorbed as the long waves. The Sun‟s energy reaches the Earth as short wave rays. Out of total 100% solar energy only 51% reaches the Earth, 35% is reflected back into space and only 14% is absorbed by the atmospheric layers including ozone during insolation. The amount of insolation received depends upon – the inclination of the Sun‟s rays, the amount of atmosphere to be crossed, duration of sunshine or length of day, amount of cloud cover, distance between the Sun and the Earth. Discuss how these factors influence the amount of insolation? (Use the space on the left to jot down points) 7
Definition 1: The solar radiation received by Earth during the day is radiated back into the atmosphere at night in form of long waves. This is referred as Terrestrial radiation. Definition 2: The Sun‟s energy absorbed by the Earth‟s surface and radiated out into space is called terrestrial radiation. Definition 3: The insolation radiated out by the Earth is called terrestrial radiation. Terrestrial radiation is in the form of long waves. These long waves are more readily absorbed by the molecules of gases in the atmosphere. Out of the 51 units of insolation that are absorbed by the Earth, all 51 units are radiated out at night. Out of the 51 units of terrestrial radiation given out by then Earth, 34 units are absorbed by the atmosphere. This means that the atmosphere absorbs only 14% of the incoming insolation but absorbs 67% (34 out of 51) of the terrestrial radiation. Thus, the atmosphere is heated more by terrestrial radiation given out by the Earth than by the incoming radiation. Not all terrestrial radiation can escape to space. Some gases in our atmosphere – mainly water vapour and carbon dioxide trap it. This helps to avoid extreme temperature during day and night. The atmosphere also gives out the heat absorbed. Thus a balance is maintained between the amount of heat that enters the Earth‟s atmosphere and that which leaves it. The atmosphere acts like a blanket. 1. At night, the atmosphere keeps the Earth‟s surface warm by preventing all the terrestrial radiation from escaping. 2. During the day, the atmosphere prevents the Earth from becoming too hot by not allowing all the insolation to reach the Earth. Thus, the atmosphere prevents extremes of temperatures. Without the atmosphere, the Earth would become very hot during the day and very cold at night.
Activity 5: Make a diagrammatic representation of insolation and terrestrial radiation
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Heat Balance or heat budget The Earth as a whole maintains a balance between the incoming and the outgoing radiations; still its ratio is not uniform all over the Earth. Due to imbalance of heat, winds and ocean currents are produced and they help in the transfer of heat. This transfer of surplus heat energy from the lower latitudes to the heat deficit zone of the higher latitudes maintains an overall heat balance over the surface of the Earth. Definition: Heat balance is defined as the state of equilibrium that exists on Earth between insolation from the Sun and the outgoing terrestrial radiation from the Earth. Transfer of Heat: The Sun is the most important source of heat for the atmosphere, but the atmosphere is not directly heated by the rays of the Sun. The atmosphere is mostly heated by the processes of Conduction, Convection, Radiation and Advection. You already know how heat is transferred by conduction, convection and radiation. (Research and write on the left side of this page as to how the transfer of heat happens in the atmosphere by these three processes) Advection – The air masses sometimes move from one region to another alongwith the winds. These winds carry temperature of one place to another. The 9
air temperature of a place thus rises or falls depending upon whether the wind is coming from a warm or cool area. For example, winds coming from warm areas, transfer the heat in cold areas. Advection is defined as the transfer of heat in the horizontal direction. Measuring temperature (Celsius and Fahrenheit, calculations) Fahrenheit, a measure for temperature is named after Gabriel Fahrenheit who invented the modern mercury and alcohol thermometer. Celsius is another measure for temperature is named after Anders Celsius, a Swedish astronomer who introduced the centigrade scale. Today almost all countries except USA use the Centigrade scale. The freezing point of water is 0˚C and the boiling point is 100˚ C. Calculations:1) Daily Mean Temperature =Max temp. Of the day + Min Temp. Of the day 2 2) Range of Temperature = Highest value in Max column – Lowest value in Min column OR Highest temp. – Lowest temp. 3) Daily or Diurnal range of temperature = Max temp – Min temp (of that day) 4) Monthly Mean Temperature = Daily mean temp of all days in the month Number of days (30/29/28/31) 5) Annual /Weekly Mean temperature = Sum of all mean monthly temperatures/daily mean temp 12 (in case of monthly) & 7(in case of weekly) 6) Temperature in degrees C = Temperature in degree (F – 32) x 5/9
Activity 6: (To be done at home) Collect newspaper cuttings related to daily temperatures from date: Stick the cutting and label it with the newspaper date on the space provided. Fill the following table and depict it on graph paper. Newspaper date
Actual date
Max temp
Min temp
Daily range
Daily mean
Activity 7: (To be done at home) Stick the newspaper cuttings and label each one with the respective date. (Use the space on the left also to stick the cuttings)
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Temperature graph
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Factors affecting temperature 1) Latitude – Temperature decreases with increase in latitude on either side of the equator due to spherical shape of the Earth. Oblique or slanting rays not only travel a longer distance but also heat a larger area. Thus they have less heating power. The sun rays that are vertical travel short distance and heat up a smaller surface leading to higher temperature. Thus, places near the equator (Torrid zone) experience sun overhead at noon and the sunrays are vertical. In higher latitudes (Frigid zone) even at noon the sun is low in the sky and its rays are slanting. Take an example of Paris and Chennai. Chennai experiences 12 hours day almost throughout the year, whereas Paris experiences 16 hours daylight in summer. Still Chennai is warmer as 12 hours of insolation from overhead sun has much more heating capacity. Most hot Cold Torrid Temperate Frigid
Activity 8 : (To be done at home) Using internet or book resources. Find atleast 4 pairs of places, whose temperatures differs due to this factor. 1) ________________________ and ______________________ 12 2) ________________________ and ______________________ 3) ________________________ and ______________________ 4) ________________________ and ______________________
2) Altitude or height above sea level
The height of a place above the mean sea level is known as altitude. * Discuss with your teacher, the meaning of the term „mean sea level‟. Normal lapse rate is defined as the decrease in temperature with increase in height at the rate of 1°C for every 165 metres of ascent. As we go higher, the air becomes less dense and hence has a lesser capacity to absorb heat. Also the Earth is more heated by terrestrial radiation, which is more in lower levels of atmosphere. Hill stations like Lonavala, Panchagani, Ooty are cooler due to the same reason. There can be inversion of temperature in a mountainous region in the temperate belt if there are no clouds and humidity is low. That means during the day the insolation takes place in the valley and by the mountain tops. With sunset, the Earth‟s surface radiates heat back into space. The mountain tops lose more heat and the cold dense air starts sinking down. The warm air over the valley floor is displaced by this cold air and the warm air reaches the mountain tops. Due to this reason, fruit growers always have their orchards on the upper slopes of hills.
Activity 9 : To gain better understanding of inversion of temperature, make a diagrammatic representation of the same. (Use the space on the left to draw)
3) Distance from the sea –
The atmosphere gets heated mainly by the radiation of the Earth, the nature of the surface of the Earth is a very important factor affecting the temperature of the place. The major temperature contrasts on the surface of the Earth are between land masses and oceans. Compared to land, water (liquid) absorbs heat slowly and loses it slowly as well. Sun‟s rays pass through water to a great depth. Water is mobile and the warm water mixes easily with cold water. The process of convection heats it and as the particles move, the heat travels both in depth and area. Land (solid) gets heated quickly because the heat obtained by the area does not mix with other areas of land. Moreover the heat remains at the surface and so when the Sun sets, it loses the heat quickly. During the day, the land is hotter than the sea. The hot air over the land becomes lighter and goes upwards. This creates a low pressure over the land. The air above the sea is cooler during this time. When this air blows towards the low pressure over the land, it lowers the temperature of the air on land. Such cool breeze that blows from the sea to land is called Sea breeze. In the night, the reverse happens. The sea remains warmer than the land because it needs more time to lose the heat. The warm air over the sea becomes lighter and goes upwards. This creates a low pressure over the sea. The air above the land is cooler during this time. When this air blows towards the low pressure over the sea, it lowers the temperature of the air over sea. Such cool breeze that blows from land to sea is called land breeze. Thus, the interchange of breeze maintains heat balance. The places near the sea have a moderate climate. Also the places close to warm sea are more humid and get more rain than places near cold seas or far inland. 13
The places near the sea have warmer winters and cooler summers whereas the places inland have hot summers and cold winters. The places near the sea experience moderating effect of seas caused due to differential heating of land and water.
LAND BREEZE Rising air helps form clouds
Landr Land cools down (heat Land Ocean is warmer compared
sink)
to land (heat source)
Activity 10: Practice land breeze
SEA BREEZE
Rising air helps form clouds
Landr Ocean cools
Llan
Land heats up (heat source)
down (heat sink)
Activity 11 : Practice sea breeze
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4) Ocean currents – A current is like a vast river within the ocean, flowing from one place to another. These currents are caused by differences in temperature, differences in salinity, and by wind. When winds blow over warm or cold currents, the warmness or coldness is carried further into lands. * Although this seems confusing right now, we will be learning about them in Std 9. 5) Winds – Winds blowing in from warmer regions increase the temperature of a place while winds blowing from colder regions reduce the temperature of the place they blow over. Winds blowing over the sea carry more moisture than the ones that blow in from the land and may affect the humidity of the place and in turn temperature. Local winds which blow over a specific region during specific period of year, also affect the temperature of the place over which they blow. 6) Soil – Type of soil also affects temperature of a place. Sandy soil heats up and loses heat very quickly whereas heavy soils like clayey soil which contains lot of moisture heat up slower and lose heat slower too. Trees also provide shade and intercept some of the heat from reaching the surface of Earth. 7) Vegetation – Every surface has different properties. A crop covered field absorbs 60 to 80% of the solar energy while a snow surface absorbs only about 20%. A forest surface absorbs about 90% of the solar energy. Thus, vegetative cover absorbs more sunshine than snow fields. Lot of moisture in the form of transpiration is released into the air. The same is the reason for increasing rainfall of a place. This explains why these areas do not become very hot. 8) Clouds – Clouds reflect a large portion of sunlight, thus limiting the amount of heat that reaches the surface of the Earth. Thus a cloudy day is cooler than a clear day. Clear nights are cooler than cloudy night as clouds prevent the heat from escaping from the Earth‟s atmosphere. 9) Slope of the land – A slope that is very steep undergoes a more rapid change in temperature as compared to a gentler slope as it is usually bare and does not support soil and vegetation. Also, in the north hemisphere, all the south facing slopes receive the direct rays of the Sun and are therefore warmer. In the south hemisphere, all the north facing slopes will receive more sunlight than the south facing slopes. 10) Direction of Mountain ranges – Mountains act as barriers to the movement of wind hence affecting both temperature and rainfall. Depending on the direction of a mountain range, they can protect from cold winds and thus keep temperature warmer. For example, Calcutta in India and Canton in China both lie on the coast and same latitude, but Canton is much colder that Calcutta in winter. Mountain ranges also bring about contrasts in rain with the wind ward side receiving heavier rainfall than the leeward side. On the windward side, the rainfall increases with height as the temperature cools and more condensation occurs.
It was believed that the Greek God Aelous kept the wind in a cave 15 with a huge rock blocking the entrance. He would open the cave at his whim which would produce storms. One such storm is a tornado which is a spinning column of air. This does not sound threatening
How do tornadoes occur? Do winds move in specific direction? What is atmospheric pressure? The quest begins…. Pressure Belts and Types of Winds The Earth is completely surrounded by air. Gravity pulls the molecules of air and other atmospheric gases downwards, towards the ground. The upper layers of the atmosphere rest on the lower layers, thus exerting pressure on the surface of the earth. This is called atmospheric pressure. Definition: Air pressure is defined as the weight of air exerted on a unit area of the Earth’s surface. Atmospheric pressure is an important factor in producing changes in weather. Contrasts in temperature cause changes in air density, which is responsible for variations in pressure. These variations cause horizontal movements of air called winds. Winds transport heat and moisture form one region to another and thus help in the occurrence of precipitation and affect both temperature and humidity. Factors affecting air pressure 1. Altitude(Height above sea level) 2. Temperature(Degree of heat in a place) 3. Rotation of the Earth from West to East Activity 1:- How do the following factors affect air pressure? (to be done in class)
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Effects of air pressure 1. Winds are caused due to differences in air pressure 2. Atmospheric pressure influences weather Activity 2:- Discuss in detail the effects of air pressure. (to be done in class)
Find out!! What is altitude sickness? How does it affect travelers?
Pressure Gradient and Isobars Definition: - Isobars are imaginary lines that show variations in the pressure of air on weather maps. These are lines joining the places having same barometric pressure. The word isobars mean equal weight. Definition: - Pressure gradient is the rate of change of atmospheric pressure between two points on the earth’s surface. On the weather chart, this is indicated by the spacing of isobars. When the isobars are far apart, it suggests a weak gradient (there is a little difference of atmospheric pressure) and winds are gentle. The weather is calm.
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When the isobars are close together, it suggests a steep gradient (there is a great difference of atmospheric pressure) over a small area and the winds are strong. The weather is stormy. Weather maps use isobars to show atmospheric pressure because the different pressure systems cause different types of weather. Activity 3:- Identify four areas of strong pressure gradient and weak pressure gradient respectively in the above weather maps. (to be done at home)
World pressure belts The atmospheric pressure is not the same throughout the year. The variation in atmospheric pressure is mainly due to the differential (unequal) heating of the Earth and its atmosphere. The pattern of pressure distribution generally corresponds with the heat zones of the Earth. Areas that are hot will always have hot rising air currents, thus forming a low pressure region. In a cold region, the air is cold and heavy. This heavy air descends downwards causing a high pressure area in that region. The distribution of atmosphere pressure across the latitude is termed global horizontal distribution of pressure. On the earth‟s surface, there are seven pressure belts. They are the Equatorial Low, the two Sub-tropical Highs, the two Sub-polar Lows and the two Polar Highs. There is a pattern of alternate high and low pressure belts over the earth. This is due to the …………… shape of the earth and ……………… heating of the …………….. by the …………... i)
Equatorial Low Pressure Belts
Location: It extends from 0 to 5° North and South of equator. Reason: Due to the vertical rays of sun at the Equator there is an intense heating. The heated air therefore, expands and rises as convection current causing a low pressure to develop here. The air here is also very warm.
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Alternative Name: This low pressure belt is also known as doldrums, because it is a zone of total calm without any breeze. The word doldrums is an old English word which means “dull”. Sailors used to avoid this region, fearing their ships would be stranded due to the lack of winds. ii)
Sub-tropical High Pressure Belts
Location: This high pressure belts is found at about 30° North and South of equator. Reason: The temperature in the sub-tropical belt is less than the Equatorial region. Hence, the warm light air which rises starts to sink near the sub-tropical regions after they become cold and heavy after reaching the upper parts of the atmosphere. Alternative name: This high pressure area is also known as Horse latitude. Winds always blow from high pressure to low pressure. So the winds from sub tropical region blow towards equator as trade winds and another wind blows towards Sub-Polar Low Pressure as Westerlies. iii)
Circum- polar or Sub polar Low Pressure Belts
Location: - The belts located between 60° and 70° in each hemisphere are known as Cirumpolar Low Pressure Belts. Reason: The two reasons for the development of low pressure belts in this region are due to the:a) descending air from the cold Polar regions and ascending air from 30°N/S high pressure belts b) rotation of Earth – The Earth‟s rotation reduces the thickness of the air by swinging the bulk of air towards the sub-tropical and polar regions. This creates a partial vacuum creating a low pressure belt.
iv)
Polar High Pressure Belt
Location: - The belt located between 70° to 90° North and South hemisphere are known as Polar High Pressure Belt. Reason: - The temperatures are always extremely low in these regions. The cold descending air gives rise to high pressure over the poles. These areas are known as Polar Highs. These regions are characterized by permanent Ice Caps. Activity 4:- Practice diagram of world pressure belts at home.
CHECKPOINT!! Complete the passage Atmospheric pressure ……………… from place to place. It is the ………………….. exerted on a …………area of the Earth‟s surface. Atmospheric ……………… is measured using an ……………. 19
named ……………….It is a long …………….. tube that is filled with ………………..The other ……….. types of mercury are …………………………. and ………………………. barometers. When the pressure falls below …………. it is said to be ……. pressure and when it is ……… 760 mm, it is said to be high pressure. These ………… in pressure cause ………………….movements of air called ………………… which in turn lead to changes in …………………. Rapid fire!!!! 1) Name the instrument used to measure atmospheric pressure. 2) Which factors affect the atmospheric pressure of a place? 3) An area of low pressure/high pressure is created by warm air. 4) There are four pressure belts on the Earth. True or False? 5) The sub-tropical high pressure belts are also called …………. latitudes. WINDS Wind is air in motion. The horizontal movement of the air is called wind. There is constant flow of wind from areas of high pressure to that of low pressure. Wind is strong and fast if there is wide difference in pressure between two places. The wind gets its name depending on the direction from which it is blowing. For example, of it is blowing from the North to the South, it is called North wind. However when the air moves in a vertical direction, it is called air current. Winds and air currents together make up a system of circulation in the atmosphere. Wind speed is measured using an anemometer. A wind vane or weather cock is used to measure direction of wind. The speed of wind is expressed in metres per second, kilometers per hour or knots. The vertical or nearly vertical movement of air is referred to as air current. Wind moving at a low speed is called breeze.
Off the beaten path Winds blow at the speed of 320km/hr at George V in Antarctica. It is the world’s windiest place. Activity 5: Math time! On weather maps, the meteorologists use the following symbols to represent specific wind speeds such as:-
Given below are the coded forms of prevailing wind speeds of some weather stations. Decode the wind speeds
For more such symbols, visit https://brianmejia.files.wordpress.com/2010/12/windbarbs.gif Major types of wind Planetary winds
Periodic winds
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Variable winds
Local winds
Meaning
Examples
Planetary winds are permanent or
Periodic winds are winds which blow
Variable winds are winds that affect
Local winds are winds that blow
prevailing winds as they blow constantly, throughout the year in the same areas.
in a definite direction during a specific season or at a particular time of the day– either day or night, summer or winter season.
only small areas and for a short period last for only a few of time over a very days. They are small area. referred as variable because they vary in time, speed and area.
Trade winds,
Land & sea breeze,
Cyclones,
Loo, Simoom,
Westerlies, polar winds
monsoons
anticyclones
Chinook, Bora, Mistral, Pampero
1) PLANETARY WINDS (Also known as permanent or prevailing winds) 1) Trade Winds These winds blow from subtropical high pressure belts (horse latitudes) to the equatorial low pressure belt (doldrums). The name „Trade” has been derived from the Latin word „trado‟ which means “to tread or blow in a constant direction”. Since these winds always blow on the same track and in the same direction regularly, they came to be known as the Trade winds. In the North hemisphere they blow from 30°N to equator and are known as North East Trade Winds. In the South hemisphere they blow from 30°S to equator and are known as South East Trade Winds. These trade winds blow from the eastern direction and hence they bring rain to the eastern margins of the continent and the western parts get very little rain. 1) Prevailing Westerlies These winds blow from the sub-tropical high pressure belts (horse latitudes) to the sub polar low pressure belts, between 30° and 60°S North and South. They are not in constant in strength and direction as the trade winds. Westerlies blow from south-west to north-east in the Northern hemisphere and from northwest to south-east in the southern hemisphere. Thus, they are called South and North Westerlies in the Northern and Southern hemispheres respectively. Wherever these winds blow on shore in the Northern hemisphere, they bring rainfall. In the south hemisphere, there is absence of land masses and therefore the North Westerlies become very strong especially between 40° and 50° and are called „Roaring Forties‟ or „Brave West Winds‟, „Screeching sixties‟ and „Furious Fifties‟. 2) Polar Winds These winds blow from the polar high pressure belts to the sub-polar low pressure belts.
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They blow from north-east to south-west in the Northern hemisphere and from south-east to north-west in the Southern hemisphere. Thus, these winds are called the North-east polar winds and South-east polar winds in the Northern and Southern hemispheres respectively. These winds are cold and dry as they blow from the cold ice-capped regions.
2) PERIODIC WINDS Periodic Winds as the name suggests blow in a definite direction during a specific season or at a particular time of the day. a) Land & Sea breeze: These are experienced in the coastal areas. During the day, land gets heated faster than the water in the sea. Warm air above the land becomes lighter and rises causing low pressure at the surface. As a result, the cooler sea breeze starts blowing from the sea to the land in the day time to fill the gap created by the warm rising air, from the land surface. As the warm air rises, it cools and finally sinks over the sea. During the night, land cools faster than the sea. Warm air rises from the sea creating low pressure over the sea. As a result, dry winds (referred as land breeze) start blowing from the land to the sea. As the warm air rises, it cools and finally sinks over the land. Activity 6:- Practice diagrams of land and sea breeze at home. You can refer to your “Atmosphere” booklet for the same.
Quick five!! a) Land takes in and gives out heat ……………… b) ................... takes a lot of time to take in and ……….. out heat. c) Land breeze blows at ……………… d) The breeze that blows at night. e) Write down two points of differences between land and sea breeze.
b) The Monsoon: The word „monsoon‟ has been derived from the Arabic word „mausim‟ which means „season‟. The term was first used in English in India, Bangladesh, Pakistan, and neighboring countries. The land and sea breeze follow a daily pattern while the monsoon winds follow a seasonal pattern. This way they differ from land and sea breezes. They are land and sea breezes on a larger scale. They blow from sea to land in summer and from land to sea in winter Definition:- Monsoons are big seasonal winds blowing from the Indian Ocean and Arabian Sea in the southwest bringing heavy rainfall to the area. Based on the time of the year that these winds bring rain to India, they can also be categorised in two rain periods called: Summer monsoon (May to September) and Winter monsoon (October to November) Summer monsoon – 1. The southwest monsoon is generally expected to begin around the beginning of June and fade away by the end of September. 2. Asia lies in the Northern hemisphere. In June, the rays of the sun fall directly on the Tropic of Cancer and the region experiences summer.
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3. The landmass of Asia gets extremely heated. A low pressure develops over the land. On the other hand, the Indian Ocean remains relatively cold during this period. high pressure develops. 4. The South-east trade winds which are the prevailing winds in the Southern hemisphere are attracted into India. 5. When these winds cross the Equator, they are deflected towards the right due to the Coriolis effect and they blow into India as the South-west monsoon winds. 6. The moisture-laden winds on reaching the southernmost point of the Indian Peninsula divide into two parts: the Arabian Sea Branch and the Bay of Bengal Branch. As these winds have lot of moisture carried by them, they bring rainfall. 7. Kanyakumari is the first place and Kerala, the first state to receive the rain. The bulk of the rainfall is received during this season in almost every part of India except Tamil Nadu.
Winter monsoon 1. During the winter season in Asia when the sun is directly over the Tropic of Capricorn (23 ½ °S), the air over the land gets cold and heavy, resulting in high pressure over the land. 2. The surrounding water bodies (seas and oceans) are relatively warmer and there is low pressure over them. This differential rate of heating of land and water causes the movement of air from high to low pressure (land to sea). 3. The winds being off-shore do not bring rainfall. They are dry except for the branch that flows over the Bay of Bengal and pours it over peninsular India and parts of Sri Lanka. 23
4) Cities like Chennai, which get less rain from the Southwest Monsoon or summer monsoons, receives rain from this Monsoon. About 50% to 60% of the rain received by the state of Tamil Nadu is from the Northeast Monsoon.
A peek
into mythology!!
In ancient Greek cosmology the River Okeanos was a great, fresh-water stream which circled the flat Earth. In a cosmological sense, the river symbolized the eternal flow of time. The Nephelai rose up from the Earth, encircling over River Okeanos, bearing water to the heavens in cloudy pitchers. The Nephelai poured the rains from these pitchers and nourished the Earth to feed the streams who were considered as their brothers and sisters. How the rain fell According to the ancient Greek mythology, the Nephelai (Nymphs in English) were the Goddesses of clouds, who inhabited the cosmos. The Nephelai were depicted as beautiful, young women flitting across the sky in billowing robes and pouring water from pitchers. They were responsible chiefly for the care of the plants and animals of their domain, and as such were closely associated with the prime gods of nature. 24
On the outline map of India, show the summer and winter monsoon winds. Remember to use different colours to indicate them.
Fact ‘o’ mania:Sailors noticed the stillness of the rising (and not blowing) air near the equator and gave the region the depressing name "doldrums." The doldrums, usually located between 5° north and 5° south of the equator, are also known as the Intertropical Convergence Zone or ITCZ for short. The Intertropical Convergence Zone is also known as the Equatorial Convergence Zone or Intertropical Front. The trade winds converge in the region of the ITCZ, producing convectional storms that produce some of the world's heaviest precipitation regions. Between about 30° to 35° north and 30° to 35° south of the equator lies the region known as the horse latitudes or the subtropical high. This 25 region of subsiding dry air and high pressure results in weak winds. Tradition states that sailors gave the region of the subtropical high the name "horse latitudes" because ships relying on wind power stalled; fearful of running out of food and water, sailors threw their horses and cattle overboard to save on provisions. The Oxford English Dictionary claims the origin of the term ‘uncertain’. Major deserts of the
Variable winds Variable winds are related to the pressure system and blow in small areas. They are called variable because they do not blow in any definite direction and their direction varies with the movement of the pressure system. There are two chief types of variable winds- Cyclones and Anticyclones. Cyclones A cyclone is a small low pressure system with winds blowing anti-clockwise spiral in the Northern hemisphere and clockwise in the Southern hemisphere. The eye or centre of the cyclone is a small area of low pressure and is stormy. Strong winds blow from all directions towards the centre. It is referred to as Tropical cyclone in Indian Ocean area, hurricanes in the Caribbean, typhoons in China and willies in Australia. Cyclones are associated with the turbulent weather conditions, cloudy weather, strong winds, rainfall and damage to life and property.
Anticyclones Anticyclones are centres of high pressure and occur in fine weather conditions. Winds blow outward in all directions from the centers of anticyclone. These winds blow clockwise spiral around an anticyclone in Northern hemisphere and anticlockwise in the southern hemisphere. Winds are usually light and there maybe periods of calm weather.
Activity 7:- Research and write a paragraph about Tornado. You can mention some incidents or some statistics related to it. (to be done at home)
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Activity 8:- Differentiate between cyclones and anticyclones.
Activity 9: Research at home about jet streams and discuss the same in class.
Local winds Local winds are restricted to a certain place only. They may be warm or cold depending upon the area from which they blow. For example, Harmattan is a hot local wind of Sahara desert. Other examples of local winds Loo: In the plains of northern India and Pakistan, sometimes a very hot and dry wind blows from the west in summer in the afternoons. This is known as loo. Foehn and Chinook: The winds which move down the northern slopes of Alps, which are hot and dry as they reach the valley bottom are called as Foehn. 27
Similar kind of winds in USA and Canada move down the west slopes of Rockies are known as Chinooks. Mistral: During the winter, areas adjacent to highlands may experience a local wind which originates over the snow capped mountains or highlands and blows down the valley. These winds have been given local names. The most famous is the mistral that blows from the Alps over the France towards the Mediterranean Sea. Activity 11: Find about a few more local winds and share in class.
References Earthscape Geography – class 8 A textbook of Geography – class 8 and 9 – Rita Rajen Frank Middle school – Geography Realms – class 8 Earthtrek Geography – Book 8 Around the World - 8 Google images Practice questions 1) Define atmosphere. 2) Illustrate and explain the fixed components of atmosphere. 3) Which are the variable components of atmosphere? 4) Name the components of the atmosphere and state which of the named ones have a significant influence on the weather phenomena. 5) Explain the importance of the following in affecting the weather phenomena? a) Carbon di oxide b) Water vapour c) Dust particles 6) How is the atmosphere divided on the basis of composition? 7) Which are the fixed components of atmosphere? 8) List any 6 significances of atmosphere. 9) How is the ozone layer formed? 10) What is the significance of the ozone layer? 11) Mention the causes of depletion of the ozone layer. 12) Illustrate using diagram the –insolation and terrestrial radiation, inversion of temperature, latitude affecting temperature, land breeze and sea breeze and aspect of slope. 13) Name the primary source of heat and light for the Earth. How is this created? 14) What is solar radiation? 15) What is the proportion of hydrogen and helium on the Sun? 16) Which are the predominant gases present on the Sun and what is their approximate proportion? 17) Write a short note on solar radiation. 18) What is the significance of solar radiation? 19) Define insolation and terrestrial radiation. 20) Differentiate between insolation and terrestrial radiation. 28
21) Write a short note on insolation and terrestrial radiation. 22) By which process is the Earth heated more? Which are the principal absorbing gases? 23) What is the significance of terrestrial radiation? 24) Explain the factors that have an influence on insolation. 25) Which factors are responsible for differential distribution of insolation on the surface of the Earth? 26) Give three reasons to explain the unequal distribution of insolation on the surface of the Earth. 27) How does transfer of heat take place in the atmosphere? 28) Explain why : Land gets heated faster that water. 29) Describe : Mean temperature and range of temperature 30) Justify : a) Canton is colder than Calcutta in winter though both lie at the same latitude. b) The windward side of the mountain receives more rainfall than the leeward. 31) Distinguish between:a) Land and sea breeze b) Insolation and terrestrial radiation 32) The daily temperatures of place A for a week have been given below :Date
Max Temp
Min Temp
15th Jan
29.5˚C
22.5˚C
th
25.0˚C
22.5˚C
th
17 Jan
29.0˚C
19.0˚C
18th Jan
16 Jan
32.5˚C
21.5˚C
th
29.5˚C
20.0˚C
th
30.0˚C
22.0˚C
st
33.5˚C
22.0˚C
19 Jan 20 Jan 21 Jan
Calculate the week‟s mean temperature, range of temperature and the date/s on which the diurnal range of temperature is highest and lowest. (also mention the value of diurnal range). Convert the week‟s mean temperature from Celsius to Fahrenheit. 33) How does latitude affect temperature of a place ?(Practice for all factors in the same way) 34) Some examples of application questions : Why do deserts have extreme temperatures in day and night? Why does Goa enjoy moderate climate throughout the year? Why is the climate of Lonavala cooler than Khopoli? How will be the temperatures in Jim Corbett National Park? 35) Refer the following sites to know about the origin of Earth‟s atmosphere : http://www.ux1.eiu.edu/~cfjps/1400/atmos_origin.html http://www.moorlandschool.co.uk/earth/earthorigin.htm http://burro.cwru.edu/Academics/Astr221/SolarSys/atmosorigin.html
36) Define the following terms. a) Pressure gradient b) Isobars c) Winds d) Air pressure 37) Name four main pressure belts of earth. 38) What is Circum –Polar Low Pressure Belt? 39) Name the different types of winds. 40) What are periodic winds? 41) What are the local winds? Name any two local winds 42) What are the two types of variable winds? 29
43) Give reasons:a. As we go higher, the atmospheric pressure decreases. b. Temperature and pressure are inversely related to each other. c. Doldrum is a low pressure belt. 44) Distinguish between the following:a. Summer and Winter Monsoons. b. Permanent and Periodic Winds c. Land and Sea breeze d. Cyclones and Anti-Cyclones 45) Draw the diagram of the following:a. Sea breeze and Land Breeze b. Pressure Belts c. Anticyclones and cyclones d. Polar winds e. Trade winds f. Prevailing westerlies _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ 30
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