Atmosphere Online Test 9th Social Science Lesson 21 Questions in English

Earth is a unique planet where life is found. The air is essential for the survival of all forms of life. The blanket of air that surrounds the Earth is called the atmosphere. It is held close to the earth by gravitational attraction.

Atmosphere is a mixture of gases, water vapor and dust particles in different proportions. Nitrogen (78%) and Oxygen (21%) are permanent gases of the atmosphere.

The remaining one percentage is occupied by Argon (0.93%), Carbon-dioxide,(0.03%), Neon (0.0018%), Helium (0.0005%), Ozone (0.00006%) and Hydrogen (0.00005%).

Water vapor (0 – 0.4%) is also found in the atmosphere which plays an important role in predicting weather phenomenon.

The solid particle present in the atmosphere includes dust particles, salt particles, pollen grains, smoke, soot, volcanic ashes etc.

In 1772 CE Daniel Rutherford discovered Nitrogen in atmosphere.

In 1774 Joseph priestly discovered oxygen in atmosphere.

Oxygen is most important for living organisms. CO2 absorbs heat and keeps the atmosphere warm by insulation and radiation.

Nitrogen acts as a diluent and is chemically inactive. Ozone helps in protecting the earth from radiation. The solid particles in the atmosphere acts as nuclei on which water vapor condense to form precipitation.

The atmosphere is thick near the earth surface and thins out until it eventually merges with space.

The five atmospheric layers are: Troposphere, stratosphere, Mesosphere, Thermosphere and Exosphere

The lowest layer of the atmosphere is the troposphere. The Greek word ‘tropos’ means ‘turns’ or change.

The troposphere layer extends up to 8 km at the poles and up to 18 km at the Equator. The temperature decreases with increasing height.

Almost all weather phenomenon’s take place in this troposphere layer. Hence it is called weather making layer.

The upper limit of the troposphere is called as Tropopause.

Stratosphere lies above the troposphere. It extends to a height of about 50km above earth surface.

The Stratosphere layer is a concentration of ozone molecules; it is also referred as ozonosphere.

The temperature increases with increase in height in the stratosphere layer. Large jet planes normally fly here. The upper limit of the stratosphere is called as Stratopause.

Mesosphere extends between 50km and 80km. The temperature increases with increasing height.

Radio waves transmitted from earth are reflected back to earth from the mesosphere layer. Most of the meteors nearing the earth get burned here. The upper most limit of the mesosphere is the Mesopause.

Thermosphere exists above the mesosphere. It extends to about 600 km.

The composition of gases in the lower thermosphere is more or less uniform; hence it is called “Homosphere”.

The upper portion of the thermosphere has uneven composition of gases and hence it is referred as “Heterosphere”. Here the temperature increases with increasing height

Ionosphere is a layer of the thermosphere that contains Ions and free electrons.

Magnetosphere lies beyond the exosphere. It is the earth’s magnetic belt where proton and electrons, coming out from the sun are trapped by the earth. The magnetic field extends to around 64,000 km above the Earth.

The uppermost layer of the atmosphere is called exosphere. This layer is extremely rarefied with gases and gradually merges with the outer space. This zone is characterized by aurora Australis and aurora borealis.

Auroras are cosmic glowing lights produced by a stream of electrons discharged from the Sun’s surface due to magnetic storms that are seen as unique multicolored fireworks hanging in the polar sky during midnight.

Weather and climate are the terms that are related to the atmospheric conditions. Weather denotes the way the atmosphere behaves every day and climate reveals the average of weather conditions over relatively long periods of time.

There are many factors that influence weather and climate: Distance from the equator, Altitude, Nearness to the sea, Nature of the prevailing winds, Mountain barrier, Cloud cover, Ocean currents and Natural vegetation

The sun’s rays fall vertically on the equator. The rays are inclined on the regions away from the equator and near the poles due to the spherical shape of the earth. The vertical rays heat up the earth more than the inclined rays. Thus, the places near the equator are warmer than the places which are far away from the equator.

Altitude refers to the height above sea level. The temperature decreases at the rate of 1° C for every 165 mt of height.

The Normal lapse rate is the temperature decrease at the rate of 1° C for every 165 mt of height. So places at the higher altitude have a lower temperature.

The climate of a place varies according to its nearness to the sea. Places near the coast experience equable climate due to the influence of the winds from the sea.

Places located in the land, far from the sea, does not experience the moderating influence of the sea, such places experience a continental type of climate.

During the day, the land masses get heated more rapidly than the oceans. Heated air ascends and this causes low pressure on the adjoining ocean. Therefore, the wind blows from ocean to land in the afternoon. This is called sea breeze.

Sea breeze helps in reducing the temperature of the coastal region especially during the summer season.

During the night, the land cools more rapidly than the ocean. Cool air sinks and forms high pressure. The wind blows from land to sea during the night, this is called land breeze.

The winds change the climate of a place based on, from where they blow. When wind blows from a warm region, it makes the place warm and cold, when blows from a colder region. The on-shore winds cause rainfall making the place cool whereas the off-shore winds bring dry weather.

The location of the mountain influences the climate of a place. The mountain chains act as natural barrier for the wind. Sometimes they prevent the entry of cold winds into the country or the escape of monsoon winds, thus having a great influence over the climate.

The windward is the side of a mountain which faces the prevailing wind. It receives heavy rainfall.

The leeward side of the mountain is the side sheltered from the wind. It receives very less rainfall.

Clouds reflect a large amount of radiation from the sun. This prevents the entry of heat to the earth’s surface. So, in areas generally of cloudless sky like the deserts, temperature is very high. On the other hand under cloudy sky, the temperature is low.

The warm ocean currents raise the temperature of the nearby coastal areas, while the cold current lower the temperature of a place.

The trees release water vapor into the air and make it cool. Thus forest areas have lower range of temperature throughout the year in contrast to no forested areas.

Elements of climate and weather depends on, Temperature, Pressure, Wind, Cloud, Precipitation and Humidity.

Temperature is a measure of the warmth of an object expressed in terms of Celsius or Fahrenheit, measured with thermometer. Sun is the chief source of energy for the Earth.

The atmosphere acts as an insulator and maintains the temperature of the earth. Without atmosphere, the earth would experience great extremes of temperatures during day and night.

Some of the processes that are responsible for atmospheric heat are radiation, Conduction, Convection and Advection.

The amount of heat received from the sun in the form of short waves is called Insolation or Incoming Solar Radiation.

The outgoing heat from the earth to space in the form of long waves is called terrestrial radiation. This is also called as re-radiation

Albedo is the fraction of solar energy reflected from the earth back into space without reaching or heating the earth surface.

There is a balance between insolation and radiation. This balance is termed as a heat budget of the earth.

Isotherms are imaginary lines drawn on maps, connecting points that have equal temperatures.

Temperature varies from place to place, season to season and continent to continent. The average global surface temperature is about 13 °C.

Conduction is the transfer of heat from hot body to a cold body through contact. Convection is transfer of heat by movement or circulation of air in a mass. Advection is the transfer of heat through the horizontal movement of air.

Based on the amount of insolation received from the sun and the heat, Earth is classified into three heat zones namely torrid zone, temperate zone and Frigid Zone.

Torrid Zone: This largest thermal zone covers almost 50% of the earth’s surface. It is located between the Tropic of Cancer (23½°N) and Tropic of Capricorn (23½°S). Torrid Zone experiences vertical sun rays almost throughout the year and is hot.

Temperate Zones: The Temperate Zone stretches out between Tropic of Cancer (23½°N) and Arctic Circle (66½°N) in the northern hemisphere and between Tropic of Capricorn (23½° S) and Antarctic Circle (66½° S) in the southern hemisphere. The sun’s rays never fall vertical in this region.

The Frigid Zone is found between Arctic Circle (66½°N) and North Pole (90° N) in the northern hemisphere and stretches out between Antarctic Circle (66½° S ) and South Pole (90°S) in the southern hemisphere. The sun’s rays fall slanting in this zone. These are the coldest regions of the world. The surface remains permanently frozen under thick snow.

The atmospheric pressure is the weight exerted by air on a particular area of the earth surface. It is measured with a mercury barometer and the unit of measurement is millibar (mb).

An isobar is an imaginary line drawn through places having equal atmospheric pressure reduced to sea level.

The distribution of atmospheric pressure on the surface of the earth is not uniform. It varies both vertically and horizontally.

Air pressure decreases with altitude. The air molecules become scattered and more widely spaced at higher altitudes. The air pressure decreases by 34 millibars per 300 metres increase in height.

The horizontal distribution of atmospheric pressure in the world is not uniform. It varies from time to time and place to place due to (i) air temperature (ii) the earth’s rotation (iii) presence of water vapor.

The pressure belts of the world are: Equatorial low, Sub tropical highs, Sub polar lows, Polar highs.

The Equatorial Low Pressure Belt: This belt extends from equator to 5° N and 5° S latitudes. At the equator, the earth gets heated by the vertical sun rays and in turn heats the air in contact with it. The heated air expands and rises upwards resulting in a low pressure belt. This belt is called doldrums due to virtual absence of surface winds.

The Inter Tropical Convergence Zone (ITCZ) is the belt of converging winds and rising air encircling Earth near the Equator.

Doldrums (the zone of calm) lies in the equatorial region with calms, light unpredictable winds and sudden cyclones.

The sub-tropical high pressure belts extend from the tropics to about 35° latitudes in both the hemispheres. The air that rises in the equatorial region, becomes cold and heavy, and starts to descend in the Sub Tropical regions. This result in sub-tropical high pressure belts referred as the Horse latitude.

HORSE LATITUDE: In olden days vessels with cargo of horses passing through sub-tropical high pressure belts found difficulty in sailing under calm conditions. With little water and food left for the humans, sailors used to throw the horses in to the sea in order to make the vessels lighter. Henceforth these belts or latitudes are also called ‘horse latitudes’.

The Sub-polar Low Pressure Belts: The sub-polar low pressure belts extend between 45°N and the Arctic Circle in the northern hemisphere and between 45°S and the Antarctic Circle in the southern hemisphere. The air present in this layer moves to the sub-tropical high pressure belt and polar high pressure belt making it free from air pressure forming the sub polar low pressure belt. This is made possible by the rotation of the earth.

The Polar High Pressure Belts: Sun rays are always slanting at poles resulting in low temperatures. Because of low temperature, air compresses and its density increases. Hence, high pressure is found here. Winds from these belts blow towards sub-polar low pressure belts.

The horizontal movement of air along the surface of the earth is called the “Wind’ while the vertical movement of air is a called an “Air Current”.

The winds always blow from a high pressure area to a low pressure area. Wind is mostly named after the direction from which it blows. For example, the wind blowing from the east is known as the easterly wind.

An “anemometer” records wind speed while a “wind vane” measures the direction of the wind. The unit of measurement is kilometer per hour or knots.

Winds are generally classified into the following four major types: Planetary winds, Periodic winds, Variable wind and Local wind.

Planetary winds: The winds which constantly blow in the same direction throughout the year are called the Planetary winds. They are also called as permanent winds or the prevailing winds. These winds include Trade winds, Westerlies and Polar Easterlies.

Trade Winds: Trade winds blow from the subtropical high pressure belt to the Equatorial low pressure belt in both the hemispheres. They blow with great regularity, force and in a constant direction throughout the year. These winds were very helpful to traders who depended on the winds while sailing in the seas. And so, they are named as Trade winds. As they travel over vast oceans, they collect more moisture and bring heavy rainfall to the East Coast of the continents of the tropics. As they move westwards, they become dry and do not give rainfall.

Westerlies: Westerlies are the permanent winds that blow from the tropical high pressure belt to the sub polar low pressure belt in both the hemispheres. They blow from South West to North East in the northern hemisphere and North West to South East in the southern hemisphere. The velocity of Westerlies become so vigorous and fast to be called Roaring Forties at 400,Furious Fifties at 500 and Screaming Sixties at 600 latitudes.

Polar Easterlies: Polar easterlies are cold and dry polar winds that blow from the polar high pressure belt to the sub polar low pressure belt. These are weak winds blowing from North East direction in the Northern Hemisphere and South East direction in the Southern Hemisphere.

Periodic winds: The periodic winds are the seasonal winds that change their direction periodically. These winds are caused by the differential heating of land and ocean.

Winds which reverse their direction with the change of seasons are called monsoons. Tropical Monsoon winds of Indian subcontinent are a best example.

The rotation of the Earth causes deflection of winds from their original path, called the “Coriolis effect.

Winds are deflected to the right in the northern hemisphere and to the left in the southern hemisphere which is known as “Ferrel’s law”. This was profounded by William Ferrel. He used “Coriolis force” named after G.G Coriolis (1792-1843) for proving Ferrel’s Law.

Variable winds: The disturbance and the changes in the local weather cause variations in the prevailing winds. These winds are known as the variable winds. Variable winds usually end up with the development of cyclones, anticyclones and storms.

Cyclones: The term cyclone is a Greek word meaning “coil of a snake”.

Cyclones are centers of low pressure where, winds from the surrounding high pressure area converge towards the center in a spiral form. Due to the rotation of the earth, the cyclonic winds in the northern hemisphere move in anti-clock wise direction, where as they move in clockwise direction in the southern hemisphere.

Cyclones can be classified into Tropical cyclones, Temperate cyclones and Extra tropical cyclones.

Tropical cyclones: Tropical cyclones develop in the Inter tropical convergence zone [ITCZ]. They are formed due to the differential heating of land and sea.

Tropical cyclones are known as ‘cyclones’ in Indian ocean, ‘typhoons’ in the western pacific ocean, ‘hurricanes’ in the Atlantic and eastern Pacific ocean, ‘Baguios’ in Philippines and ‘willy willy’ in Australia. Tropical cyclones often cause heavy loss of life and property on the coasts and become weak after reaching the landmasses.

Super Cyclone: A violent cyclone that hit Odisha on Friday, 29 October 1999, was one of the most devastating and strong storm to hit the Indian coast. Winds of up to 260 kph raged for over 36 hours. The winds caused a seven-meter tidal wave that swept more than 20 km inland and brought massive destruction and death to a number of coastal districts in the state of Odisha. It is estimated that more than 10 million people in 12 coastal belt districts were affected by the cyclone.

Temperate cyclones are formed along a front where hot and cold air masses meet in mid-latitudes between 35°S and 65°N.

Temperate cyclones do not become weak like the tropical cyclones on reaching the land. Temperate cyclone commonly occurs over the North Atlantic Ocean, North West Europe, Mediterranean basin.

Mediterranean basin’s temperate cyclones extend up to Russia and India in winter. In India it is as called western disturbances.

A front is the boundary separating warm and cold air masses. One type of air mass is usually denser than the other, with different temperatures and humidity. This meeting of air mass causes rain, snow, cold days, hot days, and windy days.

Extra tropical cyclones occur in the latitudes between 30° and 60° in both the hemispheres. They are also called as mid-latitude cyclones. They collect energy from temperature differences which are found in higher latitudes. Extra tropical cyclones produce mild showers to heavy gales, thunderstorms, blizzards, and tornadoes.

Deliberations for naming cyclones in the Indian Ocean region began in 2000 and a formula was agreed upon in 2004.

Eight countries in the Indian Ocean region Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Srilanka, and Thailand contributed a set of names which our assigned sequentially whenever a cyclonic storm develops.

Anticyclones are the opposite of cyclones. Here an area of high pressure region is found in the center surrounded by low pressure on all sides. The wind from the high pressure region move outwards to the low pressure regions in a spiral form. Anticyclones are often accompanied by cold and heat waves.

Local winds are the winds that blow only in a particular locality for a short period of time. The effect of these local winds is experienced only in that particular area.

They are mostly seasonal and have local names like, Foehn (Alps-Europe), Sirocco (North coast of Africa), Chinook (Rockies-North America), Loo (Thar Desert- India), Mistral (Mediterranean sea in France), Bora (Mediterranean sea in Italy).

Large amount of water evaporates each day from the surface of the sea. This is the principal source of atmospheric moisture.

Cool moisture laden air, gets collected around particles like dust, salt content from the sea, smoke etc., and forms clouds. Sometimes, mixing of warmer and cooler air also produces clouds. A visible mass of condensed water vapor floating in the air above the ground level is called a cloud.

The three layers of atmosphere such as troposphere, stratosphere and mesosphere are specific locations of clouds.

According to their height, clouds are classified into the following types: High clouds (6-20km Height), Middle clouds (2.5km-6km Height), Low clouds (ground surface to 25 km height). These major types of clouds are further divided into different types on the basis of shape and structure

Cirrus: Detached clouds in the form of white delicate fibrous silky filaments formed at the high sky (8000 meters to 12000 meters) are called Cirrus clouds. These clouds are dry and do not give rainfall.

Cirro-cumulus: White patched, sheet or layer like clouds composed of ice crystals. Cirro-stratus: Smooth milky transparent whitish clouds composed of tiny ice crystals.

During sunset cirrus clouds look colorful hence they are called as “Mare’s Tails”.

Middle clouds
Alto-stratus: Thin sheets of grey or blue Coloured clouds in uniform appearance consisting of frozen water droplets
Alto-cumulus: clouds fitted closely together in parallel bands, called as ‘Sheep clouds’ or wool pack clouds.
Nimbo stratus: These are clouds of dark color very close to the ground surface associated with rain, snow or sleet.

The only sphere which contains all clouds in the atmosphere is troposphere.

Low clouds Strato-cumulus:- Grey or whitish layer of non-fibrous low clouds found in rounded patches at an height of 2500 to 3000 metres, associated with fair or clear weather
Stratus:- Dense, low lying fog-like clouds associated with rain or snow
Cumulus:- Dome-shaped with a flat base often resembling a cauliflower, associated with fair weather
Cumulo-nimbus:- Fluffy thick towering thunderstorm cloud capable of producing heavy rain, snow, hailstorm or tornadoes

Falling down of condensed water vapor in different forms is called Precipitation. When the dew point is reached in the cloud water droplets become saturated and start to fall. Hence, they fall on the earth as Precipitation.

The climatic conditions/ factors influencing the forms of precipitation mainly are: Temperature, Altitude, Cloud type, Atmospheric conditions and Precipitation process.

The main forms of precipitation include drizzle, rain, sleet, snow, hail etc.

Falling of numerous uniform minute droplets of water with diameter of less than 0.5mm is called a drizzle from low clouds. Sometimes drizzles are combined with fog and hence reduce visibility.

Rain is the most widespread and important form of precipitation in places having temperature above the freezing point. It occurs only when there is abundant moisture in the air. The diameter of a rain drop is more than 5mm.

Sleet refers to a precipitation in the form of pellets made up of transparent and translucent ice. This precipitation is a mixture of snow and rain.

Snow is formed when condensation occurs below freezing point. It is the precipitation of opaque and semi opaque ice crystals. When these ice crystals collide and stick together, it becomes snowflakes.

Hails are chunks of ice (greater than 2cm in diameter) falling from the sky, during a rainstorm or thunderstorm.

Hailstones are a form of solid precipitation where small pieces of ice fall downwards. These are destructive and dreaded forms of solid precipitation because they destroy agricultural crops and human lives.

Any thunderstorm which is associated with fall of hail stones is known as hailstorm. Hailstorm is one of the most feared weather phenomenon because it has the potential to destroy plant, trees, crops, animals and human life.

Rainfall is the most predominant type of Precipitation. Moisture laden air masses raise upwards, forms clouds and bring rainfall.

Based on the mechanisms of raising the air, there are three types of rainfall. Convectional rainfall, Frontal or cyclonic rainfall and Orographic rainfall.

Earth surface is intensely heated through solar radiation during the day time. When the air near the earth surface is heated, it rises and expands. This heating results is the formation of convectional air currents. Thus the ascending moist air cools, condenses and results in convectional rainfall.

Convectional rainfall occurs regularly in the equatorial region in the evenings. It is also experienced in tropical, sub-tropical and temperate regions in the summer months and on warmer days.

Cyclonic precipitation occurs during cyclones when air masses are made to converge and move upward so that adiabatic cooling occurs. Cyclonic rainfall occurs in tropical as well as temperate regions.

When warm and cold air masses converge, condensation and precipitation takes place on the boundary between warm and cold air masses called as Frontal rainfall.

Orographic rainfall, also called relief rainfall, is caused when air is forced to rise against a high mountain. The mountain barriers lying across the direction of air flow, force the moisture laden air rise along the mountain slope. This results in the cooling of the air, which leads to the formation of clouds and rain. This rainfall is called Orographic rainfall.

The side of the mountain facing the wind is called the windward side and receives heavy rainfall. It is called the rain fed region. The other side of the mountain that does not face the wind is called the leeward side and receives less rainfall becomes rain shadow region.

Humidity is an important aspect of the atmosphere because it affects both weather and climate. The amount of water vapor present in the atmosphere is referred to as humidity. Humidity of the atmosphere is high when it has large quantities of water vapor. The amount of water vapor in the atmosphere is called absolute humidity. The ratio between the amount of water vapor in the atmosphere and the amount of water vapor it can hold is relative humidity

Hot air can hold more water vapor then cold air. Relative humidity increases when air gets cold and decreases when air gets heated up.

Mawsynram is the wettest place of India as it is located in the windward side of the Purvachal hills.

Shillong lies on the leeward side and thus receives less rainfall. This is the same, in the case of Mumbai and Pune.

Absolute humidity is expressed in terms of grams of water vapor present per cubic meter of air. Relative humidity is expressed in percentage.

When the relative humidity of the air is 100%, the air is said to be saturated. Saturated air will not absorb any more water vapor. The temperature at which air gets saturated is called dew point.

Humidity of the atmosphere is measured by the wet and dry bulb thermometer also called the Hygrometer.

The Cyclone Fani emerged from the Indian Ocean and caused huge damage in Odisha, West Bengal, Andhra Pradesh and East India in the year 2020.