Class 7 Geography
Chapter 5 Air composition and Structure
Define Atmosphere?
- Atmosphere is Blanket of air that surrounds Earth.
- Atmosphere is derived from Greek word “Atmos” meaning vapour and “Spaira” meaning sphere.
- Atmosphere is held to Earth by force of gravity.
List some important features of atmosphere?
- Atmosphere extends up to height of about 1600 Km.
- Atmosphere comprises of gases essential for living organisms, such as oxygen for humans and animals, and carbon dioxide for plants.
How atmosphere is held to Earth?
- Near the surface of Earth, gravitation force is strong, so the density of air is high near the Earth’s surface
- About 50% of the total mass lies within 6 km from earth surface, 99% lies within the height of 32 km.
- That is no boundary between atmosphere and space.
List the composition of atmosphere?
Main gases that constitute the atmosphere are
- Nitrogen 78%
- Oxygen 21%
- And remaining 1%
- Argon, Hydrogen, Helium, Ozone. Carbon dioxide.
List the structure of atmosphere?
Depending on the density and temperature of atmosphere, atmosphere is divided into
- Troposphere
- Stratosphere
- Mesosphere
- Thermosphere
- Exosphere
Differentiate Troposphere and Stratosphere?
| Troposphere | Stratosphere |
| It is the lowest and densest layer of atmosphere. | It is the next layer of Troposphere. |
| “Tropo” in Latin means change. | “Strato” means Straight. |
| It extends up to height of 8 km at poles and 18 km at equator. | It extends up to height of 50km above Troposphere. |
| Most of the weather changes occur here. | It is dry zone and free from all kinds of weather phenomenon. |
| During day, this layer protects us from enormous heat of the sun at night, it keeps the Earth warm. | This layer has ozone. It protects us from harmful ultraviolet rays. |
| Tropopause is the boundary between Troposphere and Stratosphere. | Stratopause separates the Stratosphere from the Mesosphere. |
Write a short note on Ozone?
- Ozone is concentrated lower part of Stratosphere (15 – 35 km)
- This layer is termed as Ozonosphere.
- Ozone protects us from sun`s harmful UV rays.
- Ozone is essential for protecting their life on Earth.
- But ozone layer is destroyed by the release of Chlorofluorocarbons (CFC)
- It is mainly by the activity of humans, such as industrial emissions and much more.
What are the various factors that affect the atmospheric pressure?
The factors that affect atmospheric pressure are
Temperature
Altitude
Water vapour
Write in detail about the factors affecting the atmospheric pressure?
Temperature:
- When temperature increases, air expands and density decreases, so air becomes lighter and exerts low pressure.
- On the other hand cold air is denser and exerts high pressure.
Altitude:
When altitude increases, pressure decreases
Water vapour:
- Air containing more water vapour is lighter than air without water vapour.
- So moist air exert less pressure than dry air.
Define Pressure Belts?
The region of Earth that is dominated either by high pressure or low pressure is called as Pressure Belt.
That are 4 major Pressure belts on Earth.
- Equatorial low pressure belt
- Subtropical high pressure belt
- Subpolar low pressure belts
- Polar high pressure belts
Write the difference between Equatorial low pressure belt and subtropical high Pressure belt?
| Equatorial low pressure belt. | Subtropical high pressure belt. |
| It lies around Equator. | It lies near 300N and 300 S |
| It is also termed as Doldrums. | It is also termed as Horse latitudes. |
| Air heat up quickly and becomes lighter on expanding. Here, there is no surface wind | The high temperature in Equatorial region start decreasing in these belts after losing its heat |
Write the difference between Subpolar low pressure belt and Polar high pressure belt?
| Sub Polar low pressure belt | Polar High pressure belt |
| It lies near a 500 N and 700 S latitudes. | This belt lies in extremely cold regions of North Pole and South pole. |
| Warm air rises up creating strong upward current. This current pulls down the cold air creating this layer. | Cold air is denser and cannot leave the surface so weight of cold air creates this belt. |
Define Wind?
- Wind is the horizontal movement of air from high pressure area to low pressure area.
- Moving air is called as Wind.
- Vertical movement of air is called as Wind.
What are Planetary winds?
- The wind which blow through the year in a particular direction are called as Planetary winds.
- These winds are also called as Permanent winds/ Prevailing winds.
- These winds are of 3 types. They are
- Trade winds
- Westerlies
- Polar winds.
Write the difference between Trade winds, Westerlies, Polar winds?
| Trade winds | Westerlies | Polar Winds |
| They blow from subtropical high pressure belt to Equatorial low pressure belt in both the hemisphere. | They blow from subtropical high pressure belt to subpolar low pressure in both hemispheres. | The blow from Polar high pressure belt to subpolar low pressure belt. |
| They blow from East to West. | They blow from West to East.These are also called as anti trades. | They blow from East to West.These are also called as Polar Easterlies |
What are Periodic winds?
- Periodic winds are the winds that blow in particular season.
- These are also called as Seasonal winds.
- These winds are due to unequal heating and cooling Earth’s surface.
There are 2 types of Periodic winds
- Monsoon winds
- Land and Sea breeze
What are Monsoon winds?
- Monsoon winds are seasonal winds that reverse their direction with change of seasons.
- These are rain laden winds.
- Rain laden winds (SouthWest monsoon)
- These are largest source of Irrigation in agricultural country like India.
What are local winds?
The winds that blow over limited areas for shorter duration are called as local winds.
Examples of local winds are
Loo wind – Hot dry wind that blows in Mayanand June in northern India.
Chinook means Snow eater, warm dry wind that blows in Rocky Mountain in Northern America.
Foehn – It blows during winter in Southern region of Alps. It helps in ripening of grapes.
Define Tornado, Hurricane, Typhoons?
Tornado
It is a violently rotating column of air that is in contact with surface of Earth and Cumulonimbus Cloud.
Hurricane
Hurricane is a tropical storm with winds that reached the constant speed of 74 miles per hour or more.
Typhoons
Typhoons are Hurricanes occurring in the regions of Phillippines or China Sea.
Define Humidity?
- The amount of water vapour in air is called as Humidity.
- Only 1-4% of air regulates day to day changes in the weather, formation of clouds and temperature.
- When temperature is high, humidity is also high.
- Hygrometer is used to measure humidity.
Write the difference between absolute and relative humidity?
| Absolute humidity | Relative humidity |
| It is defined as actual amount of water vapour in air column at a given temperature | It is a ratio between amount of water vapour in air column at a given temperature and maximum capacity of air to hold the moisture at that temperature. |
| It is measured in grams per cubic metre | It is expressed in Percentage |
Write the difference between Evaporation and Condensation
| Evaporation | Condensation |
| It is the process where water changes into vapour. | It is the process where water vapour converted to droplets of water. |
| It occurs before the liquid reaches its boiling point. | Condensation is a phase change regardless of temperature. |
| It occurs in low altitudes. | It occurs in high altitudes. |
| Evaporation takes place everywhere, every time and at all temperatures. | Condensation occurs where the temperature of saturated air reaches the dew point. |
What is Saturated?
When the air contains maximum amount of water vapour, it is said to be Saturated.
What is Dew point?
The temperature at which air gets saturated is called as Dew point.
What are the different forms of condensation?
Different forms of condensation are Dew, Frost, Fog, Clouds.
How Dew is formed?
During winter, when moist air comes in contact with cool surface such as grass or leaves, it condenses into droplets of water called as Dew.
How Frost is formed?
When dew drop freeze into ice due to the temperature fall, it is called as Frost.
How Fog is formed?
- Fog are formed when condensation takes place in lower the layers of atmosphere above the water bodies or close to the land surface.
- It reduces the visibility in winter.
How Clouds are formed?
Clouds are formed when water vapour in air condenses around dust particles into tiny droplets of water.
What are different types of clouds?
Nimbus – Dark clouds with full of water that brings rain.
Cumulus – White grey clouds that bring rain.
Cirrus – Feather white clouds
Stratus – Thin white sheets clouds.
Define Precipitation?
Tiny droplets of water join one another and grow in size, When the air is unable to hold them due to gravitational force, they fall down.This process is called as precipitation.
Define Rain?
The condensed water vapour in clouds fall down in the form of water droplets, it is called as Rain. It is the most important form of precipitation.
What are the Three types of rainfall?
- Convectional rainfall
- Relief rainfall
- Cyclonic rainfall
How and where Convectional rainfall occurs?
- It occurs in Equatorial tropical regions in afternoon.
- Sun rays heats the land surface, air near the surface rises in the form of Conventional air currents.
- These reaches a greater height and suddenly water vapour in these current condenses quickly and fall as heavy rainfall.
- It has accompanied by thunder and lightening.
Write short note on Relief Rainfall?
- It is the most widespread form of rainfall.
- When moisture laden winds strike a mountain/hill they rise upward , expand, cool and condenses and fall as rain on windward side of mountain.
- The leeward side of mountain get little/no rainfall.
Write about Cyclonic rainfall?
- This rainfall is linked with cyclones.
- In Cyclone, low pressure area is created at centre winds from all the sides rush towards the low pressure.
- As a result there is circular motion that causes the whirling air to rise up.
- This cools and condensers to fall as rain.
What are the advantages of rainfall?
- Rainfall gives us freshwater
- It helps in producing several crops
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