The Atmosphere Chapter Notes | Window on the Word Class 7: Solutions, Notes & Worksheets PDF Download

The Earth is surrounded by a thin layer of air known as the atmosphere, kept in place by gravity. The atmosphere is vital for supporting life on Earth. It contains crucial gases like oxygen, carbon dioxide, and nitrogen necessary for life. It shields us from the Sun's harmful ultraviolet rays. It regulates the Earth's temperature to maintain ideal conditions for life. Without it, days would be extremely hot, and nights freezing. Various weather events like rain, wind, storms, clouds, and rainbows occur in the atmosphere.

Composition of the Atmosphere

• The atmosphere is a blend of different gases. The major components of the atmosphere are nitrogen and oxygen. Other gases present include carbon dioxide, helium, ozone, argon, and hydrogen.
• In addition to gases, the atmosphere also holds aerosols, water vapor, and dust particles.

Nitrogen

• Almost 78% of the atmosphere consists of nitrogen.
• Nitrogen is crucial for plant survival.
• Plants cannot directly absorb nitrogen from the air.
• Bacteria in the soil convert atmospheric nitrogen into plant-absorbable compounds.

Oxygen

• Oxygen makes up about 21% of the air.
• Various organisms including humans, plants, and animals require oxygen for survival.
• Oxygen is vital for breaking down food into energy.
• Plants and animals breathe in oxygen to stay alive.
• Green plants maintain the oxygen balance by releasing oxygen during photosynthesis.

Carbon Dioxide

• Carbon Dioxide: Despite constituting only about 0.04% of the atmosphere, carbon dioxide is a crucial gas. It plays a vital role in supporting plant photosynthesis.
• Impact of Human Activities: Deforestation and the combustion of fossil fuels contribute to an increase in atmospheric carbon dioxide levels.
• Greenhouse Effect and Global Warming: The heightened presence of carbon dioxide in the atmosphere intensifies the greenhouse effect, leading to global warming.

The Greenhouse Effect and Global Warming

• A greenhouse is like a glass building that traps heat to create a warm environment for plants.
• The Earth's atmosphere acts similarly to a greenhouse, trapping heat to keep the planet warm.
• Energy from the Sun comes to Earth as solar radiation.
• Most of the solar radiation is absorbed by the Earth's surface.
• The absorbed energy is then radiated back as infrared radiation.
• Greenhouse gases like carbon dioxide and methane in the atmosphere trap some of this energy.
• This trapped energy keeps the Earth warm, allowing life to thrive.
• Human activities such as burning fossil fuels and deforestation have increased greenhouse gases.
• This increase in greenhouse gases traps more heat in the atmosphere.
• The trapped heat leads to a rise in the planet's average temperature, known as global warming.
• Global warming results in consequences like climate change, melting ice caps, rising sea levels, and ecosystem disruptions.

Structure of the Atmosphere

• Troposphere: The layer closest to the Earth's surface, about 13 km high. Temperature decreases with height, and weather events like rain, clouds, and winds occur here.
• Stratosphere: Located above the troposphere, extends about 50 km. It lacks weather phenomena and contains ozone that absorbs harmful solar radiation.
• Mesosphere: Extends from the top of the stratosphere up to 80 km. Meteorites burn up in this layer.
• Thermosphere: Begins from the mesosphere to around 600-1,000 km. Contains the ionosphere critical for radio communication, where auroras and satellites are found.
• Exosphere: The outermost layer, merging with space at 10,000 km. Very thin air with light gases like helium and hydrogen escaping into space.

Weather and Climate

• Weather: The state of the atmosphere at a place over a short period, like a few hours or a day. It includes elements such as temperature, moisture, precipitation, winds, and pressure. Weather changes constantly in the troposphere and includes phenomena like wind, lightning, storms, rain, and snow.
• Climate: The average atmospheric conditions of an area over a long period (around 30 years or more). It is determined by studying variations in temperature, humidity, atmospheric pressure, wind, and precipitation. Climate has a significant impact on people and is influenced by factors like temperature, pressure, winds, and precipitation.

Factors Influencing Climate

Atmospheric temperature is the measure of how hot or cold the air around us is. It plays a crucial role in determining the temperature of a place.

Temperature changes throughout the day (diurnal range) and across the year (seasonal variation).

Factors affecting temperature:

• Latitude: The closer a place is to the equator, the hotter it tends to be due to the more direct sunlight.
• Altitude: Temperature decreases with increasing altitude because the air becomes thinner, causing less heat absorption.
• Distance from the sea: Coastal areas have milder temperatures due to the sea's moderating influence.

Other factors like winds and ocean currents also impact temperature.

 Heat Zones of the Earth

• Torrid Zone: This area is situated between the equator and the Tropic of Cancer in the north, and the Tropic of Capricorn in the south. It is the hottest of the three zones.
• Temperate Zone: Found between the Tropic of Cancer and the Arctic Circle in the north, and between the Tropic of Capricorn and the Antarctic Circle in the south. This zone has pleasant weather and is neither as hot as the Torrid Zone nor as cold as the Frigid Zone.
• Frigid Zone: This zone lies between the Arctic Circle and the North Pole in the Northern Hemisphere, and between the Antarctic Circle and the South Pole in the Southern Hemisphere. It is the coldest of the three regions.

Air Pressure and Winds

The atmosphere is composed of matter such as gases, water vapor, and dust particles, giving it weight. Air pressure refers to the pressure exerted by the weight of the air on the Earth's surface. Despite its presence, we do not feel this pressure as the air pressure inside our bodies remains balanced with the external air pressure.

Major Pressure Belts of the Earth

• Pressure belts are broad zones with similar pressure levels.
• Equatorial Low Pressure Belt: Forms around the equator due to intense heating and rising air.
• Subtropical High Pressure Belts: Located around 30° north and south of the equator with sinking cold air.
• Subpolar Low Pressure Belts: Found around 60° north and south of the equator due to Earth's rotation.
• High Pressure at Poles: Near the poles with very low temperatures, resulting in high pressure.
• Permanent Pressure Belts: Determine wind patterns on Earth's surface.
• Air Pressure and Temperature: Highest at sea level, decreases with altitude. Heating makes air lighter, creating low pressure.
• Wind Formation: Air moves from high to low pressure areas, creating wind.

Types of Winds

Permanent Winds:

• Trade Winds: Blow from subtropical highs towards equatorial lows.
• Westerlies: Flow from subtropical highs to subpolar lows.
• Polar Easterlies: Move from polar highs to subpolar lows.

Periodic Winds:

• Seasonal winds like monsoons that change direction based on seasons.
• Land and Sea Breezes: Daily winds along coasts influenced by temperature differences.

Local Winds:

• Winds that affect small areas at specific times like the 'loo' wind over the Indo-Gangetic Plain.

Moisture and Condensation

Moisture:

• Water from land and water bodies evaporates, forming water vapor in the air.
• Humidity is the amount of moisture in the air; it increases as air gets warmer.
• Absolute humidity: Water vapor in a unit volume of air, measured in grams per cubic meter.
• Relative humidity: Ratio of moisture in the air to the total moisture air can hold at that temperature, expressed as a percentage.

Condensation:

• Process where water vapor cools and changes to water or ice, happening when relative humidity is high.
• Warm, humid air cools and condenses when it rises or meets cold air, forming water droplets around particles like dust.
• Water droplets combine to form clouds; condensation can also create fog, mist, frost, and dew.

Precipitation

When water droplets in clouds grow large and heavy, they fall to Earth due to gravity. This process is known as precipitation. Precipitation that reaches Earth in liquid form is called rain. In colder areas, rain freezes in clouds and falls as snow, hail, or sleet.

Rainfall Types

• Convectional: Rainfall resulting from the heating of the ground causing air to rise.
• Relief: Rainfall that occurs when warm, moist air rises over elevated terrain.
• Cyclonic: Rainfall caused by the movement of air masses associated with cyclones or low-pressure systems.

Types of Rainfall

Convectional Rainfall:

• This type of rainfall is common near the equator where humidity is high. When the land heats up, it warms the air above it, causing it to rise. As the moist air rises, it cools and condenses into towering cumulus clouds. When these clouds become saturated, rainfall occurs.

Relief Rainfall:

• This type of rainfall occurs when moist winds encounter a mountain. The wind is forced to rise over the mountain, cooling and condensing to bring heavy rain on the windward side. The leeward side, on the other hand, receives little rain due to the rain shadow effect.

Cyclonic Rainfall:

• Cyclonic rainfall happens when warm air is lifted by a mass of cold air. In tropical regions, it is associated with cyclones, intense storms formed over warm seas. These storms create heavy rainfall and strong, spiraling winds. In temperate areas, cyclonic rainfall occurs when warm and cold air masses meet, leading to rain formation.

Cyclones in Odisha - Case Study

1999 Super Cyclone (Cyclone BOB 03)

• Odislia is located along the Bay of Bengal, which makes it prone to cyclones. In 1999, it was struck by a supercyclone with speeds up to 260 km per hour. The cyclone brought heavy rain, strong winds, and caused extensive damage. Storm surges with waves exceeding 5 meters flooded areas up to 35 km inland. The disaster led to widespread destruction, including infrastructure, homes, trees, and the loss of over 10,000 lives.

2013 Cyclone Phailin

• In 2013, Odisha faced another significant cyclone, Cyclone Phailin, with winds reaching speeds of over 200 km per hour. This time, the state was better prepared. The Odisha government had built safe shelters and conducted regular drills, evacuating nearly a million people before the cyclone made landfall.