Air (Basic)

Composition Of The Atmosphere

The Earth's atmosphere is a mixture of gases that surrounds our planet. While it's a complex system, the primary gases present are:

Nitrogen (N₂): Approximately 78% of the atmosphere is nitrogen. It is relatively inert and plays a crucial role in plant growth through the nitrogen cycle.

Oxygen (O₂): Approximately 21% of the atmosphere is oxygen. It is essential for respiration in most living organisms and for combustion.

Argon (Ar): This is the third most abundant gas, making up about 0.93%. It is an inert gas.

Carbon Dioxide (CO₂): While present in a small amount (about 0.04%), carbon dioxide is a very important greenhouse gas. It absorbs and re-emits infrared radiation, trapping heat and warming the planet. It is also vital for photosynthesis.

Trace Gases: The remaining fraction of the atmosphere consists of trace gases like neon (Ne), helium (He), krypton (Kr), hydrogen (H₂), ozone (O₃), methane (CH₄), nitrous oxide (N₂O), and others. Although present in very small quantities, some of these gases, like ozone and methane, have significant impacts on the Earth's climate and atmosphere.

Water Vapour (H₂O): The amount of water vapour in the atmosphere varies greatly depending on location and altitude, typically ranging from near 0% in cold, dry regions to about 4% in warm, humid tropics. Water vapour is a powerful greenhouse gas and is crucial for weather phenomena like clouds and precipitation.

Particulates: The atmosphere also contains solid and liquid particles suspended in the air, such as dust, pollen, salt spray, smoke, and volcanic ash. These particles can influence cloud formation, visibility, and the Earth's radiation balance.

Importance of the Atmosphere:

• It provides the oxygen we breathe.
• It protects us from harmful ultraviolet (UV) radiation from the sun (ozone layer).
• It regulates Earth's temperature by trapping heat (greenhouse effect).
• It protects us from meteoroids, most of which burn up upon entering the atmosphere.
• It is essential for the water cycle and weather patterns.

Structure Of The Atmosphere

The Earth's atmosphere is divided into several layers based on temperature variations with altitude. These layers are:

Troposphere

Altitude: Extends from the Earth's surface up to an average of about 12 km. It is thinner at the poles (around 7-10 km) and thicker at the equator (around 16-18 km).

Temperature: Temperature generally decreases with increasing altitude. The average rate of decrease is about 6.5°C per kilometre, known as the normal lapse rate. This is because the troposphere is heated from below by the Earth's surface.

Characteristics:

• Contains about 75-80% of the atmosphere's total mass.
• Contains almost all of the water vapour and dust particles.
• This is where most weather phenomena, such as clouds, rain, snow, and storms, occur.
• The upper boundary of the troposphere is called the tropopause.

Stratosphere

Altitude: Extends from the tropopause up to about 50 km.

Temperature: Temperature increases with altitude in the stratosphere. This warming is due to the presence of the ozone layer, which absorbs ultraviolet (UV) radiation from the sun and re-radiates it as heat.

Characteristics:

• Contains the ozone layer, which protects life on Earth from harmful UV radiation.
• The air is very thin and dry, with few clouds.
• Commercial aircraft often fly in the lower stratosphere to avoid turbulence in the troposphere.
• The upper boundary is called the stratopause.

Mesosphere

Altitude: Extends from the stratopause up to about 80-85 km.

Temperature: Temperature decreases with altitude, reaching the coldest temperatures in the Earth's atmosphere at its upper limit (around -90°C). This is because there is very little ozone to absorb solar radiation.

Characteristics:

• Meteors burn up in this layer due to friction with the sparse air molecules, creating "shooting stars."
• The upper boundary is called the mesopause.

Thermosphere

Altitude: Extends from the mesopause upwards to about 600-1000 km.

Temperature: Temperature increases dramatically with altitude due to the absorption of high-energy solar radiation (X-rays and UV rays) by the few gas molecules present. Temperatures can reach thousands of degrees Celsius, but the air is so thin that it would not feel hot to us.

Characteristics:

• The International Space Station (ISS) orbits within this layer.
• The aurora borealis (Northern Lights) and aurora australis (Southern Lights) occur in the thermosphere when charged particles from the sun interact with atmospheric gases.
• The lower part of the thermosphere contains the ionosphere, a region where solar radiation ionizes atoms and molecules, which is important for radio wave propagation.
• The upper boundary is often considered part of the exosphere.

Exosphere

Altitude: The outermost layer of the atmosphere, extending from the top of the thermosphere to about 10,000 km and gradually merging with outer space.

Temperature: Temperatures continue to be very high, but the air is extremely tenuous.

Characteristics:

• Contains mostly hydrogen and helium.
• Gas molecules are so far apart that they rarely collide and can escape Earth's gravity into space.

Summary Table of Atmospheric Layers:

Weather And Climate

Temperature

Definition: Temperature is a measure of the degree of hotness or coldness of the air. It is determined by the amount of solar energy received by a location and how that energy is absorbed, reflected, and radiated.

Measurement: Measured using thermometers in degrees Celsius (°C) or Fahrenheit (°F).

Factors Affecting Temperature:

• Insolation: The amount of solar radiation received. This varies with latitude, time of day, season, and cloud cover.
• Altitude: Temperature generally decreases with increasing altitude (normal lapse rate) in the troposphere.
• Latitude: Equatorial regions receive more direct sunlight and are generally warmer than polar regions.
• Distance from the Sea: Coastal areas have more moderate temperatures (smaller annual range) than inland areas due to the high specific heat capacity of water, which moderates temperature changes.
• Ocean Currents: Warm currents can warm coastal areas, while cold currents can cool them.
• Landforms: Slope and aspect (direction a slope faces) can influence temperature by affecting the amount of sunlight received.
• Cloud Cover and Humidity: Clouds can reflect incoming solar radiation (cooling effect) and trap outgoing terrestrial radiation (warming effect). High humidity can also trap heat.

Air Pressure

Definition: Air pressure, or atmospheric pressure, is the weight of the air column pressing down on a unit area of the Earth's surface. It is caused by the gravitational pull of the Earth on the atmosphere.

Measurement: Measured using barometers in units like millibars (mb), hectopascals (hPa), or inches of mercury. Standard atmospheric pressure at sea level is 1013.25 mb (or hPa).

Characteristics:

• Decreases with Altitude: As altitude increases, the air column above is shorter and less dense, so air pressure decreases.
• Horizontal Variations: Air pressure varies horizontally across the Earth's surface, creating pressure gradients that drive winds.
• High Pressure Systems (Anticyclones): Areas of high atmospheric pressure where air is generally descending. They are associated with calm, clear, and fair weather.
• Low Pressure Systems (Cyclones): Areas of low atmospheric pressure where air is generally rising. They are associated with cloudy skies, precipitation, and unsettled weather.

Relationship between Temperature and Pressure: Warm air is less dense and tends to rise, creating low pressure at the surface. Cool air is denser and tends to sink, creating high pressure at the surface.

Wind

Definition: Wind is the horizontal movement of air from an area of high pressure to an area of low pressure. The greater the pressure difference (pressure gradient), the stronger the wind.

Formation: Wind is driven by differences in air pressure, which are primarily caused by uneven heating of the Earth's surface. Heat causes air to expand and rise (low pressure), while cold causes air to contract and sink (high pressure).

Types of Winds:

• Planetary Winds: Winds that blow continuously throughout the year in definite directions, such as the Trade Winds, Westerlies, and Polar Easterlies.
• Seasonal Winds: Winds that change direction with the seasons, most notably the Monsoons, which are prominent in India and Southeast Asia.
• Local Winds: Winds that blow over a short area and are often influenced by local temperature and pressure differences, such as land breezes, sea breezes, mountain breezes, and valley breezes.

Measurement: Measured using anemometers for speed and wind vanes for direction.

Beaufort Scale: A scale used to estimate wind speed based on observed conditions at sea or on land.

Cyclone – Nature’s Fury

Definition: A cyclone is a large-scale, rotating weather system characterized by low atmospheric pressure at its center and winds that spiral inwards. In the Northern Hemisphere, cyclones rotate counter-clockwise, and in the Southern Hemisphere, they rotate clockwise.

Formation: Cyclones typically form over warm ocean waters (above 26.5°C or 80°F). They begin as a cluster of thunderstorms that organize around a low-pressure center. The Coriolis effect, caused by the Earth's rotation, causes the system to spin.

Characteristics:

• Low Pressure Center (Eye): The calm, clear center of a tropical cyclone.
• Eyewall: The ring of intense thunderstorms surrounding the eye, with the strongest winds and heaviest rainfall.
• Rainbands: Spiral bands of thunderstorms extending outwards from the eyewall.
• Strong Winds: Can reach speeds of over 250 km/h, causing widespread destruction.
• Heavy Rainfall: Can lead to severe flooding.
• Storm Surge: A rise in sea level caused by the cyclone's winds pushing water towards the shore, often leading to devastating coastal flooding.

Types:

• Tropical Cyclones: Form over warm tropical or subtropical oceans (known as hurricanes in the Atlantic and Northeast Pacific, typhoons in the Northwest Pacific, and cyclones in the Indian Ocean and South Pacific).
• Extratropical Cyclones: Form over land or cooler waters in higher latitudes and are associated with weather fronts.

Impact: Cyclones are among the most destructive weather phenomena, causing significant damage to property, infrastructure, and loss of life.

Moisture

Definition: Moisture in the atmosphere refers to the presence of water in its gaseous state, as water vapour. It is a critical component of weather and climate.

Sources: Primarily from evaporation of oceans, seas, lakes, rivers, and transpiration from plants.

Measurement:

• Absolute Humidity: The mass of water vapour present in a given volume of air (e.g., grams per cubic meter).
• Specific Humidity: The mass of water vapour present in a given mass of air (e.g., grams per kilogram).
• Relative Humidity: The ratio of the actual amount of water vapour in the air to the maximum amount it can hold at a given temperature and pressure, expressed as a percentage. This is the most commonly used measure as it indicates how close the air is to saturation.

Forms of Atmospheric Moisture:

• Water Vapour: Invisible gaseous state of water.
• Clouds: Visible masses of tiny water droplets or ice crystals suspended in the atmosphere, formed by condensation or deposition of water vapour around condensation nuclei.
• Precipitation: Any form of water that falls from the atmosphere to the Earth's surface, including rain, snow, sleet, hail, and drizzle.

Condensation: The process by which water vapour in the air changes into liquid water droplets or ice crystals. This occurs when the air cools to its dew point and is often facilitated by condensation nuclei (tiny particles like dust or salt).

Importance: Water vapour is a potent greenhouse gas, influencing Earth's temperature. It is also essential for cloud formation, precipitation, and various weather processes.