Composition and Structure of Atmosphere

Learning Outcomes:

1. Understand the composition and structure of the atmosphere.
2. Identify key elements of weather and climate and their impacts on human life.
3. Explain processes like insolation, heating and cooling of the atmosphere.
4. Differentiate between various types of winds, cyclones, and precipitation.
5. Recognize global climate classifications and comprehend concepts like greenhouse effect and global warming.

Atmosphere: Composition and Structure

The atmosphere is vital to life on Earth, a mixture of gases, water vapour, and dust particles that envelops the planet. It plays a fundamental role in sustaining life, regulating the planet’s temperature, and influencing weather and climate patterns. The atmosphere’s layers are distinguished by varying densities and temperature gradients, and each layer plays a distinct role in atmospheric processes.

Composition of the Atmosphere

1. Gases: The major components of the atmosphere include nitrogen, oxygen, and carbon dioxide. Carbon dioxide is meteorologically significant because it absorbs and re-emits terrestrial radiation, contributing to the greenhouse effect. Its concentration has increased due to human activities, particularly the burning of fossil fuels, which raises global temperatures.
2. Ozone: Found between 10 and 50 km above Earth, ozone absorbs harmful ultra-violet (UV) radiation from the Sun. Without the ozone layer, life on Earth would be exposed to dangerous levels of UV radiation.
3. Water Vapour: This variable gas decreases with altitude and plays a crucial role in the regulation of temperature. In tropical regions, water vapour may constitute up to 4% of the atmosphere, while in polar areas, it is less than 1%. Water vapour absorbs part of the incoming solar radiation and prevents extreme temperature variations by trapping terrestrial radiation.
4. Dust Particles: The atmosphere also contains solid particles such as sea salt, pollen, ash, and dust. These particles, particularly abundant in subtropical and temperate regions, act as hygroscopic nuclei, aiding in cloud formation.

Structure of the Atmosphere

The atmosphere is stratified into distinct layers based on temperature profiles and density:

1. Troposphere: The lowest layer, extending approximately 8-18 km above the Earth’s surface, is where weather and climate occur. Temperature decreases with altitude at a rate of about 1°C per 165 meters. The tropopause, marking the boundary between the troposphere and stratosphere, maintains a nearly constant temperature.
2. Stratosphere: Located above the troposphere, the stratosphere extends up to 50 km and contains the ozone layer, which shields Earth from UV radiation. Temperature increases with altitude in this layer due to the absorption of radiation by ozone.
3. Mesosphere: This layer, extending from 50 km to 80 km, experiences a decrease in temperature, reaching as low as -100°C at its upper boundary, the mesopause.
4. Thermosphere: Between 80 km and 400 km, the thermosphere contains ionized particles (ions) that reflect radio waves back to Earth. Here, temperature increases with altitude.
5. Exosphere: The outermost layer, where the atmosphere gradually transitions into outer space, contains extremely rarefied gases.

Important Note: The troposphere and stratosphere are the primary layers of concern for geographers, as they directly influence climate and weather patterns on Earth.

Elements of Weather and Climate

Weather and climate are influenced by several key elements that vary over time and space. These elements, which are subject to change, have significant impacts on both ecosystems and human life.

Insolation and Temperature

1. Insolation: The solar energy received by Earth is known as insolation. The amount of insolation reaching Earth’s surface depends on the angle of incidence, which varies with latitude and season. Equatorial regions receive more direct sunlight, while polar regions experience lower angles of incidence.
2. Heat Budget: Earth maintains a heat balance through various processes:

• Conduction: Transfer of heat from the Earth’s surface to the atmosphere through direct contact.
• Convection: Vertical transfer of heat through air movements.
• Terrestrial Radiation: The Earth emits heat back into space after absorbing insolation.
• Advection: Horizontal heat transfer via wind.

1. Temperature: The distribution of temperature across the globe is affected by factors such as latitude, altitude, distance from the sea, and ocean currents. Temperature also decreases with altitude due to the lapse rate.

Important Note: The phenomenon of temperature inversion occurs when temperature increases with height rather than decreases, often leading to fog and frost in lower layers.

Atmospheric Pressure and Winds

Pressure and winds play crucial roles in shaping climate and weather patterns. Atmospheric pressure varies with altitude and temperature, and differences in pressure lead to the movement of air, or winds.

1. Pressure Belts: The Earth is divided into various pressure belts that influence wind patterns. These belts include equatorial low, subtropical high, subpolar low, and polar high regions.
2. Winds: Wind patterns are classified as:

• Planetary Winds: These include the trade winds, westerlies, and polar easterlies, which blow consistently over large parts of the globe.
• Seasonal Winds: These winds change direction seasonally, such as the monsoons in Asia.
• Local Winds: Specific regions experience unique winds like land and sea breezes and mountain and valley breezes.

Air Masses and Cyclones

1. Air Masses: Large bodies of air with relatively uniform temperature and humidity are known as air masses. They can be classified into tropical, polar, continental, and maritime air masses.
2. Fronts: The boundary between two different air masses is called a front. Cold fronts and warm fronts are associated with varying weather patterns.
3. Cyclones: Cyclones are large-scale air masses rotating around a low-pressure center:

• Tropical Cyclones: Occurring in tropical regions, these cyclones are powerful storms that can cause extensive damage.
• Extratropical Cyclones: Forming in mid-latitudes, these cyclones are associated with frontal systems and bring precipitation.

Precipitation

Precipitation is the release of moisture from the atmosphere in the form of rain, snow, sleet, or hail.

Types of Precipitation

1. Evaporation: The process by which water changes from liquid to gas, leading to the formation of water vapour in the atmosphere.
2. Condensation: When water vapour cools and forms droplets, leading to the formation of clouds and fog.
3. Dew and Frost: When condensation occurs on surfaces near the ground, it forms dew. If the temperature is below freezing, frost forms instead.
4. Rainfall: Rain is the most common form of precipitation, and its distribution varies globally. Orographic, convectional, and cyclonic rainfall are the primary types, depending on the mechanism that causes the air to rise and cool.

World Climates

Global climates can be classified based on temperature and precipitation patterns. One of the most widely used systems is Koeppen’s classification:

1. Tropical Climates: Found near the equator, characterized by high temperatures and heavy rainfall.
2. Dry Climates: These areas receive little precipitation and include deserts and semi-arid regions.
3. Temperate Climates: Characterized by moderate temperatures and distinct seasonal changes.
4. Polar Climates: Found near the poles, these regions experience extremely cold temperatures and minimal precipitation.

Greenhouse Effect and Global Warming

1. Greenhouse Effect: Certain gases in the atmosphere, such as carbon dioxide and methane, trap heat, creating a natural greenhouse effect that keeps Earth warm. However, human activities have intensified this effect, leading to global warming.
2. Climate Change: Rising global temperatures are causing climatic changes such as more frequent heatwaves, melting glaciers, and rising sea levels.

Important Note: Addressing climate change requires global efforts to reduce greenhouse gas emissions and transition to sustainable energy sources.

Comparison of Wind Types

Multiple Choice Question:
Which one of the following gases is transparent to incoming solar radiation and opaque to outgoing terrestrial radiation?
(a) Oxygen
(b) Nitrogen
(c) Helium
(d) Carbon Dioxide
Answer: (d) Carbon Dioxide