8.3 Earth’s Atmosphere

Earth's atmosphere is a complex system of layers and gases that protect and sustain life. From the troposphere where we live to the exosphere at the edge of space, each layer has unique properties and functions. Understanding these layers helps us grasp weather patterns and climate dynamics.

The atmosphere's composition is crucial for life, with nitrogen, oxygen, and trace gases playing vital roles. Weather and climate, while related, operate on different timescales. Weather is short-term and variable, while climate represents long-term averages, both impacting Earth's systems and human activities.

Earth's Atmospheric Layers and Composition

Layers of Earth's atmosphere

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• Troposphere
  • Lowest layer extends from Earth's surface to an average height of 12 km (7.5 miles)
  • Contains approximately 75% of the atmosphere's mass and 99% of its water vapor
  • Temperature decreases with altitude at a rate of about 6.5°C per kilometer (3.6°F per 1,000 feet)
  • Atmospheric pressure decreases with altitude in this layer
• Stratosphere
  • Extends from the top of the troposphere to an altitude of about 50 km (31 miles)
  • Contains the ozone layer which absorbs harmful ultraviolet (UV) radiation from the Sun
  • Temperature increases with altitude due to the absorption of UV radiation by ozone
• Mesosphere
  • Extends from the top of the stratosphere to an altitude of about 85 km (53 miles)
  • Temperature decreases with altitude reaching as low as -90°C (-130°F) at the mesopause
  • Noctilucent clouds, the highest clouds in Earth's atmosphere, form in this layer
• Thermosphere
  • Extends from the top of the mesosphere to an altitude of about 600 km (373 miles)
  • Temperature increases with altitude and can reach up to 2,000°C (3,632°F) due to absorption of intense solar radiation
  • International Space Station orbits in this layer at an altitude of about 400 km (250 miles)
• Exosphere
  • Extends from the top of the thermosphere to an altitude of about 10,000 km (6,200 miles)
  • Extremely low density with particles rarely colliding
  • Hydrogen and helium are the main components at this altitude

Composition of Earth's atmosphere

• Nitrogen ($N_2$): 78.08% by volume, essential for life as it is a key component of amino acids and proteins
• Oxygen ($O_2$): 20.95% by volume, necessary for respiration in many life forms and important for combustion
• Argon (Ar): 0.93% by volume, a noble gas that is chemically inert and does not participate in biological processes
• Carbon dioxide ($CO_2$): 0.04% by volume, a greenhouse gas that traps heat and plays a crucial role in Earth's carbon cycle
  • Atmospheric $CO_2$ levels have increased from about 280 ppm (parts per million) before the Industrial Revolution to over 400 ppm today due to human activities such as burning fossil fuels and deforestation
• Water vapor ($H_2O$): 0-4% by volume, the most abundant greenhouse gas and plays a key role in the water cycle through evaporation, condensation, and precipitation
  • Humidity is a measure of the amount of water vapor in the air
• Trace gases: Include neon (Ne), helium (He), methane ($CH_4$), krypton (Kr), and xenon (Xe), which together make up less than 0.1% of the atmosphere by volume

Weather vs climate

• Weather
  • Refers to the day-to-day state of the atmosphere in terms of temperature, humidity, precipitation, wind speed and direction, and atmospheric pressure
  • Highly variable and can change rapidly over short time scales (hours to days)
  • Influenced by local factors such as topography, proximity to water bodies, and urban heat islands
  • Examples: A thunderstorm, a heat wave, or a cold front
• Climate
  • Represents the average weather conditions in a given area over a long period, typically 30 years or more
  • Determined by factors such as latitude, altitude, proximity to large water bodies, ocean currents, and prevailing wind patterns
  • Remains relatively stable over long time scales but can change gradually due to natural factors (Milankovitch cycles) or human influences (greenhouse gas emissions)
  • Examples: Tropical rainforest climate, Mediterranean climate, or tundra climate
• Impact on Earth's systems
  1. Biosphere: Weather and climate patterns determine the distribution and diversity of ecosystems and species across the planet (biomes)
  2. Hydrosphere: The water cycle is driven by weather and climate through processes like evaporation, transpiration, condensation, and precipitation
  3. Geosphere: Weathering and erosion rates are influenced by temperature, precipitation, and wind patterns, shaping Earth's landscape over time
  4. Human society: Weather and climate affect agriculture, energy production and consumption, transportation, and human health and well-being (heat waves, natural disasters)

Atmospheric Energy Transfer and Weather Patterns

• Convection: The transfer of heat through the movement of fluids, such as air or water
  • Plays a crucial role in the formation of thunderstorms and other weather phenomena
• Radiation: The transfer of energy through electromagnetic waves
  • Solar radiation is the primary source of energy for Earth's atmospheric processes
• Air masses: Large bodies of air with similar temperature and humidity characteristics
  • Influence weather patterns as they move across regions
• Jet streams: Fast-flowing, narrow air currents found in the atmosphere
  • Play a significant role in guiding weather systems and influencing climate patterns