Understanding Earth’s Gaseous Envelope: The Atmosphere

Encyclopédie environnement - atmosphère -température selon altitude

The Earth is encompassed by a gaseous domain known as the atmosphere. This envelope of gas plays a crucial role in shaping life on our planet, influencing climate patterns and serving as a protective shield against the Sun’s radiation. While the atmosphere consists of various concentric layers, it is the troposphere, stratosphere, and mesosphere that exhibit the most distinct characteristics.

1. The Layered Structure of the Atmosphere

![Figure 1. At sunrise, from the International Space Station ISS) in June 2011, a NASA crew photographed the cross-section of the atmospheric layer. From the still very dimly lit ground, the colours change from red to orange, then to darker and darker blues, to the absolute black of space. Source : © NASA

The atmosphere, extending above the Earth’s crust, is lighter and less dense than the elements below. As we ascend in altitude, the density gradually decreases. At around 85 km above the ground, the transition to Earth’s space environment occurs, where air density is almost negligible. The atmosphere consists of constantly agitated gases that collide frequently due to their high speed. This gaseous nature enables birds and airplanes to navigate through it.

Three vital phenomena, including heat transfer, water content, and ultraviolet radiation absorption, determine the composition and behavior of the atmosphere’s concentric layers. The troposphere, reaching up to 8 km at the poles and 15 km at the equator, experiences temperature variations due to heat transfer through conduction and convection. As we ascend, the temperature gradually decreases, but the water vapor content becomes a key factor. The condensation of water vapor at higher altitudes releases latent heat, reducing the temperature drop. This layer, known as the troposphere, is the most active and agitated part of the atmosphere.

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Beyond the tropopause, which marks the upper limit of the troposphere, heat transfer from the Sun’s ultraviolet radiation becomes the primary driving force. This radiation triggers chemical reactions that transform oxygen into ozone, generating warmth in the stratosphere. The stratosphere comprises several sub-layers with varying oxygen and ozone compositions. It is stable and protected from disturbances caused by the troposphere’s turbulence. Further from the ground, the mesosphere experiences a decrease in temperature due to reduced ultraviolet radiation absorption. Figure 2 illustrates the temperature variations across these layers.

2. How Atmospheric Properties Vary with Altitude

  • An Exponentially Decreasing Pressure:
    The pressure within the atmosphere exponentially decreases with altitude. Gravity is the sole force acting on the gas, causing this pressure decrease. This exponential law ensures that pressure decrease is proportional to local pressure. Consequently, at higher altitudes, where air pressure is significantly lower, oxygen content decreases as well, affecting life forms differently. For mountaineers, reduced oxygen levels necessitate faster breathing and an increased heart rate.

  • A Homogeneous Composition:
    Dry air consists of four major gases: nitrogen (78%), oxygen (21%), argon (1%), and carbon dioxide (0.035%). These percentages remain relatively constant throughout the troposphere-stratosphere-mesosphere complex due to the molecular agitation that maintains thermodynamic balance. However, water vapor content varies based on temperature and local pressure, resulting in clouds and precipitation.

  • Where Pressure Decreases, Oxygen is Scarce:
    As pressure decreases exponentially with altitude, the oxygen content in the troposphere also declines. At high altitudes, mountaineers must breathe faster to maintain adequate oxygen supply. For instance, on Mount Blanc, oxygen levels are halved, requiring mountaineers to breathe twice as fast. The Himalayas, reaching even higher altitudes, pose significant challenges due to reduced oxygen content.

  • As Density Decreases, Flight Becomes Challenging:
    Air density, closely related to pressure and temperature, decreases exponentially as well. This explains why birds predominantly fly within the lower layers of the troposphere, where air density supports their flight. Similarly, commercial airliners rely on the denser troposphere for lift. In contrast, rockets are free from the limitations of air density, enabling them to traverse the stratosphere and beyond.

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3. Heat Transfer in the Atmosphere

Earth and Sun

The distribution of temperature within the troposphere follows a linear decrease. This temperature distribution is a result of heat exchange mechanisms between the Sun, Earth, and the atmosphere. These mechanisms form the foundation of climate analysis, intimately tied to the Earth’s radiation and energy balance.

The heat flux radiated by the Sun towards the Earth averages around 1361 W/m². However, factors such as albedo—the fraction of solar energy reflected back into space—and the Earth’s spherical shape influence this heat flux. After accounting for these influences, the average heat flux reaching the Earth’s surface is around 240 W/m². To maintain thermal equilibrium, the Earth radiates the same amount of energy back to space. This equilibrium gives rise to an average ground temperature of approximately +15°C (or 288 K).

However, the average atmospheric temperature far exceeds the predicted ground temperature of -18°C. This disparity is explained by the greenhouse effect, whereby the atmosphere absorbs a significant portion of the Earth’s infrared radiation and redirects it back to the surface. This effect raises the average temperature to around +15°C at sea level. Figures 4 and 5 depict the respective contributions of solar radiation and infrared radiation to the Earth’s energy balance.

4. Messages to Remember

  • The Earth’s atmosphere consists of three distinct layers: the troposphere, stratosphere, and mesosphere. Each layer possesses unique characteristics that influence life on Earth.
  • The air in the atmosphere mainly comprises nitrogen, oxygen, argon, and carbon dioxide, with their proportions remaining relatively constant throughout the troposphere-stratosphere-mesosphere complex.
  • Heat exchange within the atmosphere determines the Earth’s average temperature. The balance between solar radiation received and Earth’s infrared radiation emitted is crucial. Factors such as albedo and the greenhouse effect significantly influence this balance.
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