The atmosphere is the thin gaseous envelope that surrounds the Earth and extends to a height of about 20,000 kilometres, gradually merging into space. It is held to the Earth by gravity and rotates with the planet. The atmosphere is essential for life because it provides oxygen for breathing, carbon dioxide for photosynthesis, protects living beings from harmful solar radiation, and moderates the Earth's temperature by the greenhouse effect. Without the atmosphere, the surface temperature of the Earth would be far too cold to support life.
The atmosphere is a mixture of many gases, along with suspended solid and liquid particles called aerosols. Its composition, though apparently simple, is remarkably stable in the lower layers. In this chapter we study the composition of the atmosphere, the structure of its layers, and the special features of each layer such as the greenhouse effect, the ozone layer and the phenomena of meteorology. This understanding is the foundation for the study of climate, weather systems and the heat balance of the Earth discussed in later chapters.
The atmosphere is composed mainly of nitrogen and oxygen. Nitrogen accounts for about 78.08% of the total volume, and oxygen about 20.95%. The remaining gases include argon (0.93%), carbon dioxide (about 0.04%), and traces of neon, helium, methane, hydrogen, krypton and ozone. Water vapour is highly variable and is present in varying amounts, from near zero in dry deserts to about 4% in humid tropical air. Although carbon dioxide is present in tiny amounts, it is crucial because it is a greenhouse gas that absorbs and re-emits infrared radiation, keeping the Earth warm.
Water vapour is the most variable component of the atmosphere. It is present in the lower layers and decreases rapidly with height. Water vapour is the source of all clouds and precipitation and absorbs much of the outgoing terrestrial radiation. It is responsible for the greenhouse effect along with carbon dioxide. Humidity, the amount of water vapour in the air, determines weather conditions and the formation of clouds, fog and precipitation.
The atmosphere also contains dust particles, salt particles from oceans, smoke, pollen and volcanic ash, collectively called aerosols. These particles are important because they act as hygroscopic nuclei (condensation nuclei) around which water vapour condenses to form clouds. Aerosols also scatter and reflect solar radiation, affecting visibility and the Earth's radiation budget.
The atmosphere is divided into layers on the basis of temperature, which is the most convenient way to classify it. From the surface upwards, the layers are the troposphere, stratosphere, mesosphere, thermosphere (ionosphere) and exosphere.
The troposphere is the lowest layer, with an average height of about 13 kilometres. It is thicker at the equator (about 16-18 km) and thinner at the poles (about 8 km). Its average thickness is 13 km. This layer contains almost all of the water vapour, dust and clouds and is the layer in which all weather phenomena such as rain, wind, storms and clouds occur. Temperature decreases with height at an average rate of about 6.5 degrees Celsius per kilometre, known as the normal lapse rate. The upper limit of the troposphere is called the tropopause.
The stratosphere extends from the tropopause to about 50 kilometres above the surface. In its lower part the temperature is nearly constant, but in the upper part temperature increases with height because of the absorption of ultraviolet radiation by ozone. The ozone layer, which absorbs harmful UV radiation and protects life on Earth, is located in the stratosphere. The stratosphere is free of clouds and weather and is therefore ideal for jet aircraft to fly. Its upper limit is called the stratopause.
The mesosphere extends from the stratopause to about 80-85 kilometres. Temperature decreases with height, reaching the coldest part of the atmosphere, about -100 degrees Celsius, at the mesopause. Meteors burning up in the atmosphere generally occur in the mesosphere, creating "shooting stars". The upper boundary is the mesopause.
The thermosphere extends from the mesopause to about 400-500 kilometres. Temperature increases rapidly with height, reaching more than 1,000 degrees Celsius, because of the absorption of extremely shortwave solar radiation by gases like oxygen and nitrogen. Because of this ionisation, the upper part of the thermosphere is called the ionosphere, which is useful for radio communication as it reflects radio waves. The ionosphere extends from about 80 to 400 kilometres.
The exosphere is the outermost layer, extending beyond the thermosphere into space. The air is extremely rarefied and atoms and molecules can escape into space. This layer merges gradually into the interplanetary space.
The atmosphere allows incoming shortwave solar radiation to pass through and heat the Earth's surface. The Earth's surface then radiates energy back as longwave (infrared) radiation. Greenhouse gases such as carbon dioxide, water vapour, methane and nitrous oxide absorb this outgoing longwave radiation and re-emit it back towards the surface. This trapping of heat is called the greenhouse effect. It keeps the Earth's average temperature about 15 degrees Celsius warmer than it would otherwise be (about -18 degrees Celsius), making life possible. However, the rapid increase in greenhouse gases due to human activities is enhancing this effect, leading to global warming and climate change.
The atmosphere performs vital functions. It provides the gases necessary for life, protects the Earth from harmful radiation (ultraviolet by ozone, X-rays and gamma rays by the ionosphere), prevents the Earth from becoming too hot or too cold by the greenhouse effect, and is the medium for weather and climate. The atmosphere also protects us from meteorites, which burn up in the mesosphere. The heat balance and radiation processes of the atmosphere are studied in detail in the next chapter.
| Gas | Percentage by Volume | Significance |
|---|---|---|
| Nitrogen | 78.08% | Dilutes oxygen; essential for proteins |
| Oxygen | 20.95% | Supports respiration and combustion |
| Argon | 0.93% | Chemically inert |
| Carbon dioxide | About 0.04% | Greenhouse gas; photosynthesis |
| Water vapour | 0-4% (variable) | Source of precipitation; greenhouse gas |
| Layer | Extent | Temperature Behaviour | Key Feature |
|---|---|---|---|
| Troposphere | Surface to ~13 km | Decreases (6.5 C/km) | All weather phenomena |
| Stratosphere | ~13 to 50 km | Increases (ozone) | Ozone layer; jet aircraft |
| Mesosphere | ~50 to 85 km | Decreases | Coldest layer; meteors burn |
| Thermosphere | ~85 to 500 km | Increases greatly | Ionosphere; auroras |
| Exosphere | Beyond 500 km | Very hot, rarefied | Atoms escape to space |
The atmosphere is the protective gaseous envelope of the Earth, composed chiefly of nitrogen and oxygen, along with variable components like water vapour, dust and aerosols. It is organised into five layers based on temperature: the troposphere where all weather occurs, the stratosphere containing the life-protecting ozone layer, the cold mesosphere where meteors burn, the hot thermosphere with its ionosphere for radio communication, and the outer exosphere. The greenhouse effect, driven by gases like carbon dioxide and water vapour, keeps the planet warm enough for life. The composition and layered structure of the atmosphere are the basis for understanding solar radiation, heat balance, atmospheric circulation and climate, which form the subject of the following chapters.
Keywords: Troposphere, stratosphere, mesosphere, thermosphere, exosphere, ozone, lapse rate, greenhouse effect, aerosols.