The atmosphere consists of several distinct layers, each defined by variations in density and temperature. Density is highest at the Earth's surface and progressively decreases with altitude. Based on temperature variations, the atmosphere can be divided into – TroposphereStratosphereMesosphereThermosphereExosphere Exosphere The outermost layer of Earth's atmosphere extends from about 600 km above sea level to around 10,000 km. This layer is characterized by extremely low densities, and the atmosphere resembles a Nebula, where atmospheric particles are so sparse that they can travel hundreds of kilometres without colliding with one another. Hydrogen (H₂) and helium (He) are the primary gases (while oxygen, carbon dioxide, and atomic nitrogen can also be found, though in negligible concentrations) Layer is exposed to higher levels of solar and cosmic radiation. Importance of the exosphere Many satellites orbit within the exosphere The exosphere is where lighter gases can escape into space. This process is critical for understanding atmospheric loss over geological timescales. helps to understand space weather and its impact on human activities on Earth. Classification by chemical composition Homosphere Homosphere : Extends from the Earth's surface up to approximately 90 km. It includes the troposphere, stratosphere, and mesosphere. These layers are primarily defined by temperature variations. Characterized by a relatively uniform mixture of gases. This means the proportions of major gases like Nitrogen (N2), Oxygen (O2), and Argon (Ar) remain fairly constant throughout this layer. While the Proportions of gases stay consistent, the Density of air decreases with increasing altitude. Heterosphere Heterosphere : Lies above the homosphere, starting around 90 km and extending outward. Gases are no longer uniformly mixed. Instead, they stratify into distinct layers based on their molecular weight. Heavier gases remain closer to the base, while lighter gases are found at higher altitudes. Influenced by solar radiation and other space phenomena. Nitrogen layer = up to ~ 200 km. Atomic Oxygen layer = 200 to 1100 km. Helium layer : Extends to approximately 3500 km. Atomic Hydrogen layer = beyond 3500 km, gradually fading into the vacuum of space. Questions from the chapter Write on the – Different layers of atmosphere with suitable diagram [UPPSC ACF (Main) 2018, Environmental Science Optional | P1/2(a) | 20 M]. Write on the – Different layers of atmosphere with a suitable diagram [UPPSC ACF (Main) 2017, Environmental Science Optional | 8 M | 150 Words]. What do you mean by atmosphere? Describe its structure, importance, and chemical composition in detail. [UKPSC RFO (Main) 2015 Environmental Science Optional | 40 M]. Write short note on – Structure of atmosphere [UK PSC FRO (Mains) 2012 Environmental Science Optional | 15 M, 200 words]. Name the different layers of the atmosphere and state their characteristics [Arunachal PSC ACF (MAIN) 2012; Environmental Science Optional | 4 M]. Exam attributions follow the supplied chapter. Quick revision Temperature-based layers are troposphere, stratosphere, mesosphere, thermosphere and exosphere.The homosphere has relatively uniform proportions of major gases.The heterosphere has gases stratified according to molecular weight.The exosphere is very sparse and allows lighter gases to escape into space. Related concepts from Chapter 1Abiotic components of environmentPhotosynthesis and biotic–abiotic interactionsThe interdisciplinary nature of environmental scienceRelated topicsComposition of Atmosphere: Gases, Water Vapour and DustTroposphere, Stratosphere and Polar VortexMesosphere: Noctilucent Clouds, Meteors and Gravity WavesThermosphere and Ionosphere: Temperature and Radio CommunicationChapter 2 indexComposition of Atmosphere: Gases, Water Vapour and DustStructure of Atmosphere: Layers, Exosphere, Homosphere and HeterosphereTroposphere, Stratosphere and Polar VortexOzone: Formation, Depletion and Effects on PlantsMesosphere: Noctilucent Clouds, Meteors and Gravity WavesThermosphere and Ionosphere: Temperature and Radio CommunicationAurora and Magnetosphere: Solar Wind and Earth’s Magnetic FieldSodium Layer: Artificial Guide Stars and Adaptive Optics
