Earth's atmosphere is a complex system of layers, each with unique properties. The troposphere, where we live, experiences decreasing temperatures with height. Above it, the stratosphere warms due to ozone, while the mesosphere cools again.

The upper layers, including the thermosphere and exosphere, play crucial roles in Earth's climate and space weather. These regions host phenomena like auroras and noctilucent clouds, and interact with solar radiation and particles, shaping our planet's environment.

Atmospheric Layers and Their Characteristics

Layers of Earth's atmosphere

• Troposphere
  • Lowest layer extends from Earth's surface to an average height of 12 km (ranges from 8-18 km depending on latitude and season)
  • Contains about 80% of the total mass of the atmosphere and almost all water vapor and aerosols
  • Air pressure and density decrease rapidly with altitude
  • Temperature decreases with height at an average rate of 6.5℃/km (known as the environmental lapse rate) due to adiabatic cooling as air expands and rises
• Stratosphere
  • Extends from the tropopause (top of the troposphere) to an altitude of about 50 km
  • Contains the ozone layer which absorbs harmful ultraviolet (UV) radiation from the sun protecting life on Earth
  • Temperature increases with altitude due to ozone absorbing UV radiation
  • Very stable layer with little vertical mixing and low humidity
• Mesosphere
  • Extends from the stratopause (top of the stratosphere) to an altitude of about 85 km
  • Coldest layer with temperatures dropping to -90℃ at the mesopause (top of the mesosphere)
  • Noctilucent clouds made of ice crystals can form near the mesopause during summer at high latitudes (visible at twilight)
  • Most meteors burn up in the mesosphere due to friction with air molecules
• Thermosphere
  • Extends from the mesopause to an altitude of about 600 km
  • Temperature increases with height reaching up to 2000℃ due to absorption of intense solar radiation (UV and X-rays) by oxygen and nitrogen molecules
  • Highly variable temperatures depending on solar activity
  • Lower part of the thermosphere (80-550 km) is the ionosphere a region with high concentrations of ions and free electrons (important for radio wave propagation)
  • Aurora (Northern and Southern Lights) occur in the thermosphere when energetic charged particles from the solar wind are guided by Earth's magnetic field and collide with air molecules
• Exosphere
  • Uppermost layer above the thermosphere extending to about 10,000 km
  • Extremely low density (atoms and molecules are so far apart they rarely collide)
  • Atoms (mainly hydrogen and helium) can escape Earth's gravitational pull into space
  • No clear upper boundary where the exosphere merges with interplanetary space

Temperature profile of atmosphere

• Temperature changes with altitude are determined by the balance between absorption and emission of radiation and energy transfer by convection and conduction
• Troposphere
  1. Solar radiation passes through and is absorbed by Earth's surface
  2. Earth's surface heats the lower troposphere by conduction and convection
  3. Greenhouse gases (water vapor, carbon dioxide) absorb outgoing infrared radiation emitted by Earth's surface further warming the lower troposphere
  4. Temperature decreases with height at an average rate of 6.5℃/km (environmental lapse rate) due to adiabatic cooling as air parcels expand and rise
• Stratosphere
  • Temperature increases with altitude due to ozone absorbing solar UV radiation
  • Ozone is produced by photochemical reactions involving oxygen molecules and UV radiation
  • Positive lapse rate (temperature increasing with height) creates a stable layer with little vertical mixing
• Mesosphere
  • Temperature decreases with altitude due to decreasing ozone concentration and reduced solar heating
  • Coldest layer reaching -90℃ at the mesopause
• Thermosphere
  • Temperature increases with altitude due to absorption of intense solar radiation (UV and X-rays) by oxygen and nitrogen molecules
  • Highly variable temperatures (500-2000℃) depending on solar activity (higher during solar maximum)

Tropopause and Upper Atmospheric Layers

Role of tropopause

• The tropopause is the boundary between the troposphere and stratosphere
• Acts as a barrier to the vertical transport of air, moisture, and pollutants between the troposphere and stratosphere
• Tropopause height varies with latitude and season
  • Higher in the tropics (16-18 km) where warm air rises to greater heights
  • Lower in polar regions (8-10 km) where cold dense air sinks
  • Higher in summer and lower in winter due to seasonal temperature changes
• Controls the vertical extent of weather systems (thunderstorms, hurricanes) by limiting convection to the troposphere
• Affects the exchange of greenhouse gases (water vapor, ozone) and pollutants between the troposphere and stratosphere impacting climate and air quality
• Changes in tropopause height and temperature can influence jet stream patterns and the development of extreme weather events

Features of upper atmospheric layers

• Mesosphere
  • Noctilucent clouds form near the mesopause (85 km) in summer at high latitudes
    • Composed of ice crystals that nucleate on meteoric dust particles
    • Visible at twilight when illuminated by the sun below the horizon
  • Most meteors ablate (burn up) in the mesosphere due to high-speed collisions with air molecules
    • Meteor showers occur when Earth passes through the dusty trail of a comet
• Thermosphere
  • Ionosphere (80-550 km) is a region with high concentrations of ions and free electrons
    • Ionization caused by solar UV and X-ray radiation and energetic particle precipitation
    • Divided into distinct layers (D, E, and F) based on electron density profiles
    • Reflects radio waves enabling long-distance communication (AM radio, shortwave)
    • Responsible for GPS signal delays and errors that must be corrected
  • Aurora form when energetic electrons and protons guided by Earth's magnetic field collide with oxygen and nitrogen atoms and molecules causing them to emit light
    • Colors depend on the type of atom/molecule and the energy of the collision (green from oxygen, red/blue from nitrogen)
    • Auroral ovals centered around the geomagnetic poles (not the geographic poles)
    • More intense and frequent during solar storms (coronal mass ejections, solar flares)
• Exosphere
  • Transition zone between Earth's atmosphere and interplanetary space
  • Extremely low density (particles rarely collide) allowing atoms to follow ballistic trajectories
  • Lightest atoms (hydrogen and helium) can escape Earth's gravity into space
  • Exosphere is supplied with atoms from the lower atmosphere and ions from the ionosphere
  • Influenced by the solar wind (stream of charged particles from the sun) which can compress the exosphere on the dayside and create a geotail on the nightside