The envelope of gases surrounding Earth, extending ~100 km up. Contains all weather, life-supporting oxygen, the protective ozone layer, and the climate system that shapes life on Earth.
basicWhat is the Atmosphere?
The atmosphere is the layer of gases that surrounds Earth, held in place by gravity. It extends from the surface to roughly 100 kilometres in altitude (the international “Kármán line” definition of where space begins), although gases continue to fade out into space all the way to ~10,000 km.
The atmosphere is essential for life:
- Provides oxygen (21% of dry air) for respiration
- Filters harmful UV via the stratospheric ozone layer
- Regulates temperature through the greenhouse effect
- Distributes heat and moisture via wind and weather
- Contains the water cycle — evaporation, clouds, precipitation
- Protects from meteors — most burn up in the mesosphere
- Enables transmission of sound and light
For weather, all of Earth’s weather occurs in the lowest 12 km — the troposphere. Above that, the upper layers play important supporting roles but don’t generate weather directly.
Atmospheric layers
1. Troposphere (0-12 km in tropics, 8 km at poles):
- Where all weather happens
- Temperature decreases with altitude (avg 6.5°C/km lapse rate)
- Top of troposphere = tropopause
- Contains 75% of all atmospheric mass
2. Stratosphere (12-50 km):
- Temperature increases with altitude (due to ozone absorbing UV)
- Ozone layer peaks here (15-30 km)
- Stable, mostly cloud-free (commercial jets fly in lower stratosphere)
- Volcanic eruptions can inject material that affects climate for years
3. Mesosphere (50-85 km):
- Temperature decreases again
- Coldest atmospheric layer (-80 to -100°C)
- Meteors burn up here (the “shooting stars”)
- Mostly devoid of clouds (rare noctilucent clouds at top)
4. Thermosphere (85-600 km):
- Temperature very high (500-2000°C) but air so thin you wouldn’t feel it
- Aurora borealis/australis occurs here
- ISS orbits at 400 km within this layer
- Satellites operate in lower thermosphere
5. Exosphere (600+ km):
- Atmosphere fades to space
- Hydrogen and helium atoms drift away
- Geostationary satellites at 36,000 km — technically in exosphere
Composition
By volume, dry air:
- Nitrogen (N₂): 78.09% — chemically inert, doesn’t directly affect weather
- Oxygen (O₂): 20.95% — essential for respiration and combustion
- Argon (Ar): 0.93% — noble gas, inert
- Carbon Dioxide (CO₂): 0.04% (rising) — greenhouse gas, climate critical
- Trace gases: neon, helium, methane, krypton, hydrogen, ozone, nitrous oxide
Variable components (not in “dry air”):
- Water vapor: 0-4% by volume
- Dust, pollen, aerosols: highly variable
- Pollution particles (PM2.5, NOx, SO₂): variable
- Volcanic emissions: episodic
- Wildfire smoke: seasonal/episodic
The trace gases punch above their concentration weight — 0.04% CO₂ traps enough infrared radiation to keep Earth’s temperature 14°C habitable instead of -18°C. Doubling CO₂ would add roughly 3°C of warming.
Atmosphere over South Asia
The South Asian atmosphere has distinctive characteristics:
Vertical structure:
- Tropopause height: ~16-17 km (close to equator)
- Boundary layer depth: 1-3 km, varies seasonally
- Monsoon trough position at surface determines rainfall
Seasonal flow patterns:
- Summer monsoon (June-September): southwesterly flow from Indian Ocean
- Winter monsoon (December-February): northeasterly flow from continent
- Inter-monsoon (March-May, October-November): transitional, variable
Aerosol loading:
- Indo-Gangetic plain: among the highest in the world
- PM2.5 winter peaks 10-30× WHO limits
- Visibility often degraded
- Affects radiation balance, monsoon timing
Atmospheric pressure profile
Pressure decreases approximately exponentially with altitude:
| Altitude | Pressure | % of Sea Level |
|---|---|---|
| 0 km (sea level) | 1013.25 hPa | 100% |
| 2 km | 795 hPa | 78% |
| 5 km | 540 hPa | 53% |
| 8 km (Mt Everest base) | 356 hPa | 35% |
| 8.85 km (Everest summit) | 314 hPa | 31% |
| 12 km (tropopause) | 194 hPa | 19% |
| 20 km (lower stratosphere) | 55 hPa | 5% |
| 50 km (upper stratosphere) | 0.8 hPa | 0.08% |
This rapid pressure drop with altitude is why:
- Cooking takes longer at altitude (water boils at lower temperature)
- Mountain climbers need oxygen above 5,500 m
- Aircraft cabins pressurize to 2,400 m equivalent
- Weather balloons reach 30+ km altitude
The atmosphere as a weather engine
Three properties make the atmosphere a weather engine:
- Solar heating drives convection (rising warm air)
- Earth’s rotation creates Coriolis force, deflecting winds
- Moisture condenses to form clouds and precipitation
The interaction of these — combined with land-sea contrast, terrain, ocean currents, and seasonal sun angle — produces the rich variety of weather we experience.
For South Asia specifically:
- Tibetan Plateau heating in summer drives the monsoon
- Bay of Bengal moisture fuels monsoon depressions and cyclones
- Himalayan barrier blocks cold air outflow, allowing the IGP to become extremely hot in summer
- Mediterranean lows (Western Disturbances) deliver winter precipitation
Climate change and the atmosphere
Human activities are altering atmospheric composition:
- CO₂: 280 ppm (1850) → 425 ppm (2026), highest in 3+ million years
- CH₄ (methane): 700 ppb → 1920 ppb, mostly from agriculture and fossil fuels
- N₂O: 270 ppb → 335 ppb
- CFCs (banned): now declining
- Black carbon: rising; affects monsoon
These changes are causing:
- Global warming: 1.2°C above pre-industrial
- Sea-level rise: 21+ cm since 1880, accelerating
- More extreme weather: heatwaves, heavy rain, cyclones
- Glacial retreat: Himalayan ice loss accelerating
- Acidification of oceans: 30%+ increase in surface acidity
Frequently asked questions
How high is the atmosphere? The Kármán line at 100 km is the international boundary with space. But gases fade out gradually — even at 600 km (ISS orbital altitude), there’s enough atmospheric drag to slow satellites. Practically, “atmospheric weather” stops at the troposphere ceiling of 12-18 km.
Why does temperature decrease with altitude? Because air expands as it rises (lower pressure), and expanding gas cools. The standard “lapse rate” is 6.5°C per km in the troposphere. Above the tropopause, the pattern reverses due to ozone absorbing UV radiation in the stratosphere.
How does the atmosphere affect weather forecasting? Weather models must simulate atmospheric physics — pressure, temperature, humidity, winds, clouds — across the entire troposphere on a 3D grid. Higher resolution = more accurate forecasts but more computing required. ECMWF’s IFS uses 137 vertical levels.
Is the atmosphere getting “thicker”? Slightly, due to climate change — warming expands the troposphere. The tropopause has risen ~50-80 metres per decade since the 1970s. Stratosphere is also cooling and contracting.
Where can I see real-time atmospheric data? Mausam Online displays surface conditions (temperature, pressure, humidity, wind) on every city page. For upper-atmospheric data, see IMD’s radiosonde data or weather.cod.edu. Live forecasts: Delhi, Mumbai, Chennai, Kolkata, Dhaka.