What is the atmosphere?
The atmosphere is the layer of air around the Earth. Gravity holds it close. Without it there would be no breathing, no rain and no protection from harmful sun rays. Most of the air (about 97%) is packed within 30 km of the ground. Higher up, air becomes very thin.
Gases of the atmosphere
- Nitrogen ~78% — the biggest share; plants get it through soil bacteria.
- Oxygen ~21% — we breathe it; fire needs it.
- Argon ~0.93% — a gas that does not react.
- Carbon dioxide ~0.04% — tiny share, but it lets sun rays in and traps heat going out (greenhouse gas). Burning coal and oil is raising it.
- Ozone — mostly 10–50 km up; it absorbs ultraviolet (UV) rays.
Water vapour and dust
Water vapour is water in gas form. It can be up to 4% of air in hot, wet places and almost nil in cold, dry places. It traps heat and forms clouds. Dust particles (sea salt, soil, smoke, pollen) act as tiny seeds (hygroscopic nuclei) on which water vapour condenses to make drops.
Structure of the atmosphere (the five layers)
- Troposphere (0 to about 13 km; ~8 km at poles, ~18 km at the equator): all weather happens here. Temperature falls about 6.5 °C every 1 km (normal lapse rate). Its top is the tropopause.
- Stratosphere (up to ~50 km): holds the ozone layer. Temperature rises with height because ozone absorbs UV rays. Calm air.
- Mesosphere (50–80 km): temperature falls to about −100 °C, the coldest layer. Meteors burn here.
- Thermosphere (80–400 km): temperature rises again. Its lower part, the ionosphere, has charged particles that bounce radio waves back to Earth.
- Exosphere: the top layer; air is so thin it fades into space.
Elements of weather and climate
Weather = the state of the air at one place for a short time (hours or days). Climate = the average weather of a place over a long time (about 30 years). Both use the same six elements:
- Temperature
- Air pressure
- Wind
- Humidity (water vapour)
- Clouds
- Precipitation (rain, snow, hail)
Example: "Delhi is hot and dry today" is weather. "Delhi has hot summers and cool winters" is climate.
Try it at home
Keep a weather diary for 7 days. Each morning note: temperature (from a phone app), sky (clear or cloudy), wind (still, breezy, strong) and rain (yes or no). At the end, write one line about your city's weather that week. Compare it with what you know of its climate.
Key formulas and definitions
- Air = Nitrogen 78% + Oxygen 21% + Argon 0.93% + CO₂ 0.04% + traces
- Layers: Troposphere → Stratosphere → Mesosphere → Thermosphere → Exosphere
- Normal lapse rate: temperature falls ~6.5 °C per 1 km (1,000 m) in the troposphere
- Weather = short time; Climate = average of ~30 years
- Elements: temperature, pressure, wind, humidity, clouds, precipitation
Worked examples
1. It is 30 °C at the foot of a hill. What is the temperature at the top, 2 km higher?
Fall = 6.5 × 2 = 13 °C. Temperature at top = 30 − 13 = 17 °C.
2. Why does the troposphere reach 18 km at the equator but only 8 km at the poles?
At the equator strong heating makes air rise high (convection) and pushes the layer up. At the poles the air is cold and sinks, so the layer is thin.
3. A statement: "Mumbai gets heavy rain in July every year." Weather or climate?
Climate, because it describes the usual pattern over many years, not one day.
4. Why does temperature rise in the stratosphere even though we are going higher?
The ozone layer absorbs UV rays from the Sun and gets warmed. So the stratosphere heats up from within, and it is warmer higher up.
Common mistakes
- Saying oxygen is the main gas of air. Nitrogen is (78%).
- Thinking temperature always falls as we go up. It rises again in the stratosphere and thermosphere.
- Mixing weather and climate. Weather is today; climate is the long-term average.
- Thinking the troposphere has the same height everywhere. It is thick at the equator, thin at the poles.