The Atmosphere  Composition & Winds
Layers of atmosphere, ozone, pressure, trade winds, westerlies, local winds.
The Atmosphere  Composition & Winds
The Atmosphere
What you'll learn
- Atmosphere — blanket of gases surrounding Earth; essential for life.
- Composition of air: nitrogen, oxygen, other gases.
- Five layers of the atmosphere.
- Weather and climate basics.
- Types of winds: permanent, seasonal, local.
Key concepts
Composition of air
| Gas | Percentage |
|---|---|
| Nitrogen (N₂) | 78% |
| Oxygen (O₂) | 21% |
| Argon | 0.93% |
| Carbon dioxide (CO₂) | 0.04% |
| Water vapour, dust, etc. | Variable |
CO₂ and water vapour trap heat → greenhouse effect (warms Earth; too much → global warming).
Layers of the atmosphere
| Layer | Height | Key feature |
|---|---|---|
| Troposphere | 0–12 km | All weather occurs here; temperature decreases with altitude |
| Stratosphere | 12–50 km | Ozone layer (absorbs harmful UV rays); used by aircraft |
| Mesosphere | 50–80 km | Meteors burn up here; coldest layer (−90°C) |
| Thermosphere | 80–700 km | Aurora borealis/australis; very hot but thin air |
| Exosphere | Above 700 km | Merges with outer space; satellites orbit here |
Ozone layer in stratosphere: shields Earth from UV-B radiation → prevents skin cancer, cataracts. Damaged by CFCs (chlorofluorocarbons from old fridges, aerosols) → ozone hole over Antarctica.
Atmospheric pressure
- Weight of air pressing down on Earth's surface.
- Measured in millibars (mb) using a barometer; standard = 1013 mb at sea level.
- Decreases with altitude (less air above you).
- Low pressure → air rises → clouds & rain (cyclone, monsoon).
- High pressure → air sinks → clear, dry weather (anticyclone).
Temperature and insolation
- Insolation — incoming solar radiation reaching Earth.
- Troposphere heated from below (Earth absorbs sun → warms air above).
- Temperature decreases 6.5°C per 1000 m rise in altitude (lapse rate).
Winds
Air moves from high pressure to low pressure areas = wind.
Permanent (Planetary) winds
| Wind | Direction | Region | Effect |
|---|---|---|---|
| Trade winds | NE (N Hemisphere) / SE (S Hemisphere) → equator | Tropics (0°–30°) | Warm, steady; drive ocean currents |
| Westerlies | West to East | Temperate (30°–60°) | Bring rain to western Europe |
| Polar easterlies | East to West | Polar (60°–90°) | Cold, dry winds |
Seasonal winds
- Monsoon winds — reverse direction with season (summer = sea → land; winter = land → sea).
- Bring India 75–90% of its annual rainfall.
Local winds
| Wind | Region | Nature |
|---|---|---|
| Loo | North India (Punjab, UP, Rajasthan) | Hot, dry summer wind |
| Mistral | France/Mediterranean | Cold, dry winter wind |
| Chinook | Rocky Mountains, USA | Warm, dry (melts snow) |
Weather instruments
| Instrument | Measures |
|---|---|
| Thermometer | Temperature |
| Barometer | Atmospheric pressure |
| Rain gauge | Rainfall |
| Hygrometer | Humidity (moisture in air) |
| Anemometer | Wind speed |
| Wind vane | Wind direction |
Quick check
- What percentage of the atmosphere is nitrogen? Oxygen?
- Name the five layers of the atmosphere in order from Earth's surface.
- What is the ozone layer? Why is it important? What damages it?
- What is the difference between trade winds and westerlies?
- Name two local winds and the regions where they blow.
Open the Practice tab for graded questions on the Atmosphere.
Key Takeaways (TL;DR)
- What you'll learn
- Key concepts
- Quick check
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