The atmosphere envelops the earth and is essential for the survival of all organisms. Weather is the state of the atmosphere at a particular place at any given time (short-term and always changing), while climate is the average weather condition of an area over a long period. The World Meteorological Organisation (WMO) suggests data for 30 consecutive years to calculate climatic averages.
1. Weather, Climate and Composition
- The ancient Greeks called the tilt of latitude 'klima' (slope or inclination) and divided the earth into seven latitudinal regions called 'klimata' – the origin of the word 'climate'.
- Major components: nitrogen (78%), oxygen (21%) and other gases (1%) such as argon, carbon dioxide and neon.
2. Layers of the Atmosphere
The atmosphere is divided into five layers based on temperature variations.
| Layer | Extent | Key Features |
|---|---|---|
| Troposphere | About 8 km at the poles, 18 km at the equator | Lowermost and densest layer (70 to 80 percent of gases); all weather phenomena occur here; height increases in summer, decreases in winter; outer boundary tropopause is about 1.5 km thick |
| Stratosphere | Up to about 50 km | Temperature constant up to 20 km, then rises to nearly -4°C at the stratopause; lower part rich in ozone, called the 'ozonosphere' |
| Mesosphere | Up to about 80 km | Coldest layer; temperature falls with height due to absence of ozone; upper boundary mesopause; noctilucent clouds form due to cosmic dust; meteors burn here |
| Thermosphere | Up to about 400 km | Temperature rises rapidly up to 1,000 °C; its lower part is the Ionosphere, whose charged particles (ions) create auroras and reflect radio waves back to the earth |
| Exosphere | Above 400 km up to 1600 km | Uppermost layer; rarefied, mainly oxygen and hydrogen atoms; minimal gravitational pull; merges with space |