🌾 Atmosphere — Composition and Structure

The atmosphere is the medium in which all weather processes occur. A student of agrometeorology must first understand what the atmosphere is made of and how its layers differ, because rainfall, clouds, temperature change, and crop-weather interactions all begin there.

Composition of the Atmosphere

The atmosphere is a gaseous envelope surrounding the Earth and held by gravity. In dry air, the major gases remain almost constant.

Why These Gases Matter

• Nitrogen is the dominant atmospheric gas, though plants cannot directly use free atmospheric nitrogen without biological or industrial fixation.
• Oxygen is essential for respiration of plants, animals, and soil organisms.
• Carbon dioxide is present in small quantity but is extremely important because it is the raw material for photosynthesis.

Variable Components of the Atmosphere

Some components vary significantly from place to place and time to time.

• Water vapour: 0-4% by volume; highest near the surface in warm and humid regions.
• Ozone (O₃): concentrated in the stratosphere and protects life by absorbing harmful ultraviolet radiation.
• Aerosols: dust, smoke, sea salt, pollen, and ash; they act as condensation nuclei and influence cloud formation.

These variable components are especially important in agriculture because they affect rainfall formation, solar radiation receipt, temperature, and air quality.

Vertical Structure of the Atmosphere

The atmosphere is divided into layers mainly on the basis of temperature change with altitude.

1. Troposphere (0-12 km)

This is the lowest and most important layer for agriculture.

• Temperature decreases with height at an average rate of about 6.5°C per km.
• It contains about 75% of the total atmospheric mass.
• Almost all water vapour, clouds, rain, fog, storms, and other weather events occur here.
• Its upper boundary is called the tropopause.

2. Stratosphere (12-50 km)

• Temperature increases with height because ozone absorbs ultraviolet radiation.
• The ozone layer is concentrated roughly between 20 and 25 km.
• The air is relatively stable, with little vertical mixing.
• Its upper boundary is the stratopause.

3. Mesosphere (50-80 km)

• Temperature again decreases with height.
• This is the coldest layer of the atmosphere.
• Many meteors burn in this region.

4. Thermosphere (80-700 km)

• Temperature rises sharply because high-energy solar radiation is absorbed.
• It includes the ionosphere, which is important for radio communication.

5. Exosphere (Above 700 km)

• This is the outermost layer.
• It gradually merges into outer space.
• Hydrogen and helium dominate here.

Agricultural Relevance of Atmospheric Layers

For farming, not all layers are equally important.

• The troposphere is the real working layer of agriculture because weather and crop growth interactions take place here.
• The stratosphere matters indirectly because ozone protects crops and living organisms from damaging UV rays.
• Higher layers are less directly linked with day-to-day farming, but they influence communication, energy balance, and broader atmospheric processes.

Easy Way to Remember

• Troposphere: weather layer
• Stratosphere: ozone layer
• Mesosphere: cold layer
• Thermosphere: ionized hot layer
• Exosphere: outermost layer

Summary Cheat Sheet