The atmosphere is a thick gaseous envelope surrounding Earth, attached to its surface by gravitational force. It acts as a shield against the Sun’s harmful ultraviolet radiation and protects us from meteors. Containing essential gases like oxygen and carbon dioxide, the atmosphere allows shortwave solar radiation to pass but blocks outgoing long-wave terrestrial radiation. This effect maintains Earth’s surface temperature around an average of 15°C, making it hospitable for life.
Learning Outcomes:
Understand the composition and structure of the Earth’s atmosphere.
Differentiate between the various layers of the atmosphere.
Comprehend the factors affecting temperature and pressure distribution.
Explain different wind systems and cyclonic patterns.
Identify types and processes of condensation and precipitation.
Principle Gases of Dry Air
The atmosphere comprises various gases with specific volumes and concentrations. The major components include:
Constituent
% by Volume
Concentration (PPM)
Nitrogen (N2)
78.084
780,840
Oxygen (O2)
20.946
209,460
Argon (Ar)
0.934
9,340
Carbon Dioxide (CO2)
0.036
360
Neon (Ne)
0.002
18.2
Helium (He)
0.000524
5.24
Methane (CH4)
0.00015
1.5
Krypton (Kr)
0.000114
1.14
Hydrogen (H2)
0.00005
0.5
Xenon (Xe)
0.00009
0.9
Structure of the Atmosphere
The atmosphere has layers that vary in density and temperature. It consists of two primary zones:
Zones Based on Uniformity
Homosphere: Extending up to 88 km, it includes the troposphere, stratosphere, and mesosphere. Gases are uniformly mixed here due to convective currents. It contains nitrogen, oxygen, argon, and carbon dioxide.
Heterosphere: From 88 km to 10,000 km, it consists of non-uniform layers characterized by variable chemical composition. The layering of gases occurs based on their mass, with nitrogen and oxygen in the lower levels and hydrogen and helium in the uppermost levels.
Layers Based on Thermal Conditions
Troposphere: The lowest layer, extending up to 13 km on average, contains 75% of the atmospheric mass. Temperature decreases with altitude at 1°C per 165 m (normal lapse rate). Weather phenomena occur in this layer, separated from the stratosphere by the tropopause.
Stratosphere: Extending up to 50 km, it lacks turbulence and houses the ozone layer which absorbs UV radiation. The temperature increases with altitude due to this absorption, separated from the mesosphere by the stratopause.
Mesosphere: Extends up to 80 km, with temperatures dropping to -100°C. This is where meteors burn up. It is separated from the thermosphere by the mesopause.
Thermosphere: Ranges up to 640 km. The ionosphere (part of the thermosphere) contains ionic particles and free radicals. Temperature increases rapidly due to interaction with solar radiation.
Ionosphere: Extends from 80 to 640 km. It reflects radio waves back to Earth and contributes to the rise in temperature with altitude.
Exosphere: The outermost layer, extending beyond 640 km, contains rarefied gases. Molecules in this layer can escape into space due to their kinetic energy.
Important Note: The troposphere hosts most of the Earth’s weather systems, whereas the stratosphere contains the critical ozone layer.
Thermal Radiations
Atmospheric radiation is of two types:
Insolation: Short-wave radiation from the Sun. It is measured as 1.92 calories per square centimeter per minute. Factors affecting insolation include:
Sun Ray Angle: Higher insolation at the equator due to more vertical rays.
Earth-Sun Distance: Varies, with minimum insolation at aphelion and maximum at perihelion.
Atmospheric Effect: Absorption and scattering reduce direct beam radiation.
Day Length: Longer days result in more insolation.
Terrestrial Radiation: Long-wave energy emitted from Earth. It is key in the greenhouse effect, absorbed and reradiated by atmospheric gases.
Heat Balance
Earth’s temperature remains stable due to the heat balance: the equilibrium between incoming insolation and outgoing terrestrial radiation.
Heat Budget: Earth does not gain or lose heat on a net basis, maintaining its temperature through the balance of incoming and outgoing radiation.
Concept: Areas between 40°N and 40°S receive more solar energy than they lose, while areas poleward lose more energy than they gain. This imbalance drives heat transfer through air masses and ocean currents.
Temperature
Measured in Celsius, Fahrenheit, or Kelvin, temperature is influenced by insolation and the surface. Isotherms connect regions of equal temperature on maps.
Factors Affecting Temperature Distribution
Insolation: Direct solar energy affects regions’ temperatures.
Latitude: Temperature decreases from the equator towards the poles.
Surface Nature: Rough surfaces heat faster than smooth ones; water heats and cools slower than land.
Altitude: Higher altitudes have lower temperatures.
Distance from Sea: Coastal areas have moderated temperatures compared to interior regions.
Air Masses and Winds: Transport heat, influencing temperature changes.
Horizontal and Vertical Distribution of Temperature
Horizontal: Represented by isotherms; close spacing indicates steep gradients, while wide spacing shows gentle gradients.
Vertical: Temperature decreases with altitude at an average rate of 6.5°C/km, known as the normal lapse rate.
Spatial Distribution
Torrid Zone: High temperatures due to near-vertical solar rays.
Temperate Zone: Moderate temperatures, no extreme heating or cooling.
Frigid Zone: Low temperatures due to slanting sun rays.
Atmospheric Pressure
Defined as the weight of air above a unit area at sea level. The isobars connect regions of equal pressure.
Pressure Belts
Equatorial Low: Located between 5°N and 5°S, characterized by intense heat and convergence of trade winds.
Sub-Tropical High: Found between 25°-35° in both hemispheres, characterized by descending air and clear weather.
Subpolar Low: Between 60°-65°, more developed in the Southern Hemisphere.
Polar High: Persistent high pressure due to very low temperatures.
Concept: The Coriolis force affects wind direction, causing cyclonic and anticyclonic circulations.
Atmospheric Circulations
Planetary Winds: Include Trade Winds, Westerlies, and Polar Winds.
Seasonal Winds: Monsoon winds change direction with the seasons due to differential heating of land and water.
Local Winds: Include Chinook, Foehn, Sirocco, etc., affecting regional climates.
Wind Systems
Trade Winds: Blow from sub-tropical high pressure to equatorial low pressure, contributing to rainfall in Eastern continents.
Westerlies: Blow from high to temperate low-pressure zones, more consistent in the Southern Hemisphere.
Polar Easterlies: Blow from polar high-pressure areas towards the subpolar lows.
Important Note: Winds are named based on the direction from which they blow.
Special Wind Patterns
Land and Sea Breezes: Occur due to differential heating of land and water.
Katabatic and Anabatic Winds: Downhill and uphill winds influenced by topography and temperature changes.
Jet Streams: High-speed winds in the upper atmosphere, influencing weather systems.
Condensation and Precipitation
Condensation transforms water vapor into liquid or solid forms, creating phenomena like dew, fog, clouds, and frost. Essential factors include nuclei presence, temperature falling to dew point, and sufficient water vapor.
Types of Condensation
Dew: Forms when air cools below its dew point.
Fog: Microscopic water droplets suspended near the ground.
Frost: Ice crystals formed when the dew point is below freezing.
**Cloud
s**: Aggregates of water droplets or ice particles in the air, caused by *adiabatic cooling*.
Cloud Type
Characteristics
Cirrus
High, fibrous, indicate fair weather
Altostratus
Middle clouds, thin sheets
Stratus
Low, fog-like, light drizzle
Cumulus
Dense, dome-shaped, fair weather
Cumulonimbus
Dark, heavy, thunderclouds
Rainfall
Occurs when water droplets become large due to coalescence. Precipitation distribution is influenced by factors like latitude, temperature, and landforms.
Types of Rainfall
Convectional: Occurs in equatorial regions due to surface heating.
Cyclonic/Frontal: Results from air convergence at fronts, typical in temperate and tropical regions.
Orographic: Caused by moist air ascending mountain slopes.
Air Masses and Fronts
An air mass is a large body of air with uniform temperature and humidity, originating over homogeneous regions. Fronts form where air masses of different characteristics meet.
Types of Fronts
Stationary: No movement between air masses.
Cold: Cold air advances, undercutting warm air.
Warm: Warm air overrides cold air.
Occluded: Forms when a cold front overtakes a warm front.
Cyclones
Temperate Cyclones: Develop in middle latitudes due to contrasting air masses.
Tropical Cyclones: Originate over warm tropical waters, with high wind speeds.
Comparison Table: Tropical vs. Temperate Cyclones
Aspect
Tropical Cyclone
Temperate Cyclone
Origin
Over sea, summer
Land/sea, winter
Isobar Shape
Circular, steep gradient
‘V’ shaped, low gradient
Movement
East to West
West to East
Rainfall
Heavy, short duration
Slow, long duration
MCQ: Which layer of the atmosphere contains the ozone layer?