What causes air density to be greatest at the surface?

Air density is greatest at the surface because gravity pulls air molecules toward the Earth, compressing them and increasing the number of molecules in a given volume.

How does air pressure change with altitude?

Air pressure decreases with increasing altitude. This is because there are fewer air molecules exerting force as you move higher in the atmosphere.

What is the average atmospheric pressure at sea level?

The average atmospheric pressure at sea level is 1013.25 hPa, 1013.25 mb, or 29.92 inches of mercury (in Hg).

Why does air density decrease rapidly at first and then more slowly with altitude?

Air density decreases rapidly at lower altitudes due to greater compression from the weight of the air above. As altitude increases, the compression effect diminishes, causing a slower rate of density decrease.

What is the temperature lapse rate in the lower atmosphere?

The average temperature lapse rate in the lower atmosphere (troposphere) is 6.5°C per 1000 meters or 3.6°F per 1000 feet.

What is a temperature inversion?

A temperature inversion occurs when air temperature increases with height, contrary to the usual decrease. This condition traps cooler air below and can affect weather and pollution dispersion.

Where is the tropopause located, and how does its height vary?

The tropopause separates the troposphere from the stratosphere and varies in height. It is higher over equatorial regions and lower over the poles. It also rises in summer and falls in winter.

What causes the increase in temperature in the stratosphere?

The temperature in the stratosphere increases with height due to the absorption of ultraviolet (UV) radiation by the ozone layer.

Why is the top of the mesosphere extremely cold?

The top of the mesosphere is extremely cold, with temperatures reaching -90°C, due to the low density of air and minimal heat absorption from solar radiation.

How hot can the thermosphere get, and why doesn’t it feel hot?

Temperatures in the thermosphere can exceed 500°C due to solar radiation absorption by oxygen molecules. However, it doesn’t feel hot because the air is so thin that there are very few molecules to transfer heat to a human body.

What is the exosphere, and what happens there?

The exosphere is the outermost layer of the atmosphere where light, fast-moving molecules can escape Earth’s gravity into space. It marks the transition between the Earth's atmosphere and outer space.

What is the difference between the homosphere and the heterosphere?

The homosphere, extending up to the thermosphere, has a uniform composition of gases. The heterosphere, above the thermosphere, is stratified by molecular weight, with heavier atoms settling lower and lighter ones higher.

What is the ionosphere, and why is it important?

The ionosphere is an electrified region with a high concentration of ions and free electrons. It is crucial for radio communication as it reflects and modifies radio waves, especially AM signals, enabling long-distance communication.

How does the D region of the ionosphere affect AM radio waves?

The D region of the ionosphere absorbs AM radio waves during the day, weakening them. At night, this region disappears, allowing AM waves to penetrate higher layers, like the E and F regions, for better long-distance transmission.

What elements are considered important in climatology?

Key elements of climatology include air temperature, air pressure, humidity, clouds, precipitation, visibility, and wind. These factors influence the Earth’s climate and weather patterns.

What is air pressure and how is it measured?

Air pressure is the force exerted by the weight of air molecules on a surface area. It is commonly measured in units like hPa, mb, or inches of mercury (in Hg).

How does temperature in the troposphere typically change with altitude?

In the troposphere, temperature generally decreases with altitude because the Earth's surface absorbs solar energy, warming the air above it.

What happens to air pressure at 50 km altitude?

At an altitude of 50 km, the air pressure is about 1 mb, indicating that 99.9% of the Earth's air molecules are below this height.

Why is the mesosphere considered the coldest layer of the atmosphere?

The mesosphere is the coldest layer of the atmosphere, with temperatures reaching -90°C at its top, due to the thin air and lack of heat absorption.

What defines the tropopause, and how does its elevation vary?

The tropopause is the boundary between the troposphere and the stratosphere, where temperature stops decreasing with altitude. It is higher over the equator and lower near the poles, and it varies with seasons.

CHAPTER:1 Earth and It’s Atmosphere Part 2

November 8, 2024
7:29 am

Vertical Structure of the Atmosphere

Table of Contents

Air molecules (as well as everything else) are held near Earth by gravity. This strong, invisible force pulling down on the air squeezes (compresses) air molecules closer together, which causes their number in a given volume to increase. The more air above a level, the greater the squeezing effect or compression.

As air density is the number of air molecules in a given volume, therefore air density is greatest at surface and decreases upward. The density of air (or any substance) is determined by the masses of atoms and molecules and the amount of space between them.

As air near the surface is compressed, so air density decreases rapidly at first, then more slowly as we move up from the surface.

The weight of all the air around Earth is a stag gering 5600 trillion tons, or roughly 5.08X10¹⁸ kg. The weight of the air molecules acts as a force upon Earth. The amount of force exerted over an area of surface is called atmospheric pressure or, simply, air pressure. As the number of air molecules decreases with height, so atmospheric pressure decreases with increasing height. Like density, air pressure decreases rapidly at first, more slowly at higher levels.

At sea level, the average or standard value for atmospheric pressure is

1013.25b = 1013.25 hpa = 29.92 in Hg

At an altitude of 50km, the air pressure is 1mb, which means 99.9% of all the air molecules are below 50 km.

Layers of the Atmosphere

We know both air pressure and density decrease with height- rapidly at first, then more slowly. But air temperature has a more complicated vertical structure.

Air temperature normally decreases from Earth’s surface up to an altitude of about 11 km, which is nearly 36,000 ft, or 7 mi. This decrease in air temperature with increasing height is due primarily to the fact that sunlight warms Earth’s surface, and the surface, in turn, warms the air above it.

The rate at which the air temperature decreases with height is called the temperature lapse rate. The average (or standard) lapse rate in this region of the lower atmosphere is about 6.5⁰C for every 1000 m or about 3.6⁰F for every 1000-ft increase in altitude. (Keep in mind that these values are only averages)

If air becomes colder more quickly with height, then the lapse rate steepens and when the air cools slowly with increasing height the lapse rate is less. Occasionally the air temperature increases with height, producing a condition called Temperature inversion. The lapse rate fluctuates, varying from day to day, season to season, and place to place.

When the temperature decreases with height, the lapse rate is positive (in Troposphere and Mesosphere).

When the temperature increases with height, the lapse rate is negative (in Stratosphere and Thermosphere). This region of circulating air extending upward from Earth’s surface to where the air stops becoming colder with height is called the troposphere— from the Greek tropein, meaning “to turn” or “to change.”

Above 11 km the air temperature normally stops decreasing with height. Here, the lapse rate is zero. This region, where, on average, the air temperature remains constant with height, is referred to as an isothermal (equal temperature) zone.

The boundary separating the troposphere from the stratosphere is called the tropopause. The height of the tropopause varies. It is normally found at higher elevations over equatorial regions, and it decreases in elevation as we travel poleward. The tropopause is higher in summer and lower in winter at all latitudes.

In stratosphere the air temperature begins to increase with height, producing a temperature inversion condition. This inversion restricts the air of the troposphere to spread out vertically. the air temperature is increasing with height, the air at an altitude of 30 km (about 100,000 feet or 19 mi) is extremely cold, averaging less than -46⁰C (-51⁰F).

The reason behind this increase in temperature in stratosphere is the absorption of UV rays by ozone. Above the stratosphere is the mesosphere. The air is extremely thin here and atmospheric pressure is quite low. With a temperature of -90°C, top of the mesosphere is coldest place of our atmosphere. The hot layer above mesosphere is the thermosphere. Here the oxygen molecules absorb the energetic solar rays, warming the air. Although the temperature exceeds 500⁰C in thermosphere, but a person shielded from the sun would not necessarily feel hot because there are very few molecules are present in this region to bump against something.

Above thermosphere many of the lighter, faster-moving molecules travelling in the right direction escape the earth’s gravitational pull. The region where atom and molecules shoot off into space is called Exosphere. Below the thermosphere the composition of the atmosphere is uniform (78% nitrogen and 21% oxygen). This lower well mixed region is called homosphere.In thermosphere due to very less number of air molecules stratification of air molecules (heavier atoms settle to the bottom and lighter float at the top). This stratified layer is called heterosphere.

Ionosphere

Ionosphere is an electrified region in the upper atmosphere, where large concentration of ions and free electrons exist. The ionosphere starts about 60 km above the earth’s surface. So major portion of the ionosphere is in the thermosphere. The ionosphere plays an important role in radio communication. The lower D region/layer absorbs the AM radio waves, thereby weakening the same. During night the D regions disappears and AM radio waves are able to penetrate higher into the ionosphere (E & F regions). After reflecting from this AM waves are able to travel for many hundred kilometers at night. Layers of the atmosphere based on temperature, composition and electrical properties are given in

Important elements of climatology

air temperature—the degree of hotness or coldness of the air
air pressure—the force of the air above an area
humidity—a measure of the amount of water vapor in the air
clouds—visible masses of tiny water droplets and/or ice crystals that are above Earth’s surface
precipitation—any form of water, either liquid or solid (rain or snow), that falls from clouds and reaches the ground
visibility—the greatest distance one can see
wind—the horizontal movement of air

CSIR NET Exam: EARTH, ATMOSPHERIC, OCEAN AND PLANETARY SCIENCES

Exam Pattern: EARTH, ATMOSPHERIC, OCEAN AND PLANETARY SCIENCES

	PART A	PART B	PART C	TOTAL
Total questions	20	50	80	150
Max No. of Questions to attempt	15	35	25	75
Marks for each correct answer	2	2	4	200
Marks for each incorrect answer (Negative marking for part A & B is @ 25%, and part C is @ 33%)	0.5	0.5	1.32	–

The candidate is required to answer a maximum of 15, 35, and 25 questions from Part-A, Part-B, and Part-C, respectively. If more than the required number of questions are answered, only the first 15, 35, and 25 questions in Part A, Part B, and Part C, respectively, will be taken up for evaluation.

Below each question in Part A, Part B, and Part C, four alternatives or responses are given. Only one of these alternatives is the “correct” option to the question. The candidate has to find, for each question, the correct or the best answer.

Syllabus

EARTH, ATMOSPHERIC, OCEAN AND PLANETARY SCIENCES

PAPER I (PART B)

The Earth and the Solar System
- Milky Way and the solar system.
- Modern theories on the origin of the Earth and planetary bodies.
- Earth’s orbital parameters, Kepler’s laws of planetary motion.
- Geological Time Scale; space and time scales of processes in the solid Earth, atmosphere, and oceans.
- Radioactive isotopes and their applications.
- Meteorites: chemical composition and primary differentiation of the Earth.
- Basic principles of stratigraphy.
- Theories about the origin of life and fossil records.
- Earth’s gravity, magnetic fields, and thermal structure: Geoid and spheroid concepts; Isostasy.
Earth Materials, Surface Features, and Processes
- Gross composition and physical properties of important minerals and rocks.
- Properties and processes responsible for mineral concentrations.
- Distribution of rocks and minerals in Earth’s units and India.
- Physiography of the Earth; weathering, erosion, and soil formation.
- Energy balance of Earth’s surface processes.
- Physiographic features and river basins in India.
Interior of the Earth, Deformation, and Tectonics
- Basic concepts of seismology and Earth’s internal structure.
- Physico-chemical and seismic properties of Earth’s interior.
- Stress and strain concepts; rock deformation.
- Folds, joints, and faults; causes and measurement of earthquakes.
- Interplate and intraplate seismicity; paleomagnetism.
- Sea-floor spreading and plate tectonics.
Oceans and Atmosphere
- Hypsography of continents and ocean floors: continental shelves, slopes, abyssal plains.
- Physical and chemical properties of seawater; residence times of elements.
- Ocean currents, waves, tides, thermohaline circulation, and conveyor belts.
- Major water masses, biological productivity, and fluid motion.
- Atmospheric structure and heat budget; greenhouse gases and global warming.
- General circulation, monsoon systems, ENSO, cyclones, and local systems in India.
- Marine and atmospheric pollution, ozone depletion.
Environmental Earth Sciences
- Properties of water and the hydrological cycle.
- Energy resources: uses, degradation, alternatives, and management.
- Ecology, biodiversity, and natural resource conservation.
- Natural hazards and remote sensing applications.

PAPER I (PART C)

I. Geology

Mineralogy and Petrology
- Point group, space group, and lattice concepts.
- Crystal field theory, mineralogical spectroscopy, and bonding in mineral structures.
- Genesis, properties, and crystallization of magmas.
- Metamorphic structures, textures, and thermobarometry.
- Petrogenesis of Indian rock suites: Deccan Traps, charnockites, ophiolites, and more.
Structural Geology and Geotectonics
- Stress and strain analysis; Mohr circles.
- Geometry and mechanics of folds, faults, and ductile shear zones.
- Plate boundaries, mantle plumes, and Himalayan orogeny.
Paleontology and Applications
- Life origin theories, evolution models, and mass extinctions.
- Applications of fossils in age determination, paleoecology, and paleogeography.
- Micropaleontology in hydrocarbon exploration.
Sedimentology and Stratigraphy
- Classification of sediments and sedimentary rocks.
- Sedimentary environments and basin evolution.
- Stratigraphic principles, correlation methods, and sequence stratigraphy.
- Phanerozoic stratigraphy of India.
Marine Geology and Paleoceanography
- Ocean floor morphology, ocean circulation, and thermohaline processes.
- Factors influencing oceanic sediments and paleoceanographic reconstruction.
Geochemistry
- Atomic properties, periodic table, thermodynamics of reactions, and isotopes in geochronology.
- Applications of stable isotopes in Earth processes.
Economic Geology
- Ore formation processes, mineral deposit studies, and petroleum geology.
- Coal and unconventional energy resources.
Precambrian Geology and Crustal Evolution
- Evolution of Earth systems and Precambrian characteristics of India.
- Precambrian–Cambrian boundary.
Quaternary Geology
- Quaternary stratigraphy, climate variability, and human evolution.
- Dating methods and tectonic geomorphology.
Applied Geology

Remote sensing and GIS.
Engineering properties of rocks; construction investigations.
Methods of mineral exploration and groundwater studies.

II. Physical Geography

Geomorphology: Landform processes, DEM analysis, extraterrestrial geomorphology.
Climatology: Radiation balance, wind systems, ENSO, and climate classification.
Biogeography: Plant and animal associations, Indian biogeography, and conservation.
Environmental Geography: Man-land relationships, hazards, and ecological balance.
Geography of India: Physical geography, climatology, agriculture, and population characteristics.

III. Geophysics

Signal Processing: Fourier transforms, filters, and signal analysis.
Field Theory: Newtonian potential, Green’s theorem, and seismic wave propagation.
Numerical Analysis and Inversion: Least squares, optimization, and pattern recognition.
Gravity and Magnetic Methods: Data interpretation and anomaly analysis.
Seismic Methods: Ray theory, reflection/refraction techniques, seismic stratigraphy.
Well Logging: Techniques for lithology, porosity, and fluid saturation interpretation.

(IV) METEOROLOGY

1) Climatology

Same as under Geography.

2) Physical Meteorology

Thermal Structure of the Atmosphere and Its Composition.
Radiation:
- Basic laws – Rayleigh and Mie scattering, multiple scattering.
- Radiation from the sun, solar constant, effect of clouds, surface and planetary albedo.
- Emission and absorption of terrestrial radiation, radiation windows, radiative transfer, Greenhouse effect, net radiation budget.
Thermodynamics of Dry and Moist Air:
- Specific gas constant, adiabatic and isentropic processes, entropy and enthalpy.
- Moisture variables, virtual temperature, Clausius–Clapeyron equation.
- Adiabatic processes of moist air, thermodynamic diagrams.
Hydrostatic Equilibrium:
- Hydrostatic equation, variation of pressure with height, geopotential, standard atmosphere, altimetry.
Vertical Stability of the Atmosphere:
- Dry and moist air parcel and slice methods, tropical convection.
Atmospheric Optics:
- Visibility and optical phenomena – rainbows, haloes, corona, mirage, etc.

3) Atmospheric Electricity

Fair weather electric field in the atmosphere and potential gradients.
Ionization in the atmosphere, electrical fields in thunderstorms.
Theories of thunderstorm electrification, structure of lightning flash, mechanisms of earth-atmospheric charge balance, and the role of thunderstorms.

4) Cloud Physics

Cloud classification, condensation nuclei, growth of cloud drops and ice-crystals.
Precipitation mechanisms: Bergeron–Findeisen process, coalescence process.
Precipitation of warm and mixed clouds, artificial precipitation, hail suppression, fog and cloud dissipation.
Radar observation of clouds and precipitation:
- Radar equation, rain drop spectra, radar echoes of hailstorms, tornadoes, hurricanes, and rainfall measurements.

5) Dynamic Meteorology

Basic Equations and Fundamental Forces:
- Pressure, gravity, centripetal and Coriolis forces.
- Continuity and momentum equations (Cartesian and spherical coordinates).
- Scale analysis, inertial flow, geostrophic and gradient winds, thermal wind.
- Divergence and vertical motion, Rossby, Richardson, Reynolds, and Froude numbers.
Atmospheric Turbulence:
- Mixing length theory, planetary boundary layer equations, Ekman layer, eddy transport of heat, moisture, and momentum.
Linear Perturbation Theory:
- Internal and external gravity waves, inertia waves, gravity waves, Rossby waves, wave motion in the tropics, barotropic and baroclinic instabilities.
Atmospheric Energetics:
- Kinetic, potential, and internal energies; conversion into kinetic energy; available potential energy.

6) Numerical Weather Prediction (NWP)

Computational instability, filtering of sound and gravity waves.
Filtered forecast equations, barotropic and baroclinic models.
Objective analysis, data assimilation techniques, satellite applications in NWP.

7) General Circulation and Climate Modelling

Observed zonally symmetric circulations, meridional circulation models.
General circulation modelling principles: grid-point and spectral GCMs.
Climate variability phenomena: ENSO, QBO, MJO, etc.
Ocean-atmosphere coupled models.

8) Synoptic Meteorology

Weather observations and transmission, synoptic charts.
Synoptic weather forecasting, prediction of weather elements, and hazardous weather phenomena.
Tropical Meteorology:
- ITCZ, monsoons, tropical cyclones, jet streams.
Extra-Tropical Features:
- Jet streams, extratropical cyclones, anticyclones.
Air masses and fronts: sources, classification, frontogenesis, and associated weather.

9) Aviation Meteorology

Meteorological role in aviation, weather hazards during takeoff, cruising, and landing.
In-flight hazards: icing, turbulence, visibility issues, gusts, wind shear, thunderstorms.

10) Satellite Meteorology

Polar orbiting and geostationary satellites.
Applications in identifying synoptic systems, cyclones, temperature estimation, rainfall prediction, and temperature/humidity soundings.

(V) OCEAN SCIENCES

1) Physical Oceanography

T-S diagrams, mixing processes, characteristics of water masses.
Wind-generated waves, shallow and deep-water wave dynamics.
Coastal processes: wave reflection, refraction, diffraction, littoral currents, rip currents, tsunami, and more.
Ocean Circulation:
- Global conveyor belt circulation, Ekman’s theory, upwelling processes.

2) Chemical Oceanography

Composition of seawater, chemical exchanges, and classification of elements.
Element chemistry under special conditions (estuaries, vents, etc.).
Carbonate chemistry, biological pumps, and sedimentary deposit factors.

3) Geological Oceanography

Topics as listed under “Marine Geology & Paleoceanography.”

4) Biological Oceanography

Classification of marine environments and organisms.
Primary and secondary production, factors affecting biodiversity.
Human impacts on marine communities and climate change effects.

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