Composition & Structure of the Earth’s Atmosphere | UPSC Notes

• 99 % of total Atmospheric mass = 32 km 
• % by Volume : N2 – 78% ; O2 – 21% ; Ar – 0.93% ; CO2 – 0.036% ; H2 – (5 * 10^-5 %)
• CO₂ is meteorologically imp gas : transparent to Incoming Short Solar Radiation ; opaque to outgoing Long Terrestrial Radiation 
  • Helped in increasing the global temperature during initial days of earth formation | during ice age
• Ozone : filter for sun’s UV ; 10-50 km
• Water Vapour : 4%v/v in Tropics & <1% over deserts & poles

—> Water-Vap also decreases from Equator towards poles

—> Absorbes parts of insolation from Sun & preserves Earth’s radiated heat : thus acting like a blanket  —> neither too hot nor too cold

• Dust : 
  • Sources : sea-salts, fine soil, smoke-soot, ash, pollen , dust, meteors
  • Higher conc of dust particles is found in subtropical & temp regions due to dry winds in comparison to equatorial & polar regions

Structure of Atmosphere : 

1. Troposphere : 
   • Avg ht = 13 km (18 km near equator , 8 km near poles)
   • Ht greatest at Equator : Heat (hence water vapour) transported to great heights by strong convectional currents 
   • Most imp layer for all biological activity. All changes in climate & weather take place in this layer
   • Temp <| with ht @ 1’C for every 165 m of ht
   • Tropopause : Temp nearly constant
2. Stratosphere : 
   • Extends upto 50 km, has ozone layer 
   • Temp |> with ht (inversion due to Ozone)
3. Mesosphere : 
   • Extends upto 80 km
   • Temp <| with ht
   • Hits Mesopause
4. Ionosphere/Themosphere : 
   • Extends upto 400 km 
   • Contains electrically charged ions & transmit Radio waves back to Earth
   • Temp |> with ht (inversion due to outer radiations)
5. Exosphere

Reason for temp decrease as we move upward ?

ans ) Cos we are moving farther from hot earth’s surface i.e. we are farther from earth’s terrestrial radiation viz caught by GHG which is lesser as we move up cos Atmosphere (GHG) gets thinner

 —> Statement 2 is wrong

Aurora Borealis :

~ Sun ki Solar Winds interacting with our Earth’s Magnetic Lines (Magnetosphere) —> Hence Aurora seems concentratated at poles cos our Magnetic Lines are conc at poles

(Northern Light and Southern Lights)

Solar Radiation, Heat Balance & Temperature

• Insolation : Incoming Solar Radiation —> Energy received by earth 
• During Revolution :
  • Farthest from Sun : Aphelion | 152 mil km | 4th July
  • Nearest to Sun : Perihelion | 147 mil km | 3rd Jan
• Factors causing variation in insolation : 
  • Rotation of Earth : Earth’s axis makes an angle of 66 1/2 with plane of its orbit round the sun —> Effect on insolation at diff lats
  • Angle of inclination of Sun’s rays : Depends on Lat ; higher the Lat, less is the angle with surface of Earth —> slant sun rays

—> Also slant rays are required to pass through greater depth of Atmosphere —> more absorption, scattering & diffusion

• Length of day 
• Transparency of Atmosphere

—> Max insolation : sub-tropical deserts where cloudiness is the least

—> At same Lat, insolation — continent > water

• Heating the Atmosphere : 
  • Earth after being heated up by insolation, transmits heat to Atmosphere by Long wave form 
  • Conduction : process for heating the lower layers of Atmosphere
  • Convection : Vertical heating of Atmosphere
  • Advection : Transfer of heat through Horizontal movt of air

—> More imp than Convection 

—> In middle Lats, most of diurnal variation in daily weather —> caused by advection alone

—> Local winds in summer season ‘Loo’

• Terrestrial Radiation :
  • Earth after being heated by Short wave itself becomes Radiating body —> emits Long wave to heat Atmosphere from below
  • Long Wave absorbed by Greenhouse Gases —> Hence Atmosphere is indirectly heated by Earth’s Radiation
  • The Atmosphereosphere in turn radiates & transmits heat to space.
• Heat Budget of Earth : 
  • Amt of  Heat received through insolation = Amt of heat lost through TR hence Temp of earth remains constant
  • Albedo of Earth : All Reflection from Lith + Hydro + Atmosphereosphere —>  35 units are reflected back to space (27: clouds & 2 : snow ; 6 : scattered to space)
  • Remaining 65 units —> 14 absorbed w/i Atmosphere + 51 by earth’s surface
  • Now Earth returns 51 units by TR —> 34 abs by Atmosphere + 17 radiated directly to space 
  • Hence TR from earth & Atmosphere is —> 34 + 14 +17
  • Hence Earth never warms up or cools down despite huge Energy transfer

• Variation in Net Heat Budget
  1. Between 40’N & S, Net Radiation balance : surplus ; Near Poles : Deficit
  2. Surplus Heat Energy is redistri from Equator —> Tropics
  3. Ensuring Tropics don’t get too heated up or poles too frozen
• Factors controlling Temp Distribution :
  1. Latitude
  2. Altitude : The Atmosphere is indirectly heated by TR from below —> Hence places near sea-level experience more heat than at higher elevations. Normal Lapse rate : 6.5’C / km
  3. Distance from Sea : Land heats up & cools down quicker —> hence variation of Temp over Land is more
  4. Air mass & Ocean currents 
• Deviation from Temp Isotherms is more in Jan in N hemisphere
  1. Due to larger ( land surface area > sea ) in N Hemisphere —> effects of Landmass & Ocean currents are well pronounced 
  2. In Jan, isotherms deviate N over ocean ( warm Ocean currents in Winter ) & S over Continents ( Land cools much quickly ) 
  3. Effect of Oceans well pronounced in S —> hence isotherms // to Lats

Inversion of Temp —> Inversion of normal Lapse Rate     

1. Long winter nights with clear skies & still air is ideal situation for inversion e.g. desert night , Polar
   1. Heat of day is radiated during night, by morning earth is cooler than Atmosphere
   2. Surface Inversion promotes stability in lower layers of Atmosphere —> Smoke & dust particles collected beneath inversion layer —> Dense Fogs
2. Valley Inversion ( Chilled air descends down the Valley )
3. Inversion b/w Tropopause & Stratosphere

• Air Drainage: 
  • Cold air at hills & mtns produced during night flows under influence of Gravity
  • Being heavy & dense, cold air acts like water —> moves down slope to pile up in pockets & valleys with warm air above
  • It protects plants from frost damages

Atmosphere Circulation & Weather Systems

• Decrease in Pressure : 1 mB for every 10 m
• Why do we not experience strong upward winds ? —> Vertical Pressure Gradient is balanced by opposing Gravitational Force
• Distribution of Sea-Level pressure : Equatorial Lows ; Subtropical Highs (30′) ; Sub Polar Lows (60’) ; Polar Highs

—> These Pressure belts are not permanent ; they oscillate with the apparent movt of Sun

• Factors affecting Velocity & Direction of wind :
  • Pressure-Gradient Force 
  • Frictional Force
  • Coriolis Force :
    • Force prop to Lat ==> Max at poles & 0 at Equator 
    • Deflection is more when wind velocity is high
    • Coriolis Force acts _|_ to pressure Gradient Force

• Why are Tropical Cyclones not formed near Equator ?
  • At Equator —> No Coriolis Force hence Low pressure gets filled instead of getting intensified 
  • PGF _|_  isobar & CF _|_ Wind direction ( ~ Press Grad )
  • As result, both forces are _|_ to each other —> in LowP areas, wind blows around it, not into it
• Geo-strophic Wind : Wind direction 90º to Pressure Gradient
  • When Coriolis Force makes Upper Tropospheric winds (starting from Equator , aiming for Poles) deflect by 90 degrees rather than ususal 45
  • These are Upper-Tropospheric winds which blow W —> E at v.high speeds ( hence called Upper Troposheric Westerlies )
  • UPPER-TROPO WESTERLIES Winds always from West to East at 13 km due to HP at Equator & LP at Pole 
  • When isobars are straight —> Pressure Grad is balanced by Coriolis Force & resultant wind blows // to isobar

• Jet Stream : Very fast winds flowing inside Geo-stropic winds
  • 4 perma Jet streams on Earth ( above STH & SPL , West to East )
  • Also called Travelling Depression (due to Wavy nature : High & Low Pressure created alternatively )
  • https://www.youtube.com/watch?v=Lg91eowtfbw See this!
• Over LowP : Cyclonic Circulation         —> Air will Converge & Rise

Over HighP : Anti-Cyclonic Circulation —> Air will subside from above & diverge

• General Circulation of Atmosphere depends upon :
  • Lat variation of Atmosphere heating 
  • Emergence of Pressure Belts 
  • Migration of Belts following apparent path of Sun
  • Distribution of continents & Ocean
  • Rotation of Earth
• General circulation of Atmosphere also sets in motion the ocean water circulation which influences the Earth’s Climate
• GENERAL CIRCULATION OF ATMOSPHERE 
  • Air at ITCZ rises by Convection due to high Insolation —> LowP created
  • Winds from Tropic converge —> now winds Rise along Convective Cell —> reaches top Troposphere & moves towards Poles
  • Near land surface, Easterlies converge to Equator in ITCZ

• Red arrows are Surface Winds (NE & SE Trade Winds , Westerlies)
• Black arrows are winds in Upper Troposphere : Permanent Jetstream : 2 upar 2 neeche : ST Jetstream & Polar Jetstream
•  

—> Warm T/ST oceans, Hot ST deserts, Cold high Lat Oceans, Cold high Lat Conti , Perma ice covered Conti

• Fronts :
  • Boundary Zone when two diff air masses meet ; Process —> Frontogenesis
  • Cold Front : Cold air strongly towards warm air mass —> causing warm airmass to rise drastically —> hence cumulo nimbus clouds
  • Warm Front : Warm air moves towards cold air mass, but rise of warm airmass is gradual —> hence light drizzle
  • Occluded Front : If warm air mass is fully lifted above land surface & both 2 cold airmass meet || when a cold front overtakes a warm front. When this occurs, the warm air is separated from the cyclone center at the Earth’s surface

• Temperate / Extra Tropical Cyclone / Depression :
  • Dynamically Induced NOT Thermally : Develop in mid & high Lats, beyond the tropics
  • Along Polar Front, Press drops —> Warm air from N , Cold air from S moves —> Anti-Cl cyclonic circulation with warm & cold front
  • Pockets of warm air wedged betw forward & rear Cold air
  • Warm air glides over Cold air —> clouds appear ahead of Warm Front & cause precipitation
  • Cold Front approaches Warm air from behind —> pushes Warm air up —> cumulus clouds develop along Cold Front 
  • Cold Front ultimately overtakes Warm Front —> Warm air is completely lifted up  —> Front becomes occluded —> Now no more  upliftment of any air —> Cyclone dissipates

Pehle Low Pressure banta hai in SPLP : Phir Air Masses aata hai till Low Pressure Point —> Stationary Front banta hai —> Coriolis force —> anti-clock movt —> Cold Fornt & Warm Front —> Warm Air starts lifting up () giving rise to Low Pressure below  —> Till all Warm air lifted —> Occluded Front

Temperate Cyclone vs Tropical Cyclone

TEMPERATE CYCLONE	TROPICAL CYCLONE
Temperate Regions : 30-40 º : henc also from W—> E (Westerlies) Originate over both Land & Sea Frontal System Dynamically Induced —> lesser wind velocity	Tropical Region 8-20º : Hence also form E—> W (Easterlies) Originate only over Sea Due to High Sea Surface Temp SST Thermally induced —> Stronger wind velocity
Can originate over Land & Sea ; Develop in Winter Weaker : cos Dynamically Induced	Originate only over Sea ; dissipate on reaching land ; LATE Summer ??? : since this is when sea-surface get heated up —> lot of evaporation —> low pressure formed  Stronger : cos Thermally Induced
Covers a much larger area
Wind Velocity lower	Wind Velocity is much higher ; More destructive
Always Move from West to East  : cos of Westerlies in Temperate Region in mid-latitude region where they originate	Move from East to West

TROPICAL CYCLONES  : 

• Indian Ocean : Cyclones ||  Atlantic : Hurricanes ||  W Pacific & South China Sea : Typhoons || Western Australia : Willy-willies
• Originate & intensify over warm Tropical Oceans 
• Conditions favourable for formation & intensification of Tropical Storms :
  • Large sea surface with Temp > 27º C
  • Presence of Coriolis Force
  • Pre-existing weak LowP area
  • Small variations in Vertical Wind Speed
  • Upper divergence above sea-level system
• On reaching Land, moisture supply cut off —> storm dissipates
• Landfall of Cyclone : Place where Tropical cyclone crosses the coast 
• Cyclones which cross 20º N latitude recurve —> more destructive 
• Energy that intensifies the storm :  ( Condensation process in towering CuNb clouds surr centre + Moisture from Sea )

• Eye : Region of calm with Subsiding Warm Air
• Eye Wall :
  • Surrounds the eye
  • Strong spiralling ascent of air to greater heights reaching Tropopause  
  • Wind reaches Max velocity in this region : 250 km/hr
  • Torrential rain occurs here
• Rain Bands : Radiate from Eye wall & trains of Cu & CuNb clouds may drift into outer region
• Thunderstorms : 
  • Well grown CuNb cloud producing thunder & lightening 
  • Caused by intense convection on moist hot days 
  • Char by intense updraft of rising warm air —> causes clouds to grow bigger & rise to greater height —> causing precipitation
  • If insufficient moisture, Thunderstorms can create dust storms .
  • If clouds reach ht of sub-zero temps —> hail formed —> hailstorm
• Tornados : 
  • From severe Thunderstorms, spiralling wind descends with great force & LowP at centre —> causing massive destruction
  • Water Spouts : Tornados originating over Sea
• Violent Storms are manifestation of Atmosphere’s adjustment to varying energy distri