Weather and Climate

The atmosphere is a protective gaseous mantle held bound to the Earth by gravitational attraction, extending several thousand kilometers into space. Pure dry air near sea level possesses a remarkably uniform chemical composition:

Based on temperature lapse rates and molecular dynamics, the atmosphere is structured into five concentric thermal layers:

• 1. Troposphere (ক্ষুব্ধমণ্ডল): Lowest layer directly in contact with Earth's surface. Extends up to $18 ext{ km}$ at the equator (due to intense thermal convection) and $8 ext{ km}$ at the poles (mean height $pprox 12 ext{ km}$). Contains over $75\%$ of atmospheric mass and virtually $99\%$ of atmospheric water vapor. Characterized by the Normal Lapse Rate, where temperature decreases steadily with elevation at a rate of $6.5^\circ ext{C}$ per $1,000 ext{ m}$ ($1^\circ ext{C}$ per $165 ext{ m}$). The boundary capping the troposphere is the Tropopause, where temperature drops to $-80^\circ ext{C}$ at the equator and $-45^\circ ext{C}$ at the poles.
• 2. Stratosphere (শান্তমণ্ডল): Extends from the tropopause up to $50 ext{ km}$. Temperature remains isothermal (constant at $-56^\circ ext{C}$) in its lower part, but progressively increases upward to $0^\circ ext{C}$ at the Stratopause. This temperature inversion is caused by the Ozonosphere ($15–35 ext{ km}$), where ozone ($O_3$) molecules absorb lethal solar ultraviolet radiation (UV-B and UV-C) via the Chapman mechanism ($O_2 + h u o O + O$; $O + O_2 o O_3$). The complete absence of turbulent convection, clouds, and water vapor makes the lower stratosphere ideal for long-range supersonic jet aviation.
• 3. Mesosphere (মেসোস্ফিয়ার): Extends from $50 ext{ km}$ to $80 ext{ km}$. Temperature drops sharply with height, reaching approximately $-100^\circ ext{C}$ at the Mesopause, representing the coldest thermal zone in the planetary envelope. Incoming interplanetary meteoroids burn up here due to frictional ablation. Noctilucent (night-shining) ice-crystal clouds occasionally form in polar summer mesopause regions.
• 4. Thermosphere / Ionosphere (আয়নোস্ফিয়ার): Extends from $80 ext{ km}$ to $400 ext{ km}$. Intensely heated by shortwave solar X-rays and extreme UV, temperature rises rapidly exceeding $1,500^\circ ext{C}$, though the air is so rarefied that sensible heat transfer is negligible. Solar radiation ionizes gas molecules, forming stratified electrical conductibility layers:
  • D-Layer ($80–99 ext{ km}$): Absorbs medium and high-frequency radio waves; disappears at night.
  • E-Layer / Kennelly-Heaviside Layer ($99–150 ext{ km}$): Reflects medium radio waves back to Earth.
  • F-Layer / Appleton Layer ($150–400 ext{ km}$): Reflects short radio waves for global communication.
  • Auroral Phenomena: High-energy solar wind protons and electrons trapped in Earth's geomagnetic field collide with nitrogen and oxygen atoms, generating luminous ethereal displays: Aurora Borealis (Northern Lights) and Aurora Australis (Southern Lights).
• 5. Exosphere & Magnetosphere (চৌম্বকমণ্ডল): Outermost tenuous boundary extending from $400 ext{ km}$ to $10,000 ext{ km}$, gradually merging into the interplanetary solar plasma. Dominated by ultra-light hydrogen and helium atoms. Beyond lies the Magnetosphere with the Van Allen Radiation Belts shielding the biosphere from deadly cosmic ray bombardment.

The radiant solar energy intercepted by the Earth is termed Insolation (Incoming Solar Radiation), transmitted across space in the form of electromagnetic shortwave radiation ($0.2 ext{ to }4.0 ext{ \mu m}$, peaking in the visible spectrum). The flux of solar energy received at the top of the atmosphere on a surface normal to solar rays at Earth's mean orbital distance is the Solar Constant, valued at approximately $1.94 ext{ to }2.0 ext{ calories/cm}^2/ ext{min}$ ($1361 ext{ W/m}^2$).

• 1. Angle of Incidence (আপতন কোণ): Higher angles of sun rays deliver energy over a smaller surface area with greater intensity ($ ext{Energy} \propto \sin heta$), while oblique low-angle rays spread energy over a vast area and must penetrate a thicker atmospheric air-mass ($m = 1/\sin heta$), suffering greater scattering and absorption.
• 2. Duration of Daylight (দিবালোকের স্থায়িত্ব / Photoperiod): Governed by Earth's axial tilt ($23.5^\circ$). Longer summer daylight hours accumulate greater net thermal inputs.
• 3. Sun-Earth Distance (কক্ষপথের দূরত্ব): Perihelion on January 3 ($147 ext{ million km}$, receiving $7\%$ more solar radiation) versus Aphelion on July 4 ($152 ext{ million km}$).
• 4. Atmospheric Transparency: Cloud cover, moisture, and aerosols attenuate direct beam radiation.

Over an annual cycle, the Earth neither heats up nor cools down progressively; it maintains a state of dynamic radiative equilibrium. Assuming incoming solar shortwave insolation equals 100 units:

Atmospheric pressure is the weight of the overlying air column per unit area, measured by mercury barometers in hectopascals ($ ext{hPa}$) or millibars ($ ext{mb}$). Standard sea-level pressure is $1013.25 ext{ mb}$ ($760 ext{ mm}$ of Hg). Wind motion is governed by four vector forces:

• 1. Pressure Gradient Force (PGF): Driven by spatial differences in barometric pressure ($-rac{1}{ ho}rac{\Delta P}{\Delta x}$). Acts perpendicular to isobars from high toward low pressure.
• 2. Coriolis Force: An apparent deflecting force caused by Earth's rotation ($F_c = 2\Omega v \sin\phi$, where $\Omega$ is angular velocity, $v$ is wind velocity, $\phi$ is latitude). As formalized in Ferrel's Law, winds deflect to their Right in the Northern Hemisphere and to their Left in the Southern Hemisphere. Coriolis force is zero at the equator and peaks at the poles.
• 3. Centripetal / Frictional Force: Surface friction retards wind speed within the Planetary Boundary Layer ($0–1 ext{ km}$), reducing Coriolis deflection and causing surface winds to blow obliquely across isobars.
• 4. Geostrophic Wind (জিওস্ট্রফিক বায়ু): In the upper friction-free troposphere, when the Pressure Gradient Force exactly balances the Coriolis Force, air parcels blow parallel to straight isobars as a geostrophic wind.

The global distribution of atmospheric pressure features seven alternating latitudinal belts originating from thermal and dynamic mechanisms:

• Trade Winds (আয়ন বায়ু): Blow persistently from the Sub-Tropical Highs toward the Equatorial Low. Deflected into North-East Trades in the Northern Hemisphere and South-East Trades in the Southern Hemisphere. Exceptionally steady (derived from old Saxon 'trade' meaning track or path).
• Westerlies (পশ্চিমা বায়ু): Blow from the Sub-Tropical Highs toward the Sub-Polar Lows (South-Westerly in the Northern Hemisphere, North-Westerly in the Southern Hemisphere). In the vast unobstructed Southern Oceans ($40^\circ ext{ to }60^\circ ext{ S}$), they rage with immense fury, historically designated by mariners as the Roaring Forties ($40^\circ ext{S}$), Furious Fifties ($50^\circ ext{S}$), and Screaming Sixties ($60^\circ ext{S}$).
• Polar Easterlies (মেরু বায়ু): Cold, dry, gale-force winds blowing outward from the Polar Highs toward the Sub-Polar Lows.
• Jet Streams (জেট প্রবাহ): Discovered during World War II by B-29 bomber pilots, Jet Streams are narrow, meandering, ribbon-like cores of high-velocity geostrophic westerly winds located in the upper troposphere ($9–14 ext{ km}$, speeds exceeding $150–400 ext{ km/h}$). Major types include the Sub-Tropical Westerly Jet (STWJ) and the Polar Front Jet (PFJ). Their seasonal north-south migration controls regional monsoon regimes: the northward shift of the STWJ over the Tibetan Plateau in June triggers the explosive 'burst' of the Indian Summer Monsoon.

Water vapor is the driving fuel for all atmospheric thermodynamics through its release and uptake of latent heat ($540 ext{ cal/g}$ for condensation/evaporation). Humidity is quantified in three principal formats:

• 1. Absolute Humidity: Weight of water vapor per unit volume of air, expressed as $ ext{grams/m}^3$. Varies with air temperature and volumetric expansion.
• 2. Specific Humidity: Mass of water vapor per unit mass of total air (including vapor), expressed in $ ext{grams/kilogram}$. Remains constant as an air parcel expands or contracts adiabatically without mass exchange.
• 3. Relative Humidity (আপেক্ষিক আর্দ্রতা / RH): The ratio of the actual amount of water vapor present in the air to the maximum amount of water vapor the air could hold at that specific temperature to achieve saturation, expressed as a percentage: $$RH = rac{ ext{Actual Vapor Pressure } (e)}{ ext{Saturation Vapor Pressure } (e_s)} imes 100\%$$ When $RH = 100\%$, the air is saturated and reaches its Dew Point (শিশিরাঙ্ক).

When an air parcel rises, it encounters lower ambient barometric pressure, expands, does work on surrounding air, and cools mechanically without exchanging heat with its external environment (Adiabatic Process):

• Dry Adiabatic Lapse Rate (DALR): The rate of cooling of unsaturated rising air ($RH < 100\%$), constant at $9.8^\circ ext{C}$ per $1,000 ext{ m}$ ($pprox 10^\circ ext{C}/km$).
• Saturated / Wet Adiabatic Lapse Rate (SALR): Once the rising air cools to its dew point, condensation commences, releasing latent heat of condensation into the parcel. This internal heat release counteracts expansion cooling, reducing the effective lapse rate to $5^\circ ext{C} ext{ to } 6^\circ ext{C}$ per $1,000 ext{ m}$.
• Atmospheric Stability Criteria:
  • Absolute Instability: Environmental Lapse Rate (ELR) $>$ DALR ($10^\circ ext{C}/km$). Rising air parcel is always warmer and less dense than surrounding air; vigorous convective updrafts produce towering cumulonimbus clouds and thunderstorms.
  • Absolute Stability: ELR $<$ SALR ($5^\circ ext{C}/km$). Rising air parcel is always colder and denser than surrounding air; air resists upward motion, trapping pollutants and creating foggy, layered stratus skies.
  • Conditional Instability: SALR $<$ ELR $<$ DALR. Air is stable when dry, but becomes unstable once lifted to saturation level (LCL).

• Dew & Frost: Formed on cold subaerial surfaces when ground temperature falls below dew point. If dew point is above $0^\circ ext{C}$, liquid droplets (Dew / শিশির) condense; if below $0^\circ ext{C}$, water vapor deposits directly into delicate ice crystals (Frost / তুহিন).
• Fog & Mist: Suspension of microscopic water droplets reducing visibility below $1 ext{ km}$ (Fog / কুয়াশা) or between $1 ext{ and }2 ext{ km}$ (Mist / কুয়াশিকা). Radiation Fog occurs on clear winter nights over land; Advection Fog occurs when warm moist air blows over a cold ocean current (e.g., Grand Banks of Newfoundland).
• International Cloud Classification (Luke Howard, 1803):
  • High Clouds ($>6,000 ext{ m}$): Cirrus (feathery ice filaments), Cirrostratus (produces halos around sun/moon), Cirrocumulus (mackerel sky).
  • Middle Clouds ($2,000–6,000 ext{ m}$): Altocumulus (flattened globular sheets), Altostratus (translucent watery sun).
  • Low Clouds ($0–2,000 ext{ m}$): Stratocumulus, Stratus (low gray uniform blanket), Nimbostratus (dark rain cloud producing continuous drizzle).
  • Clouds of Vertical Development: Cumulus (cauliflower flat-based fair weather cloud) and Cumulonimbus (mountainous thunderhead with flattened cirrus anvil crest, producing ferocious cloudbursts, lightning, squalls, and hailstorms).

• 1. Convectional Rainfall (পরিচলন বৃষ্টিপাত): Intense solar heating of moist ground creates powerful thermal updrafts. Warm air ascends, expands adiabatically, condenses into cumulonimbus clouds, and yields torrential showers with thunder and lightning in the late afternoon (the classic '4 o'clock rain' of the Equatorial Congo and Amazon basins).
• 2. Orographic / Relief Rainfall (শৈলোৎক্ষেপ বৃষ্টিপাত): Moist onshore winds encounter a transverse mountain barrier and are forced to ascend. As air rises on the Windward Slope (প্রতিবাত ঢাল), it cools adiabatically, condenses, and produces heavy precipitation (e.g., Mahabaleshwar on the Western Ghats receiving over $6,000 ext{ mm}$, Cherrapunji-Mawsynram on Khasi Hills). As dry air descends the Leeward Slope (অনুবাত ঢাল), it compresses adiabatically and warms up, causing relative humidity to plunge and creating a dry Rain-Shadow Area (বৃষ্টিচ্ছায় অঞ্চল) (e.g., Pune on the Deccan Plateau, Shillong receiving only $1,400 ext{ mm}$).
• 3. Cyclonic / Frontal Rainfall (ঘূর্ণবাত বৃষ্টিপাত): Warm, light air masses are forced to ascend over cold, dense air masses along sloping frontal surfaces or converging low-pressure troughs, yielding widespread prolonged cyclonic rains.

An Air Mass is an extensive body of air covering thousands of square kilometers that possesses essentially uniform physical properties of temperature and moisture in any horizontal direction. Air masses acquire their characteristics from expansive, topographically uniform Source Regions. Classified into: Continental Polar ($cP$ - cold, dry), Maritime Polar ($mP$ - cold, moist), Continental Tropical ($cT$ - hot, dry), and Maritime Tropical ($mT$ - hot, humid).

When two air masses with contrasting physical properties collide, they do not mix freely; instead, a sloping transition boundary zone termed a Front (সীমান্ত) develops. Fronts are classified into:

• Cold Front (শীতল সীমান্ত): Dense cold air advances aggressively, wedging underneath warm air and forcing it upward steeply, triggering squall lines and towering cumulonimbus clouds with violent rain.
• Warm Front (উষ্ণ সীমান্ত): Warm air glides gently upward over a retreating wedge of cold air along a low slope ($1:100 ext{ to }1:200$), producing extensive cirrostratus, altostratus, and nimbostratus clouds with prolonged steady drizzle.
• Occluded Front (অন্তর্ধৃত সীমান্ত): Rapid cold front overtakes the warm front, lifting the warm sector completely off the ground surface.
• Stationary Front: Surface position of the front shows no significant movement.

Cyclones are low-pressure systems surrounded by closed isobars where winds circulate inward in a counter-clockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere:

High-pressure systems where atmospheric pressure is highest at the center and decreases outward. Winds blow divergent and clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. Characterized by slowly subsiding dry air, light winds, zero cloud development, and stable, sunny, pleasant weather.

The most widely utilized quantitative empirical climate classification was devised by the Russian-German climatologist Wladimir Köppen. Köppen based his system on annual and monthly averages of temperature and precipitation, correlating these meteorological parameters directly with world vegetation zones established by French botanist Alphonse de Candolle (1874).

Köppen established five major planetary thermal groups designated by capital letters (A, C, D, E based on temperature, and B based on aridity):

Anthropogenic combustion of fossil fuels, industrial emissions, and rapid deforestation have elevated atmospheric $CO_2$ concentrations from pre-industrial levels of $280 ext{ ppm}$ to over $420 ext{ ppm}$, alongside surges in methane ($CH_4$) and nitrous oxide ($N_2O$). This amplified Greenhouse Effect traps excess outgoing terrestrial infrared radiation, resulting in a global mean temperature rise of $pprox 1.1^\circ ext{C}$ since 1880.

Specific Climate Vulnerabilities in West Bengal:

• 1. Sea-Level Rise in the Sundarbans Delta: As the global ocean rises ($3.7 ext{ mm/year}$), the active estuarine islands of the Sundarbans (e.g., Ghoramara, Mousuni, Sagar) suffer accelerated coastal erosion, seawater intrusion contaminating agricultural paddies, and loss of mangrove habitats.
• 2. Increased Frequency and Intensity of Bay of Bengal Supercyclones: Elevated sea surface temperatures ($>29–30^\circ ext{C}$) provide boundless latent heat fueling rapid intensification of catastrophic tropical storms (e.g., Cyclone Aila 2009, Amphan 2020, Yaas 2021), inflicting widespread destruction on coastal embankments.
• 3. Himalayan Glacier Retreat & Flash Floods: Accelerated melting of North Bengal and Sikkim Himalayan glaciers (e.g., Zemu glacier feeding the Teesta River) elevates the risk of Glacial Lake Outburst Floods (GLOFs) and extreme erratic monsoon cloudbursts in Darjeeling and Jalpaiguri districts.