Climatology & Climatic Regions

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### Latitudes and Longitudes - **Definition:** Imaginary lines (coordinate system) used to determine a place's location on Earth. #### Latitude or Parallel - **Definition:** Angular distance of a place N or S of the equator, measured in degrees from Earth's center. - **Linear Distance:** Slightly longer at poles (111.7 km) than at equator (110.57 km) due to Earth's flattening. - **Important Parallels:** - Equator (0°) - Tropic of Cancer (23½° N) - Tropic of Capricorn (23½° S) - Arctic Circle (66½° N) - Antarctic Circle (66½° S) - North Pole (90° N) - South Pole (90° S) #### Earth's Shape and Latitudinal Heat Zones - **Shape:** Geoid (flattened at poles, bulged at equator). - **Gravity:** Greater near poles, less at equator (closer to center, lower centrifugal force). - **Heat Zones:** - **Torrid Zone:** Between Tropics of Cancer & Capricorn; receives maximum heat (sun overhead at least once a year). - **Temperate Zones:** Between Tropics and Circles; moderate temperatures (sun's rays decreasingly slant). - **Frigid Zones:** Beyond Arctic/Antarctic Circles; very cold (sun's rays always slanting, low above horizon). - **Temperature Gradient:** Temperature falls from equator to poles due to Earth's spherical shape and sun angle. #### Longitude or Meridian - **Definition:** Angular distance of a place E or W of the Prime Meridian, measured in degrees from Earth's center. - **Representation:** Semi-circles from pole to pole, passing through the equator. - **Prime Meridian:** 0° longitude, passes through Greenwich, London. - **Length:** All meridians are of equal length. - **Function:** Determines local time relative to Greenwich Mean Time (GMT). #### Longitude and Time - Earth rotates 360° in 24 hours (15°/hour or 1°/4 minutes). - East of Greenwich: gain time (15° E = +1 hour). - West of Greenwich: lose time (15° W = -1 hour). #### Standard Time and Time Zones - **Standard Time:** Time corresponding to a chosen longitude for a country. - **Multiple Time Zones:** Countries with large longitudinal extent (e.g., Russia, USA, Canada) have multiple time zones. #### Indian Standard Time (IST) - **Reference:** 82.5° E longitude (near Prayagraj). - **Relation to GMT:** IST = GMT + 5 hours 30 minutes. - **Chaibagaan Time:** Historical time zone in Assam, 1 hour ahead of IST, for tea planters. #### The International Date Line (IDL) - **Definition:** Imaginary line in the Pacific Ocean where the date changes by one day. - **Crossing:** - East to West: lose a day. - West to East: gain a day. - **Reason for Zigzag:** To avoid dividing countries/island groups into different dates. - **Examples:** Samoa, Christmas Island, Tonga (first to New Year); Baker Island, Howland Island (last to New Year). #### Comparison: Latitude vs. Longitude | Feature | Latitude | Longitude | |-----------------|-----------------------------------------------|---------------------------------------------------| | Distance Type | Angular distance N/S of equator | Angular distance E/W of Prime Meridian | | Naming | N/S of Equator | E/W of Prime Meridian | | Other Name | Parallels | Meridians | | 0° Reference | Equator | Prime Meridian | | Length | Equator has max length, others vary | All are equal in length | | Important Lines | Tropics, Circles, Poles | Prime Meridian, IDL | | Purpose | Both determine location (coordinates) | | ### The Motions of The Earth and Their Effects #### Rotation of Earth - **Definition:** Spinning movement of Earth around its axis (west to east). - **Duration:** Approximately 24 hours (23h 56m 4s). - **Axis:** Imaginary line through North Pole, Earth's center, South Pole. - **Effects:** - Days and nights. - **Circle of Illumination:** Divides day from night. - **Axial Tilt:** Earth's rotational axis is tilted 23.5° to the normal (66.5° to orbital plane). #### Revolution of Earth - **Definition:** Earth's orbit around the Sun. - **Orbital Plane:** Ecliptic. - **Duration:** 365¼ days (1 year). - **Leap Year:** Every 4 years, February has 29 days (366-day year). #### Solstice - **Summer Solstice (Northern Hemisphere - June 21st):** - Northern Hemisphere tilted towards Sun. - Sun's rays fall directly on Tropic of Cancer. - Longest day, shortest night in Northern Hemisphere. - Arctic region has continuous daylight. - Winter in Southern Hemisphere. - **Winter Solstice (Northern Hemisphere - December 22nd):** - Tropic of Capricorn receives direct sun rays. - Longest night, shortest day in Northern Hemisphere. - Summer in Southern Hemisphere. - **Midnight Sun:** Sun visible 24/7 at high latitudes in local summer, increasing towards poles. Polar day at poles (6 months). - **Daylight Saving Time (DST):** Advancing clocks in summer months (temperate regions) to exploit sunlight. #### Equinox - **Dates (March 21st, September 23rd):** - Direct sun rays fall on the equator. - Neither pole tilted towards Sun. - Equal days and equal nights globally. - **Seasons:** - March 21st: Spring in Northern Hemisphere, Autumn in Southern Hemisphere. - September 23rd: Autumn in Northern Hemisphere, Spring in Southern Hemisphere. - **Days Longer than Nights at Equator:** Due to atmospheric refraction, sun's apparent image appears above horizon even when actual sun is below. #### Perihelion and Aphelion - **Earth's Orbit:** Elliptical, Sun at one focus. - **Perihelion (around Jan 3rd):** Earth closest to Sun (~147.3 million km). - **Aphelion (around July 4th):** Earth farthest from Sun (~152.1 million km). - **Effect on Weather:** Minor impact on total energy received. Affects season duration slightly (summer longer in NH due to slower orbital velocity at aphelion). #### Eclipse - **Definition:** When a celestial body blocks the light of another. - **Types:** Solar (new moon), Lunar (full moon). - **Apparent Size:** Sun and Moon appear similar in size from Earth due to distance/diameter ratio. #### Phases of The Moon - **Tidally Locked:** Same side of Moon always faces Earth. - **Phases:** New moon, crescent, first-quarter, gibbous, full moon, then waning phases back to new moon. - **Synodic Month:** ~29.53 days (time for phases to repeat). - **Sidereal Month:** ~27.3 days (orbital period relative to fixed stars). - **Perigee:** Moon closest to Earth. - **Apogee:** Moon farthest from Earth. - **Supermoon:** Full/New Moon near perigee. - **Micromoon:** Full/New Moon near apogee. #### Solar Eclipse - **Definition:** Moon blocks Sun's light, casting shadow on Earth. - **Total Solar Eclipse (Umbra):** Moon completely obscures Sun; Sun's corona visible. Rare for a single location. - **Diamond Ring Effect:** Peripheral sunlight shines through lunar topography at start/end of totality. - **Annular Solar Eclipse:** Moon's apparent size smaller than Sun's (Moon at apogee); Sun appears as bright ring. - **Partial Solar Eclipse (Penumbra):** Moon partially obscures Sun. - **Umbra:** Region of total shadow. - **Penumbra:** Region of partial shadow. #### Lunar Eclipse - **Definition:** Earth blocks Sun's light from reaching Moon. - **Penumbral Eclipse:** Moon passes through Earth's lighter shadow. - **Umbral Eclipse (Total Lunar Eclipse):** Moon passes through Earth's darker shadow. - **Blood Moon:** Moon appears reddish during total lunar eclipse due to Earth's atmosphere refracting red light onto it. #### Why not every month? - Moon's orbit is tilted ~5.1° to Earth's ecliptic plane. Eclipses occur only when planes align (eclipse season, at nodal crossings). ### Earth’s Atmosphere - **Definition:** Deep blanket of gases enveloping Earth, held by gravity. #### Composition of The Earth’s Atmosphere - **Dry Air:** 78.08% Nitrogen, 20.95% Oxygen, 0.93% Argon, traces of other gases (CO2, Methane, Ozone). - **Trace Gases:** Greenhouse gases (CO2, CH4, N2O, O3). - **Pollutants:** Chlorine, fluorine compounds, elemental mercury, sulfur compounds (H2S, SO2). #### Evolution of The Earth’s Atmosphere - **First Atmosphere:** Primarily hydrogen from solar nebula. - **Hadean Eon (4.54-4 bya):** - Volcanic outgassing created primordial atmosphere (heavy CO2, H2, N2, inert gases, water vapor). - Oceans formed, removed most CO2. - Early atmosphere almost no oxygen. Lighter gases like H2, He escaped. - **Archean Eon (4-2.5 bya):** - Atmosphere without oxygen (~10-100 atm pressure). - Nitrogen became major part (from volcanism). - Late Archean: photosynthesizing cyanobacteria began producing O2. - **Proterozoic Eon (2.5 bya - 550 mya):** - Free oxygen developed (~2.4 bya). - O2 reached >15% by end of Proterozoic. - **Phanerozoic Eon (550 mya - present):** - O2 peaked ~30% (280 mya), supporting rapid animal development. - Today: 21% O2. - **Processes Governing O2:** Plants use CO2, release O2. Pyrite breakdown/volcanic eruptions release sulfur (reduces O2), but volcanism also releases CO2 (plants convert to O2). #### Permanent Gases of The Atmosphere - Nitrogen (78.08%), Oxygen (20.95%), Argon (0.93%), Carbon dioxide (0.036%), Neon, Helium, Krypton, Methane, Xenon, Hydrogen. - Heavier gases (N2, O2) concentrated lower; lighter gases (H2, He) escape. - Proportion of O2 negligible above 120 km. CO2 and water vapor up to 90 km. #### Important Constituents of The Atmosphere - **Oxygen:** Essential for life, combustion, forms oxides. - **Nitrogen:** Inert, important for organic compounds, controls combustion (dilutes oxygen). - **Carbon Dioxide:** Efficient heat absorber, crucial for heat budget. Increasing levels contribute to global warming. - **Ozone (O3):** - Type of oxygen molecule (3 atoms). - Forms 0.00005% of atmosphere, unevenly distributed. - **Ozone Layer:** 20-30 km altitude (lower stratosphere), absorbs harmful UV radiation. - **Ozone-Oxygen Cycle:** UV splits O2 to O, O combines with O2 to O3. UV splits O3 to O2 + O. - **Depletion:** Caused by CFCs, bromofluorocarbons, increasing Cl/Br radicals. - **Water Vapour:** - Variable (0.02-4% volume). - 90% moisture below 6 km. - Significant for insulating action (absorbs long-wave terrestrial and some short-wave solar radiation). - Source of precipitation, clouds. Releases latent heat of condensation (drives storms). - **Solid Particles (Aerosols):** - Sand, pollen, soot, ocean salts, meteor fragments. - Absorb, reflect, scatter radiation (responsible for red/orange sunsets/sunrises, length of dawn/twilight, blue sky). - Hygroscopic: act as condensation nuclei for clouds, fog, hail. - **Impact on Monsoons (UPSC Mains 2015):** Humanizing landscapes (urbanization, deforestation) increase GHGs/aerosols, affecting global climate and monsoon patterns (e.g., El Nino, IOD). Local pollution (condensation nuclei) can shift rainfall patterns regionally. #### Structure of The Earth’s Atmosphere - **Layers:** Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere. - **Speed of Sound:** Follows temperature profile in atmosphere (depends on temperature, not pressure/density in ideal gas). ##### Troposphere (0 to 12 km) - **Altitude:** 8 km at poles, 18 km at equator (due to rising heated air). - **Tropopause:** Topmost layer of troposphere, constant temperatures. - **Temperature:** Falls upwards at 6.5 °C/km (positive lapse rate). -45°C at poles, -80°C at equator (due to greater thickness). - **Characteristics:** Turbulence, eddies, all weather phenomena (cyclones, rainfall, fog, hailstorm). Convective region. ##### Stratosphere (12 to 50 km) - **Beyond Tropopause:** Temperature constant initially, then rises (negative lapse rate) to 0°C at 50 km. - **Ozone:** Rise in temperature due to UV absorption by ozone. - **Weather:** Almost free from clouds and weather phenomena (ideal for airplanes). - **Polar Stratospheric Clouds (PSCs):** Form in lower stratosphere (15-25 km) in frigid regions, contain water/nitric acid/sulfuric acid. Enhance ozone depletion. ##### Ozonosphere (20 to 55 km) - **Location:** Spans stratosphere and lower mesosphere. - **Ozone Layer:** Highest concentration 20-30 km. Absorbs/reflects harmful UV. - **Temperature:** Raises temperature (negative lapse rate) at 5°C/km. - **Chemosphere:** High chemical activity (ozone-oxygen cycle). - **Depletion:** Caused by CFCs, bromofluorocarbons. ##### Mesosphere (50 to 80 km) - **Meteors:** Most burn up here due to friction with gases. - **Temperature:** Drops with increasing altitude to -85°C (mesopause - coldest place on Earth). - **Noctilucent Clouds:** Polar-mesospheric, made of scattered ice crystals, visible at sunset/sunrise. ##### Thermosphere (80 to 700 km) - **Temperature:** Rises rapidly (negative lapse rate) due to solar radiation. Air extremely rarefied. - **Orbits:** International Space Station, some satellites orbit here. - **Auroras:** Observed in lower parts. - **Kármán Line (100 km):** Boundary between Earth's atmosphere and outer space. ##### Ionosphere (80 to 400 km) - **Part of Thermosphere:** EUV and x-ray solar radiation ionize atoms/molecules, creating electron layer. - **Radio Communication:** Reflects and modifies radio waves (skywaves). - **Microwaves/Some Radio Waves:** Not reflected (absorbed or too high frequency). ##### Exosphere (700 to 10,000 km) - **Uppermost Layer:** Air extremely rarefied, temperature gradually increases. - **Satellites:** High/mid Earth orbit satellites here (low atmospheric drag). - **Atmospheric Escape:** Light gases (H2, He) lost to space. Earth's magnetic field reduces this. #### Importance of Earth’s Atmosphere - **Life-giving gases:** CO2 for plants, O2 for animals, N2 for nutrients. - **Regulates solar radiation:** Absorbs/filters specific frequencies. - **Temperature balance:** Keeps surface temperature within limits, prevents extreme day/night variations. - **Blocks harmful radiation:** Protects from UV, solar wind, cosmic rays. - **Shields from impacts:** Meteors burn up (mesosphere). - **Weather and climate:** Drives natural processes, influences human settlements. - **Water in liquid state:** Pressure from atmosphere allows liquid water on surface. ### Horizontal Distribution of Temperature #### Ways of Transfer of Heat Energy - **Radiation:** Heat transfer without medium (Sun to Earth). - **Insolation:** Amount of Sun's energy received by Earth (short waves - UV, visible). - Earth radiates heat back as long-wave (infrared). - **Conduction:** Heat transfer by molecular activity in a medium (no movement of medium). - **Convection:** Heat transfer by actual movement of matter/substance (convection cells in atmosphere/oceans). #### Factors Affecting Temperature Distribution - **Angle of Incidence (Inclination) of Sun's Rays:** - Equator: near vertical rays, max heat. - Poles: near horizontal rays, least heat. - **Duration of Sunshine:** Day/night, clear/overcast sky, season. - **Transparency of Atmosphere:** - Affected by aerosols, dust, water vapor, clouds. - **Scattering:** Wavelength > particle radius (most light is scattered). - **Reflection:** Wavelength Oceans. Snow-covered areas highest (70-90%). - Albedo of Mercury (0.14) 27°C. Land >30°C in subtropical Asia. Poleward bend over continents (land overheats), equatorward bend over oceans (oceans cooler). Lowest temps over Greenland. - SH: Regular gradient, slight equatorward bend at continent edges. #### Latitudinal Heat Balance - **Energy Surplus:** Equator to 40° N/S (more heat gained than lost). - **Energy Deficit:** Beyond 40° N/S (more heat lost than gained). - **Balance:** Atmosphere and oceans transfer excess heat from tropics to poles, maintaining overall balance. Most transfer in mid-latitudes (stormy weather, jet streams). #### Heat Budget - Earth receives insolation (short waves), radiates terrestrial radiation (longwave). Maintains constant temperature. ### Vertical Distribution of Temperature #### Latent Heat of Condensation - **Latent Heat:** Energy absorbed/released during phase change without temperature change (e.g., water to steam, ice to water). - **Latent Heat of Vaporization:** Heat absorbed when liquid turns to gas. - **Latent Heat of Fusion:** Heat absorbed when solid turns to liquid. - **Latent Heat of Condensation:** Heat released when gas turns to liquid. - **Significance:** Released when water vapor condenses, supplies energy for thunderstorms, tropical cyclones. #### Lapse Rate - **Definition:** Rate of change of atmospheric temperature with altitude. - **Positive:** Temperature decreases with elevation (troposphere). - **Zero:** Temperature constant (tropopause). - **Negative (Inversion):** Temperature increases with elevation (stratosphere, temperature inversion). - **Cause of Fall with Altitude:** Atmosphere mostly transparent to incoming shortwave, absorbs outgoing longwave. GHG concentration highest at surface, decreases with altitude. - **Environmental Lapse Rate:** Rate of non-rising air, highly variable, ~6-6.5 °C/km in troposphere. #### Adiabatic Lapse Rate (ALR) - **Definition:** Rate of temperature change of a rising/falling air parcel *adiabatically* (no heat exchange with surroundings). - **Rising Air Parcel:** Expands (due to falling ambient pressure), cools (adiabatic), aids condensation. Condensation releases latent heat. - **Falling Air Parcel:** Compressed (due to increasing ambient pressure), warms (adiabatic). E.g., Katabatic wind. - **Dry Adiabatic Lapse Rate (DALR):** For dry/unsaturated air. Higher fall (9.8 °C/km) as less latent heat released. Associated with stable conditions. - **Wet Adiabatic Lapse Rate (WALR):** For saturated air. Slower fall (average 4 °C/km) as latent heat released. Associated with unstable conditions. - **Stability:** - **Absolute Stability:** ALR > DALR (little moisture, no rain). - **Conditional Stability:** WALR ### Pressure Systems and Wind System #### Atmospheric Pressure - **Definition:** Weight of air column per unit area from sea level to top of atmosphere. - **Sea Level:** 1034 gm/cm², ~1013.25 millibars (mb). - **Pressure Cells:** - **Low Pressure:** Air heated, expands, pressure falls. - **High Pressure:** Air cooled, compresses, pressure rises. - Influence of continents/oceans: Continents colder (high pressure) in winter, warmer (low pressure) in summer; inverse for oceans. - **Isobars:** Lines connecting places of equal atmospheric pressure. - **Spacing:** Close = steep/strong pressure gradient; Wide = weak gradient. - **Vertical Variation:** Pressure decreases rapidly with height (~34 mb/300m). Not constant due to temperature/water vapor variability. - **Vertical Pressure Gradient:** Much larger than horizontal, balanced by gravity (no strong upward winds). #### Factors Affecting Wind Movement - **Wind:** Horizontal movement of air. **Currents:** Vertical movement of air. - **Pressure Gradient Force:** Air moves from high to low pressure. Greater difference = greater speed. Perpendicular to isobars. - **Buoyant Force:** - Low-pressure cells: air rises (less dense). Associated with convergence, unstable weather (cyclonic). - High-pressure cells: air sinks (denser). Associated with divergence, stable conditions (anticyclonic). - Rising air also caused by convection, orographic uplift, fronts. - **Frictional Force:** Irregularities of Earth's surface resist wind. Strongest at surface (1-3 km influence), minimal over sea. - **Coriolis Force:** - Deflection of winds due to Earth's rotation. - NH: deflected right (Ferrell's Law). SH: deflected left. - Increases with wind velocity and latitude. Zero at equator, max at poles. - **Myth:** Does not cause water rotation in sinks/toilets. - **Centripetal Acceleration:** - Acts on air flowing around circulation centers. - Creates force inward, producing circular flow (vortex) around high/low pressure. - **Cyclone:** Low pressure, NH: anti-clockwise, SH: clockwise. - **Anticyclone:** High pressure, NH: clockwise, SH: anti-clockwise. - **Tropical Cyclones at Equator:** Not formed due to zero Coriolis effect; low pressure filled by vertical uplift. #### Horizontal Distribution of Pressure - **Pressure Belts:** Seven zones (equatorial low, sub-tropical highs, sub-polar lows, polar highs). - **Pairs:** All except equatorial low form matching pairs. - **Oscillation:** Shift with apparent movement of Sun (south in winter, north in summer in NH). - **Equatorial Low-Pressure Belt ('Doldrums'):** - 10° N to 10° S (varies 5° N/S to 20° N/S). - Zone of convergence of trade winds (ITCZ). - **Formation:** Intense heating, rising air (thermally formed). - **Climate:** Calm air, vertical currents. High humidity, cumulonimbus clouds, convectional rain. No cyclones 5° N/S due to negligible Coriolis. - **Sub-Tropical High-Pressure Belt ('Horse Latitudes'):** - Tropics to 35° N/S. - **Formation:** Subsidence of dry, cold air from equatorial/subpolar regions (dynamically formed). - **Climate:** Calm, feeble winds (anticyclonic). Warm, dry air (deserts along this belt). - **Sub-Polar Low-Pressure Belt:** - 45° N to Arctic Circle, 45° S to Antarctic Circle. - **Formation:** Dynamically produced by Coriolis force and convergence of westerlies/polar easterlies. - **Seasonal:** Broken in winter (high land-sea contrast), more regular in summer. - **Climate:** Area of contrast, produces polar jet streams. - **Polar High-Pressure Belt:** - Around poles, 80-90° N/S. - **Formation:** Cold, dry air subsides (thermally formed). - **Factors Controlling Pressure Systems:** - **Thermal:** Heating (low pressure), cooling (high pressure) - e.g., equatorial low, polar high. - **Dynamic:** Pressure gradient, apparent sun movement, Earth's rotation (Coriolis). - **Seasonal Shifts:** Pressure belts shift north in NH summer, south in NH winter (less shift in SH due to water). #### Pressure systems and General Circulation - **General Circulation:** Pattern of planetary winds. - **Depends On:** Latitudinal heating, pressure belts, sun's apparent path, continents/oceans distribution, Earth's rotation. - **Hadley Cell:** Equatorial low-pressure belt: air rises, moves poleward in upper trop., accumulates at 30° N/S, sinks (subtropical high). Surface winds (easterlies) flow equatorward, converge at ITCZ. - **Ferrel Cell:** Mid-latitudes: sinking cold air from poles, rising warm air from subtropical high. Surface winds: westerlies. - **Polar Cell:** Polar latitudes: cold dense air subsides, blows towards mid-latitudes (polar easterlies). - **Purpose:** Transfers heat from lower to higher latitudes. ### Classification of Winds - **Permanent/Primary/Prevailing/Planetary Winds:** Trade winds, westerlies, polar easterlies. - **Secondary or Periodic Winds:** Seasonal (monsoons), periodic (land/sea breeze, mountain/valley breeze). - **Local Winds:** Specific period/area (Loo, Mistral, Foehn). #### Primary winds or Prevailing Winds or Planetary Winds - Blow almost same direction year-round, larger global areas. - **The Trade Winds:** - Sub-tropical high to equatorial low (30° N/S). - NH: North-eastern trades. SH: South-eastern trades. - Descending/stable at origin, humid/warmer at equator. Converge at ITCZ, rise, cause heavy rainfall. - Eastern parts with cool ocean currents drier, more stable. - **The Westerlies:** - Sub-tropical high to sub-polar low. - NH: Southwest to northeast. SH: Northwest to southeast. - Wet spells, variable weather. - Best developed 40°-65° S (Roaring Forties, Furious Fifties, Shrieking Sixties). Stronger/persistent in SH (vast water). - **The Polar Easterlies:** - Dry, cold. - From high-pressure polar areas to sub-polar lows. - NH: North-east to south-west. SH: South-east to north-west. #### Secondary or Periodic Winds - Change direction with season. - **Monsoons:** - Seasonal reversal of wind direction (large-scale land/sea breezes). - **Summer:** SH trade winds pulled northward (apparent sun movement, intense low pressure over Indian subcontinent). Deflected right by Coriolis, approach Asia as SW monsoons (heavy rainfall). - **Winter:** High pressure over Indian subcontinent. Divergent winds (NE monsoons) travel southward (apparent sun movement). Bring some rain to SE India. - Stronger winter monsoon in China/Japan. - **Land Breeze and Sea Breeze:** - **Day:** Land heats faster, low pressure. Sea cooler, high pressure. Wind from sea to land (sea breeze). - **Night:** Land cools faster, cooler than sea. Wind from land to sea (land breeze). - **Valley Breeze and Mountain Breeze:** - **Day:** Slopes heat, air moves upslope (valley breeze). - **Night:** Slopes cool, dense air descends into valley (mountain breeze). Katabatic wind. #### Tertiary or Local Winds - Local differences in temperature/pressure. - **Cold Winds:** - Pampero (Argentina), Gregale (Malta), Bora (Adriatic Coast), Mistral (Rhone valley, France), Tramontane (Alps to Italy). - **Warm Winds:** - Foehn (Switzerland): Dry hot, leeward side of mountains, melts snow, ripens grapes. - Chinook (Snow Eaters, USA/Canada): Foehn-like, eastern Rockies slopes. - Loo (Indo-Gangetic plains): Hot, dry, May/June. - Sirocco (North Africa/Southern Europe): Hot, dry, dusty from Sahara. Can cause "blood rain" (red dust). - Harmattan (West Africa): Dry, dusty, NE trade wind. - Zonda (Argentina): Foehn-like on Andes. ### Hydrological Cycle (Water Cycle) - Continuous exchange of water between atmosphere, oceans, continents. - Processes: Evaporation, transpiration, condensation, precipitation. - Maintains balance between evapotranspiration and precipitation. #### Humidity - **Definition:** Water vapor in air. - **Measurement:** Hygrometer. - **Significance:** Absorbs radiation (heat budget), latent energy for storms, affects sensible temperature. - **Absolute Humidity:** Actual amount of water vapor (weight per unit volume). Varies by place, greater over oceans. Warm air holds more moisture. - **Relative Humidity:** Percentage of moisture vs. full capacity at given temperature. - RH = (Actual water vapor / Max capacity) * 100. - Changes with adding moisture (evaporation) or changing air temperature (saturation point). - Saturated air = 100% RH. - Greater over oceans, least over continents. - **Dew Point:** Temperature at which saturation (100% RH) occurs. - **Specific Humidity:** Weight of water vapor per unit weight of air. Not affected by pressure/temperature changes. Only by adding/removing moisture. #### Evaporation - Water to gaseous state. Oceans contribute 84%, continents 16%. - **Highest:** Sub-tropics of western North Atlantic/Pacific (Gulf Stream, Kuroshio), trade wind zone of southern oceans. Equatorial regions (high insolation, luxuriant vegetation). - **Factors Affecting Rate:** - Water availability, surface area, temperature. - **Relative Humidity:** Low RH = increased evaporation. - **Wind:** Replaces saturated air with unsaturated, increases evaporation. - **Air Pressure:** Lower pressure = higher evaporation. - **Water Composition:** Inverse to salinity (freshwater evaporates faster). - **Plants:** Evaporate faster than land. #### Condensation - Water vapor to liquid (or solid - desublimation). Caused by heat loss (latent heat). - Occurs when moist air cools to dew point or moisture is added. - **Hygroscopic Condensation Nuclei:** Dust, smoke, pollen, salt particles absorb water, aiding condensation. - **Processes of Cooling:** - **Adiabatic:** Air rises, expands, cools (no heat exchange). Major cause of temp changes in rising air. - **Non-Adiabatic:** Cooling by radiation, conduction, mixing. Produces dew, fog, frost (not substantial precipitation). #### Forms of Condensation - **Based on Dew Point Temperature:** - Below freezing: white frost, snow, hailstones, cirrus clouds. - Above freezing: dew, fog, clouds. - **Based on Location:** - Near surface: dew, white frost, fog, mist. - Free air: clouds. - **Dew:** Water droplets on cool surfaces (dew point > freezing). Clear sky, calm air, high RH, cold long nights. - **White Frost:** Ice crystals on cold surfaces (dew point at/below freezing). Same conditions as dew. - **Fog:** Cloud with base near ground (visibility 1 km but 0.5 mm. - **Virage:** Rain evaporates before reaching Earth. - **Drizzle:** Light rain, drops 200 cm/year):** Equatorial belt, windward slopes of mountains (cool temperate zones), monsoon lands. - **Moderate Rainfall (100-200 cm/year):** Interior continental areas. - **Low Rainfall ( ### Thunderstorm - Severe, short-duration, localized storms with rapid convection. - Involve thunder, lightning, heavy rain/hail. - Occur where high temperature and good humidity. Less frequent over water. #### Formation of A (Thermal) Thunderstorm - **Stage 1: Cumulus Stage:** - Intense ground heating creates low pressure. - Rapid uplift of warm, humid air (convection). - Air rushes in to fill low pressure. Towering cumulonimbus cloud builds. - **Stage 2: Mature Stage:** - Condensation releases latent heat, air warms, uplifts further. - Intense updraft grows cloud. Void filled by fresh moist air. - Cumulonimbus anvil cloud forms (reaches stratospheric stability). - Incoming storm: violent gust of cool wind (downdraft) and rain. - **Speed:** Isolated storms ~20 km/h, supercells 65-80 km/h. - **Downbursts:** Macrobursts (>4 km, 215 km/h), Microbursts ( 1 hr), highly organized, tilted/rotating updraft. Most large/violent tornadoes from supercells. - **Mesocyclone:** Rotating vortex within supercell, can produce tornadoes. #### Tornado - Small-diameter whirling vortex in convective cloud, contact with ground. - Strongest winds on Earth (500 km/h). - Occur mid-latitudes, spring/summer. - **Tornadogenesis:** Warm/humid air meets cold air, creates horizontally spinning winds (vertical wind shear). Updrafts lift spinning air, rotates vertically (mesocyclone). Downdraft pulls mesocyclone to ground, tornado forms. - **Waterspout:** Tornado over water, weaker than land tornadoes. - **Distribution:** All continents except Antarctica. USA most violent, Canada second. Bangladesh most prone in Indian subcontinent. #### Lightning And Thunder - **Process:** Ice crystals move up/down in cumulonimbus cloud, collide, release electrons (ionization). - Top layer positive (cations), middle/bottom negative (anions). - Huge electrical potential difference (10^9-10^10 volts). Current flows (10^5-10^6 amperes). - Heats air column (red glow), expands, produces shock waves (thunder). - **Thunderbolt:** Flash of lightning + crash of thunder. - **Cloud to Earth:** Earth becomes positively charged relative to cloud's middle layer. Current flows (20-15%) to Earth, hits taller objects. - **Most Lightning Activity:** Lake Maracaibo, Venezuela. - **Deaths:** Increasing trend. Ground currents main cause. Avoid trees, metal, water during storms. #### Hailstorm - Solid precipitation: frozen pellets from cumulonimbus. Layered irregular ice lumps. - **Favorable Conditions:** Strong updraft, high liquid water content, great vertical extent of cloud, good portion of cloud below freezing, high surface temperatures. - **Formation:** Water droplets freeze on condensation nuclei, strong updrafts carry them up, acquire layers of supercooled water, grow until too heavy for updraft, fall to ground. - **Size:** Up to 15 cm, >0.5 kg. - **Less Common in Tropics:** Atmosphere warmer at higher altitudes. #### Hazards Posed by Thunderstorms - Flash flooding (cloudburst). - Fires (lightning). - Crop/property damage, livestock deaths (hail). - Trees/power lines down (strong winds). - Aircraft risk (downbursts). - Destruction (tornadoes). - **Pyrocumulonimbus Clouds:** Thunderstorms formed from smoke plume of wildfires. Intense heat causes rapid air rise, drawing in air (strong winds), electrical charge builds (lightning, new fires). Intensifies wildfires. ### Tropical Cyclones - Rapidly rotating violent storms over tropical oceans in late summers. - **Characteristics:** Closed low-pressure center (steep gradient), closed low-level atmospheric circulation (cyclonic), strong winds, spiral thunderstorms (heavy rain). #### Conditions Necessary for The Formation of A Tropical Cyclone - Large sea surface with temperature > 27°C (60-70m deep warm water layer). - Sufficient Coriolis force (not at equator). - Pre-existing weak low-pressure area/cyclonic circulation. - Low wind shear. - Upper-level divergence. - **Why Western Margins of Oceans?** Warm ocean currents pile water here (easterly trade winds). - **Why Rare on Eastern Margins?** Cold currents lower surface temps. - **Why Weaken on Landfall?** Cut off from moisture/latent heat supply. - **Why Late Summers in NH?** Oceans reach max temp in August, ITCZ farthest from equator (increased Coriolis). #### Low-level Disturbances - Low-pressure trough moving east to west (easterly wave disturbances) in ITCZ. - Seedling circulations for tropical cyclones. #### Why Fewer Cyclones in Arabian Sea vs. Bay of Bengal? - **More Low-Level Disturbances in BoB:** In-situ formation + remnants of typhoons from NW Pacific. - **Monsoons:** Drive away moisture from AS, prevent full development. - **Higher SST in BoB:** Generally 1-2°C warmer. - **Salinity:** AS higher salinity (high evaporation, low river runoff) -> less stable stratification. BoB lower salinity (Ganga, Brahmaputra inflow) -> higher stratification, inhibits vertical mixing, favors cyclone development. - **Temperature Contrast:** Convergence of air masses with different temps needed for low-level disturbances to intensify. #### Wind Shear - **Vertical Wind Shear:** Difference in wind speeds at different altitudes. - **Low Wind Shear:** Favors tropical cyclone development (allows towering cumulonimbus). - **High Wind Shear:** Inhibits development. - **Confinement to Tropics:** Weak vertical wind shear in tropics (below subtropical jet stream). Temperate regions have high shear (westerlies). - **Few Cyclones during SW Monsoon:** Strong westerly winds in lower troposphere, strong easterly in upper troposphere -> large vertical wind shear. Also, short oceanic stay in BoB. #### Upper Air Disturbance - **Upper Tropospheric Westerly Trough (UTWT):** Remnants of westerlies moving into tropical latitudes. - **Role:** Can assist cyclogenesis/intensification by providing forced ascent (divergence on eastern side -> rising motion at surface). - **Forecasting:** Fast UTWT inhibits strengthening, slow UTWT drives cyclones eastward/northeastward. #### Convective Cyclogenesis (Development of Tropical Cyclones) - **Cyclogenesis:** Development/strengthening of cyclonic circulation. - **Stage 1: Tropical Disturbance:** Thermal origin over tropical seas (late summers). Multiple thunderstorms merge, create intense low-pressure system. - **Stage 2: Tropical Depression ( 119 kmph):** Intensifies, develops an eye (area of calm, lowest pressure). #### Vertical Structure of A Tropical Cyclone - **Inflow Layer (lowest, up to 3 km):** Drives storm. - **Main Cyclonic Storm (middle, 3-7 km).** - **Outflow Layer (above 7 km):** Anticyclonic movement. #### Mechanism - Thunderstorm: warm air rises, cools, condenses, releases latent heat, further uplift. - Over oceans: intensifies, sucks air faster. Rushing air deflects (Coriolis) -> cyclonic vortex. - **Eye Formation:** Centripetal acceleration forces air in vortex to form calm eye. Tangential force on high-speed wind. - Air in eye sinks, warms (compressional warming). Eye temp 10°C+ warmer aloft, 0-2°C at surface. - **Eyewall:** Surrounds eye, most violent region, deep convection, heaviest rainfall, maximum sustained winds. - **Central Dense Overcast (CDO):** Cirrus cloud shield from eyewall thunderstorms. - **Rain Bands (Spiral Bands):** Convection organized into long, narrow bands, spiral into center. Cumulonimbus clouds (highest rainfall), nimbostratus/cumulus at periphery. #### Breeding Grounds for Tropical Cyclones - Coincide with tropical regions with warm ocean currents. - **Major Areas:** Western Pacific (typhoons), Western Atlantic/Eastern Pacific (hurricanes), Bay of Bengal/Arabian Sea (cyclones), SE African coast/Madagascar, NW Australia (willy-willies). #### Tropical Cyclone Scale - **Saffir-Simpson Hurricane Wind Scale (US):** Categories 1-5 based on wind speed (kmph), damage. - **IMD Scale (India):** Depression, Deep Depression, Cyclonic Storm, Severe Cyclonic Storm, Very Severe Cyclonic Storm, Extremely Severe Cyclonic Storm, Super Cyclonic Storm (based on wind speed in kmph/knots). #### Damage Associated with Tropical Cyclones - **Floods:** Heavy precipitation (50 cm/day common). - **Wind:** Uproots trees, damages structures, disrupts communication. - **Storm Surge:** Abnormal sea level rise at landfall (convergence of winds, high water column). Destructive, inundates coast, increased soil salinity. - **Storm Tide:** Combination of storm surge and astronomical tide. - **Vulnerable States (India):** AP, Odisha (highest risk), TN, WB, Gujarat. #### Positive Effects of Tropical Cyclones - **Heat Balance:** Regulate ocean/planet temperature. - **Rainfall:** Bring rain to rain shadow and parched regions. - **Break Up Red Tide:** Mix/break up algal blooms. - **Replenish Barrier Islands:** Move sand/sediment. - **Speed Dispersal:** Wind blows spores/seeds inland. #### Naming of Tropical Cyclones - WMO divides oceans into basins, assigns naming responsibility to regional bodies. - **Purpose:** Reduce confusion, quick info exchange. - **Northern Indian Ocean:** IMD names cyclones from list contributed by 8 countries (alphabetical, neutral gender). #### Changing Cyclone Patterns in The Indian Ocean - Increased frequency of high-intensity storms, off-season cyclones, unusual paths (e.g., Idai, Titli). - **Global Warming:** Rising ocean temperatures (26.5°C to 30-32°C). - **El Nino Modoki:** Creates conditions conducive for cyclones in Arabian Sea, suppresses in Bay of Bengal. #### Twin Cyclones - Pair of tropical cyclones on each side of equator (e.g., Asani & Karim in 2022). - Fuelled by Madden-Julian Oscillation (MJO) and Rossby waves. - **Fujiwhara Effect:** Two cyclones spinning in same direction revolve around common center. - **Hybrid Cyclones:** Characteristics of both tropical/extratropical. #### Madden-Julian Oscillation (MJO) - Eastward moving trough of thunderstorms in tropics, recurs 30-60 days. - **Phases:** Enhanced rainfall convective phase, suppressed rainfall convective phase. - **Impact:** Modulates monsoons, influences tropical cyclone numbers/strength, can affect jet stream (cold air outbreaks, heat events). - **Twin Cyclones:** MJO provides conducive environment (active thunderstorms, enhanced rotation). Low-level westerly wind bursts along equator (on backside of MJO) combine with easterlies to enhance rotation. #### Warning of Tropical Cyclones - **Parameters:** Fall in pressure, increased wind velocity, storm track. - **Monitoring:** Aircraft (weather radar), satellites (radiometers for cloud image). - **Early Warning:** 48 hours in advance possible. - **IMD 4-Stage System:** Pre-Cyclone Watch (72 hrs, depression formation), Cyclone Alert (48 hrs, yellow code), Cyclone Warning (24 hrs, orange code), Post-landfall outlook (12 hrs, red code). - **'Tsunami Ready' Tag:** UNESCO program for tsunami preparedness (implemented by INCOIS in India). ITEWC (Indian Tsunami Early Warning Centre) is Regional Tsunami Service Provider (RTSP). ### Jet Streams - Circumpolar, narrow, concentrated bands of upper tropospheric, westerly, geostrophic streams, high velocity, meandering. - Formed by pressure differences between air masses and Coriolis Force. #### Explanation of Jet Streams - **Geostrophic Wind:** Winds 2-3 km above surface, free from friction. Balanced by pressure gradient and Coriolis force, blow parallel to isobars. - **Upper Tropospheric Westerlies:** - Produced by winds from tropics to poles in upper troposphere. - Flow west to east in both hemispheres (due to Coriolis deflection to the right in NH, left in SH). - **Polar Jet Stream:** Between polar and temperate air masses. - **Subtropical Jet Stream:** Between temperate and tropical air masses. - **Different Altitudes:** Polar jets (6-9 km), Subtropical jets (10-16 km) - due to troposphere thickness variation. - **High Velocity:** - Temperature contrast determines velocity (greater difference = faster). - Low friction in upper troposphere. - Average 120 kmph in winter, 50 kmph in summer; up to 400 kmph in core. - **Meandering (Rossby Waves):** - Straight path with max temperature contrast. - Meanders (wavy path, poleward/equatorward component) when contrast reduces. - **Rossby Waves:** Meandering jet streams. Polar air moves equatorward, tropical air poleward. Explain low/high pressure cells. #### Permanent Jet Streams - **Subtropical Jet Stream (STJ):** - Nearly continuous in both hemispheres in winter. Year-round in SH. Intermittent in NH summer. - Can merge with polar front jet (related to cloudbursts). - Connected to Indian/African summer monsoons. - **Polar Front Jet (PFJ):** - Strongest jet streams. More variable position than STJ. - Shifts poleward in summer, equatorward in winter. Strong/continuous in winter. - Related to polar front. Influences mid-latitude climates, path/speed/intensity of temperate cyclones. #### Temporary Jet Streams - Appear in specific seasons, lower troposphere (Low-Level Jets - LLJs). - **The Somali Jet:** South-westerly, summer over N Madagascar/Somalia. Most intense June-Aug. Influences Indian Monsoons. - **The Tropical Easterly (TEJ) Jet (African Easterly Jet):** - Dominant in NH summer over S Asia/N Africa (5°-20°N). - Persistent June-Oct. - Induces secondary circulations, enhances convection over S India, upper-level venting for SW monsoon. - Linked to high temps/heights over Tibetan Plateau. Decreasing trend (bad for Indian monsoons). #### Influence of Jet Streams on Weather - **Heat Balance:** Help maintain latitudinal heat balance by air mass exchange. - **Weather Systems:** Push air masses, move weather systems (can cause heat waves, drought, floods). - **Temperate Regions (PFJ):** - Separates cold polar/warm mid-latitude air. - Determines path/intensity of frontal precipitation/cyclones. - **Troughing/Ridging:** Drives weather. Ridges (warm air, high pressure), Troughs (cold air, low pressure). - **Jet Streaks:** Form in response to localized temperature gradients. - **Divergence (exiting trough/jet streak):** Creates void, air rushes up, low pressure, cyclonic rotation. - **Convergence (entering jet streak):** High pressure aloft, divergence at surface, anticyclonic rotation, clear weather. #### Polar Vortex - Cold, upper tropospheric (sometimes lower stratospheric) cyclonic parcel of air. - Up to 2,000 km wide. Sits over polar region in winter, weaker in summer. - Not seasonal, can form quickly, unpredictable movement, lasts days to weeks. Most frequent over N Russia/Siberia. - **Polar Vortex Cold Wave (Sudden Stratospheric Warming):** - Vortex within polar front. Strong vortex/westerlies = stays in place. - Weak vortex = intrudes mid-latitudes (buckling wind pattern), causes cold outbreaks (US/Canada). - **How it slips:** Meandering jet (Rossby waves) creates alternating low/high pressure cells. High pressure pushes/displaces polar cyclone south. #### Heat Domes - Atmospheric high-pressure zone. - Rossby Waves lock dome in place, air sinks, heats by compression. - High temps, violent thunderstorms, wildfires. - **Omega Block:** Blocking pattern in atmosphere, shaped like Greek letter omega. Low pressure on W/E, high pressure (heat dome) in middle. #### Jet Streams And Aviation - Used by aviators for tailwinds, avoided for headwind. - Can cause bumpy flights (unpredictable, sudden movement). - Volcanic ash plumes can be sucked into jet streams. ### Temperate Cyclones - **Definition:** Cyclonic systems in mid/high latitudes (35°-65° N/S). - Also called mid-latitude, extratropical, frontal, or wave cyclones. - **Origin:** Dynamic (frontal cyclogenesis - interaction of air masses, Coriolis force), unlike tropical (thermal origin). - **Prerequisites:** Air masses, fronts. #### Air Masses - Large body of air with little horizontal variation in temp/moisture. - Integral to planetary wind system. Extends surface to lower stratosphere, thousands of km. - **Source Regions:** Homogenous areas where air masses form (vast ocean, plains, plateaus). High-pressure belts (subtropics, poles). - Source region establishes heat/moisture equilibrium. - Air masses stable (stagnant air, no convection). - **Ideal Source Regions:** Extensive, gentle/divergent air circulation (anticyclonic). High pressure, low pressure difference. - **No Major Source Regions in Mid-Latitudes:** Dominated by frontal cyclones. - **Classification (based on moisture/source region):** - **Moisture:** c (continental - dry), m (maritime - moist). - **Source Region:** T (tropical - warm), P (polar - cool), A (arctic/antarctic - cold). - **Cold Air Mass:** Colder than underlying surface. - Cold Source Regions: Arctic Ocean (cold/moist), Siberia (cold/dry), N Canada (cold/dry), S Ocean (cold/moist). - **Warm Air Mass:** Warmer than underlying surface. - Warm Source Regions: Sahara (warm/dry), Tropical Oceans (warm/moist). - **Examples:** - **Continental Polar (cP):** Arctic basin, N America, Eurasia, Antarctica. Dry, cold, stable. Frigid, clear winters. - **Maritime Polar (mP):** Oceans 40°-60° lat. Cool, moist, unstable. High humidity, overcast, fog/precip in winter. - **Continental Tropical (cT):** Sahara, W Asia, Australia deserts. Dry, hot, stable. - **Maritime Tropical (mT):** Tropical/subtropical oceans (Mexican Gulf, Pacific, Atlantic). Warm, humid, unstable. Mild temps, overcast/fog in winter. High temps, humidity, convective rain in summer. - **Influence on World Weather:** - Properties (temp, moisture, stability) influence weather. - Carry moisture (oceans to continents), transport latent heat (heat balance). - Origin of cyclones/storms at contact zone (fronts). #### Fronts - Typical mid-latitude feature (30°-65° N/S). Uncommon in tropics/poles. - **Definition:** 3D boundary zone between two converging air masses of different temp/humidity/density. Don't merge due to circulation, properties. - **Frontogenesis:** Formation of a front (war between air masses). Can lead to temperate cyclones. - **Frontolysis:** Dissipation of a front. - **General Characteristics:** - Temp contrast inversely proportional to frontal zone thickness. - Associated with cloudiness/precipitation (warm air ascent, adiabatic cooling, condensation). - Wind shift (due to pressure gradient and Coriolis). - **Classification:** - **Stationary Front:** Surface position doesn't change (draw between air masses). Wind parallel to front. Can cause heavy rainfall/flooding. - **Cold Front:** Cold air mass replaces warm air mass (cold air wins). Steep slope. - **Weather:** Narrow band of clouds/precip. Severe storms, sharp temp drop. - **Clouds:** Cirrus, altocumulus, altostratus, nimbus, cumulonimbus (heavy showers). - **Warm Front:** Warm air moves over cold air (warm air too weak). Gentle slope. - **Weather:** Gradual temp/wind changes. Moderate to gentle precipitation over large area. - **Clouds:** Cirrus, cirrostratus, altostratus, nimbus. No cumulonimbus. Halo around sun/moon. - **Occluded Front:** Cold front overtakes warm front, warm air forced upwards. Warm sector diminishes. - **Weather:** Complex, mix of cold/warm front types. - **Clouds:** Combination of cold/warm front clouds. - **Temperature Inversion:** Cold, Warm, Occluded fronts are examples of temperature inversion. #### Origin and Development of Temperate Cyclones - **Polar Front Theory:** - Warm-humid air from tropics meets dry-cold air from poles -> polar front (surface of discontinuity). - Pressure drops along front, warm air moves N, cold air S -> anti-clockwise cyclonic circulation (NH, Coriolis). - Warm air glides over cold -> clouds, precipitation. Cold front pushes warm air up -> cumulus clouds. - Warm sector between fronts. Cold front moves faster, overtakes warm front -> occluded front -> cyclone dissipates. - **Seasonal Occurrence:** Mostly winter, late autumn, spring. Shift north in summer. - **Distribution:** USA/Canada (Sierra Nevada, Great Lakes), Iceland to Barents Sea (Russia/Siberia), Baltic Sea, Mediterranean basin (Western Disturbances in India), Antarctic frontal zone. #### Characteristics of Temperate Cyclones - **Size and Shape:** Asymmetrical, inverted 'V'. 500-600 km, up to 2500 km. Height 8-11 km. - **Structure:** NW sector cold, NE sector warm (NH). - **Orientation and Movement:** - 3-10 days lifespan. West-to-east movement (westerlies). - Movement controlled by polar jet stream. - East-west storm front: center moves swiftly east. Northward storm front: center moves north. Southward storm front: center moves south (Mediterranean region). - **Associated Weather:** - Approach: falling temp, wind shifts, halo, cirrus clouds. - Light drizzle, then heavy downpour. - Warm front arrival: temp rises, rain stops, clear weather. - Cold front arrival: temp falls, clouds, rain, thunder. Then clear weather. - More rainfall with slow movement, marked temp/rainfall difference. - Anticyclones often accompany. - **Macro-climatic changes:** Air mass interaction, fronts influence cold/heat waves, climatic shifts. #### Tropical Cyclones vs. Temperate Cyclones | Feature | Tropical Cyclone | Temperate Cyclone | |-----------------------|-------------------------------------------------|-----------------------------------------------------| | Origin | Thermal Origin (latent heat) | Dynamic Origin (Coriolis, air mass movement) | | Latitude | 10-30º N/S | 35-65º N/S | | Frontal System | Absent | Present (frontogenesis, occluded front) | | Formation | Only on seas >26-27°C, dissipate on land | Both land/sea | | Season | Seasonal (late summer) | Irregular, more in winters | | Size | Small area (100-500 km diameter) | Larger area (300-2000 km diameter) | | Shape | Elliptical | Inverted ‘V’ | | Rainfall | Heavy, short duration | Slow, long duration | | Wind Velocity/Destruction | Much greater (100-250 kmph), more destructive | Comparatively low (30-150 kmph), less destructive | | Isobars | Complete circles, steep pressure gradient | 'V' shaped, low pressure gradient | | Lifetime | ### El Nino, La Nina & El Nino Modoki #### Normal Conditions - **Pacific Ocean:** Low-pressure over N Australia/Indonesia, high-pressure over Peru. - **Trade Winds:** Strong east to west flow. - **Warm Water:** Accumulates in Western Pacific (Indonesia/Australia), causes convective storms. - **Cold Water Upwelling:** Along Peru coast (cold, nutrient-rich water replaces warm water pulled west). - **Walker Circulation (Normal Years):** Pressure gradient force (high E Pacific, low Indonesia) drives circulation. Indirectly causes upwelling off Peru/Ecuador. #### El Niño - **Definition:** Occasional development of warm ocean surface waters along Ecuador/Peru coast. - **Conditions:** Air pressure drops over central Pacific/S America. Weak high in W Pacific (Southern Oscillation). - **Trade Winds:** Reduced (weak Walker Cell, sometimes reversed). - **Equatorial Counter Current:** Accumulates warm water along Peru/Ecuador, replacing cool Peruvian current. - **Thermocline:** Drops in E Pacific, cuts off cold water upwelling. - **Climate:** Drought in W Pacific (Indonesia/N Australia), rains on equatorial S America coast, convective storms/hurricanes in central Pacific. - **Timing:** Normally around Christmas, lasts weeks to months. #### El Nino Southern Oscillation (ENSO) - **Link:** El Niño (ocean circulation) linked with Southern Oscillation (atmospheric pressure fluctuation over Indo-Pacific). - **ENSO =** (Warm water in E Pacific + Low pressure in E Pacific) + (Cold water in W Pacific + High pressure in W Pacific). - **Effects:** Large-scale weather variations. - Heavy rainfall in arid S America west coast, California, Ecuador, Gulf of Mexico, China. - Severe droughts in Australia, Indonesia, India, S Africa. - Lower fish catches off Peru/Ecuador. - **Impact on Indian Monsoons:** Inversely related. Strong ENSO years often linked to reduced monsoon rainfall in India. Affects agrarian economy (rice, sugarcane, cotton, oilseeds). #### Southern Oscillation Index (SOI) - **Measurement:** Difference in pressure between Tahiti (Central Pacific) and Port Darwin (Eastern Pacific). - **Positive SOI:** Tahiti pressure > Port Darwin. Drought in E Pacific, good rainfall in W Pacific. **Good for Indian Monsoons.** - **Negative SOI:** Reverse. **Bad for Indian Monsoons.** #### Indian Ocean Dipole (IOD) - **Definition:** Seesaw ocean-atmosphere system in Indian Ocean. Difference in SST between W Indian Ocean (Arabian Sea) and E Indian Ocean (south of Indonesia). - **Equatorial Indian Ocean Oscillation (EQUINOO):** Atmospheric component of IOD. - **Positive IOD:** Winds blow east to west. Arabian Sea warmer, E Indian Ocean colder/dry. **Negates El Nino effect, increases monsoon rains in India.** - **Negative IOD:** Reverse. Indonesia warmer/rainier. - **Impact on Cyclogenesis:** - Positive IOD: More cyclones in Arabian Sea. - Negative IOD: Stronger cyclogenesis in Bay of Bengal, suppressed in Arabian Sea. #### El Nino Modoki - **Conventional El Niño:** Strong anomalous warming in eastern equatorial Pacific. - **El Niño Modoki:** Strong anomalous warming in central tropical Pacific, cooling in eastern/western tropical Pacific. - **Walker Circulation:** Anomalous two-cell Walker Circulation (wet in central Pacific, dry in western/eastern Pacific). - **Effects (similar to normal El Nino):** Suppresses rainfall/hurricane activity in Atlantic. #### La Niña - **Definition:** Strong trade winds, abnormal accumulation of cold water in central/eastern Pacific. - **Effects:** - Strong La Niña (e.g., 1988) linked to drought in central N America. - Active hurricane seasons in Atlantic (e.g., 1998, 1999). - Heavy monsoons in India. - Cool/wet winter in SE Africa, wet in E Australia. - Cold winter in W Canada/NW US. - Winter drought in S US. - **Indian Monsoons:** Enhances monsoon circulation. ### Climatic Regions #### Koppen’s Scheme of Classification of Climate - **Empirical:** Based on observation/experience. - **Premise:** Close relationship between vegetation distribution and climate. - **Criteria:** Temperature and precipitation values. - **Major Climatic Groups (Capital Letters):** - **A:** Tropical Humid Climate (coldest month ≥ 18°C). - **C:** Warm Temperate (Mid-latitude) Climate (coldest month > -3°C but precipitation). - **Subdivisions (Small Letters):** - **Seasonality of Precipitation:** f (no dry season), m (monsoon), w (winter dry), s (summer dry). - **Temperature Severity:** a, b, c, d. - **Tropical/Temperate:** h (tropical), k (temperate). - **Dry Climates (B):** S (steppe/semi-arid), W (desert). #### Major Climatic Types According to Koppen | Climatic Group | Type | Code | Characteristics | |-------------------------|-----------------------------|------|------------------------------------------------------| | **A-Tropical Humid** | Tropical Wet | Af | No dry season | | | Tropical Monsoon | Am | Monsoonal, short dry season | | | Tropical Wet and Dry | Aw | Winter dry season | | **C-Warm Temperate** | Humid Subtropical | Cfa | No dry season, warm summer | | | Mediterranean | Cs | Dry hot summer | | | Marine West Coast | Cfb | No dry season, warm and cool summer | | **D-Cold Snow-Forest** | Humid Continental | Dfb | No dry season, severe winter | | | Subarctic | Dfc | Winter is very severe | | **E-Cold Climates** | Polar Tundra | ET | No true summer | | | Polar Ice Cap (Frost) | EF | Perennial ice | | **H-Highland** | Highland | H | Highland with snow cover | | **B-Dry Climate** | Subtropical Steppe | BSh | Low-latitude semi-arid or dry | | | Subtropical Desert | BWh | Low-latitude arid or dry | | | Mid-Latitude Steppe | BSk | Mid-latitude semi-arid or dry | | | Mid-Latitude Desert | BWk | Mid-latitude arid or dry | #### A – Tropical Humid Climates - Between Tropics of Cancer & Capricorn. Hot, humid, low annual temp range, high annual rainfall. - **Tropical Wet Climate (Af): Equatorial Rainforest Climate** - **Distribution:** 5° N/S of Equator (Amazon, Congo, Malaysia, Indonesia). - **Temperature:** Uniform all year (~27°C), narrow diurnal range. No winter. - **Precipitation:** Heavy (avg > 150 cm), well-distributed, no dry month. Two max rainfall periods (April, Oct - after equinoxes). - **Vegetation:** Luxuriant tropical rainforest (selvas). Dense canopy, undergrowth not dense. Evergreen trees (mahogany, ebony). Mangrove forests in coastal/brackish swamps. - **Commercial Extraction:** Difficult due to species diversity, heavy timber. - **Soil:** Poor, leached, rapid deterioration. - **Life/Economy:** Sparsely populated. Hunter-gatherers, shifting cultivation. Plantation farming (rubber, cocoa, oil palm, coffee, tea, spices). Health issues (heat, humidity, disease). Jungle hinders development. - **Tropical Monsoon Climate (Am):** - **Distribution:** 5°-30° N/S (Indian subcontinent, SE Asia, N Australia). - **Characteristics:** Distinct wet/dry seasons, seasonal wind reversal. Floods/droughts common. - **Temperature:** Monthly mean > 18°C. Summer 30-45°C. Winter 15-30°C. - **Precipitation:** Annual 200-250 cm (some up to 350 cm). Concentrated in rainy season (SW Monsoon). - **Cool, Dry Season (Oct-Feb):** NE Monsoon (dry). Some rain in Punjab (Western Disturbances), SE India (Bay of Bengal moisture). - **Hot Dry Season (Mar-mid-June):** Temp rises sharply, little rain. Hailstorms. - **Rainy Season (mid-June-Sept):** SW Monsoon, torrential downpours. - **Retreating Monsoon:** Rains decrease, moves south. - **Tropical Marine Climate:** Just outside monsoon zone, year-round on-shore trade winds. More even rainfall. Favorable but prone to cyclones. - **Vegetation:** Tropical Monsoon/Dry-deciduous forests. Broad-leaved, shed leaves in dry season. More open than equatorial. Thorns/savanna in drier areas. - **Population/Economy:** High density. Subsistence farming (rice, maize, millet). Plantation crops (cane sugar, jute, cotton). Lumbering (teak). Highland plantations (coffee, tea). Shifting cultivation. - **Savanna or Tropical Wet and Dry Climate (Aw):** - **Characteristics:** Alternate wet/dry seasons. Rains in warm summer. Floods/droughts. - **Distribution:** Confined within tropics, best developed in Sudan (Sudan Climate). Transitional between rainforests/hot deserts. African, South American (Llanos, Campos), Australian, Indian savannas. - **Rainfall:** Less annual rainfall (80-160 cm). Decreases with distance from equator. Rainy season May-Sept (NH), Oct-Mar (SH). - **Temperature:** Mean annual > 18°C. Monthly 20-32°C. Highest temps before rainy season. High diurnal range. - **Winds:** Trade winds. Strongest in summer. Dry by continental interiors/western coasts. - **Vegetation:** Tall, coarse grass (6-12 ft) and short trees. Bush-veld. Grasses green in rainy season, die in dry. Trees deciduous (acacias), shed leaves. Long roots, small/waxy/hairy/needle-shaped leaves. - **Animal Life:** Rich diversity, seasonal migration. 'Big game country'. - **Life/Economy:** Many tribes (Masai pastoralists, Hausa cultivators). Plantation agriculture (cotton, cane sugar, coffee), cattle rearing. Poor lateritic soils, unreliable rainfall. #### B – Dry Climate - **Grasslands:** Subtropical savanna (BSh), temperate steppe (BSk). - **Deserts:** Hot (BWh), temperate (BWk). - **Hot Desert Climate (BWh):** - **Distribution:** Western coasts of continents, 15°-30° N/S. - **Aridity:** Off-shore trade winds, rainshadow, subtropical high-pressure cells. - **Examples:** Sahara, Great Australian, Arabian, Thar, Kalahari, Namib, Mojave, Sonoran, Californian, Mexican, Atacama (driest). - **Rainfall:**