Short notes 03 : Temperature

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Radiation Laws (Wien & Stefan–Boltzmann)

• Wien’s Law:
  • Wavelength of maximum radiation (λmax) is inversely proportional to absolute temperature.
  • Hotter body → shorter wavelength.
  • Sun (very hot) → short-wave radiation.
  • Earth (cooler) → long-wave radiation.
  • λmax ∝ 1 / T.
  • Sun’s peak emission ≈ 0.5 μm (visible light).
• Stefan–Boltzmann Law:
  • Total radiation emitted ∝ T⁴.
  • Hot bodies emit much greater intensity of radiation.
  • All bodies warmer thank Zero Kelvin ( absolute zero) radiate energy.

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Solar vs Terrestrial Radiation

• Solar radiation:
  • Short-wave radiation.
  • Mostly < 2 μm.
  • Components:
    • Infrared: ~46–49%
    • Visible: ~44–45%
    • Ultraviolet: ~7–9%
• Terrestrial radiation:
  • Long-wave (infrared).
  • Wavelength range ≈ 4–80 μm.
  • Emitted continuously by the Earth.

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Heating of the Earth & Atmosphere

• Earth’s surface is heated by insolation (incoming short-wave solar radiation).
• Atmosphere is heated indirectly by earth surface from below.
• Main heating processes:
  • Conduction: Heat transfer from warm ground to adjacent air.
  • Convection: Vertical movement of warm air.
  • Terrestrial radiation: Absorbed by water vapour and CO₂.
  • Latent heat: Released during condensation.
• Latent heat ≈ 77%, Sensible heat ≈ 23% of total heat transfer.

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Albedo

• Albedo = percentage of reflected solar radiation.
• Earth–atmosphere average albedo ≈ 30%.
• Snow: Very high albedo (75–95%).
• High albedo → low surface heating → colder temperatures.

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Specific Heat & Temperature Variation

• Specific heat: Heat required to raise temperature by 1°C or 1K.
• Water: High specific heat → heats/cools slowly.
• Land: Low specific heat → heats/cools rapidly.
• Effects:
  • Large diurnal & seasonal variation over land.
  • Minimal diurnal variation over sea (< 1°C).
  • Deserts show maximum diurnal variation.
• Order (low → high specific heat):
  • Rock → Concrete → Dry soil → Wet soil → Ocean → Snow.

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Diurnal Variation of Temperature

• Diurnal variation = Tmax − Tmin.
• Maximum diurnal variation:
  • Clear skies.
  • Calm or light winds.
  • Dry land surfaces.
• Minimum diurnal variation:
  • Cloudy conditions.
  • Windy conditions.
  • Over sea and snow surfaces.

Timing

• Minimum temperature: ~30–60 min after sunrise.
• Maximum temperature: ~1500 LMT (India ~1400 LMT).

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Effect of Wind

• Wind causes turbulent mixing.
• Daytime: Reduces Tmax.
• Nighttime: Raises Tmin.
• Net effect → reduces diurnal variation.

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Effect of Clouds

• Day: Clouds reflect solar radiation → lower Tmax.
• Night: Clouds absorb & re-radiate terrestrial radiation → higher Tmin.
• Overall → reduced diurnal range.
• Clear nights cool faster than cloudy nights.

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Temperature Measurement

• Surface temperature:
  • Measured in Stevenson screen.
  • Height: 4 ft / 1.22–1.25 m above ground.
  • Protects instruments from radiation and precipitation.
• Upper-air temperature:
  • Measured by radiosonde.
  • Aircraft temperature less accurate due to compressibility & lag.

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Thermometers

• Minimum thermometer: Uses alcohol.
  • Freezing point ≈ −130°C.
• Mercury thermometers: Used in Stevenson screens.

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Temperature Scales

• Celsius–Fahrenheit coincidence: −40°C = −40°F.
• Conversions:
  • F = (9/5 × C) + 32
  • C = (F − 32) × 5/9
  • K = C + 273
• Key points:
  • Absolute zero = 0 K = −273°C
  • Freezing point = 273 K
  • Boiling point = 373 K

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Dew Point & Condensation

• Dew point: Temperature at which air becomes saturated.
• Indicates actual moisture content.
• Condensation level: Height where rising air cools to dew point.

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Lapse Rate & Stability

• Normal lapse rate: ~2°C / 1000 ft.
• Isothermal layer: Temperature constant with height (zero lapse rate).
• Inversion: Temperature increases with height.
• Inversions indicate strong stability.

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Radiation Inversion

• Forms on clear, calm nights.
• Ground cools rapidly by terrestrial radiation.
• Cold air near surface, warmer air above.
• Strongest near sunrise.
• Favors fog, frost, poor visibility.

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Adiabatic Process

• Air is a poor conductor of heat.
• Rising air cools, sinking air warms without heat exchange.
• Fundamental to cloud formation and stability analysis.

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Key Exam One-Liners

• Sun → short-wave radiation; Earth → long-wave radiation.
• Wien’s law: Hotter → shorter wavelength.
• Stefan–Boltzmann: Radiation ∝ T⁴.
• Atmosphere heated from below.
• Clouds and wind reduce diurnal variation.
• Radiosonde gives most accurate upper-air temperature.

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