📘 Class 11 Geography – Chapter 9
🌞 Solar Radiation, Heat Balance and Temperature
☀️ Solar Radiation
- The Sun is the main source of energy on Earth 🌍.
- It is a hot gaseous (गैसीय) body with a surface temperature of 6000°C.
- The constant heat radiated by the Sun into space is called Solar Radiation (सौर विकिरण).
🌈 Albedo (अल्बीडो)
- Out of incoming solar radiation:
- 27 units reflected by clouds ☁️
- 2 units reflected by snow-covered areas ❄️
- This total reflected radiation = Earth’s Albedo (प्रतिफलन क्षमता) = 29 units.
🔆 Solar Constant (सौर नियतांक)
- Earth receives 1.94 calories/cm²/minute of energy at the upper atmosphere.
- This fixed value is called Solar Constant.
🔥 Calorie (कैलोरी)
- Heat required to raise temperature of 1 gram water by 1°C.
📐 Planck’s Law (प्लांक का नियम)
- Hotter an object → more energy it radiates.
- Wavelength (तरंग दैर्ध्य) becomes shorter.
🌡️ Specific Heat (विशिष्ट ऊष्मा)
- Heat required to raise temperature of 1 gram substance by 1°C.
🗺️ Isotherms (समताप रेखाएँ)
- Imaginary lines on a map joining places of equal temperature.
🌍 Aphelion & Perihelion
- Aphelion (अपसौर) → 4 July → Earth farthest from Sun (152 million km).
- Perihelion (उपसौर) → 3 January → Earth closest to Sun (147 million km).
☀️ Insolation (आवर्तित सौर विकिरण)
- Energy received by Earth from Sun → in short wave form.
🌡️ Insolation vs Temperature
🔹 Insolation:
- Energy received from the Sun.
- Measured in calories.
- Example: 1.94 cal/cm²/min.
🔹 Temperature:
- Measurement of heat effect.
- Measured in Celsius (°C), Kelvin (K), Fahrenheit (°F).
📌 Factors Affecting Insolation
- Inclination of Sun’s Rays (सूर्य किरणों का झुकाव) 🌞
- Vertical rays heat more, oblique rays spread over large area → less heating.
- Atmosphere Effect (वायुमंडल का प्रभाव) 🌫️
- Clouds, dust, humidity absorb, reflect, scatter solar energy.
- Land & Water Contrast (स्थल-जल भिन्नता) 🏔️🌊
- Land heats quickly, water heats slowly.
- Length of Daylight (दिन की लम्बाई) ⏳
- Longer day → more insolation.
- Shorter day → less insolation.
- Slope of Land (भूमि का ढलान) 🏞️
- Sun-facing slopes receive more insolation.
- Earth’s Distance from Sun 🌍☀️
- 3 Jan (Perihelion) → more insolation.
- 4 July (Aphelion) → less insolation.
✅ Exam Tip:
- Always remember difference between Insolation vs Temperature (often asked in short notes).
- Diagrams of Aphelion & Perihelion are important for exams.
🌡️ Ways of Heating & Cooling the Earth
🔁 Conduction
- Heat transfer by direct contact.
- Energy flows from hot → cold body until balance.
🌪️ Convection
- Heat transfer vertically in gases & liquids.
- Example: warm air rises, cool air sinks.
🌬️ Advection
- Heat transfer in horizontal direction.
- Example: wind & ocean currents moving heat across regions.
🌍 Terrestrial Radiation
- Earth absorbs solar radiation (short waves) & re-emits it as long waves.
- This process:
- Heats atmosphere indirectly 🌫️.
- Absorbed by greenhouse gases.
- At night, clear skies → faster cooling.
🌡️ Factors Affecting Temperature Distribution
- Latitude 🌐 – Higher latitude = lower temperature.
- Altitude ⛰️ – Higher altitude = lower temperature.
- Distance from Sea 🌊 – Coastal areas = moderate temperature; interiors = extremes.
- Air Masses & Ocean Currents 🌬️🌊 – Warm currents/air = higher temp, cold currents/air = lower temp.
📉 Temperature Inversion
- Normally, temperature decreases with altitude in Troposphere.
- But sometimes, temperature increases with height (opposite condition).
- This is called Temperature Inversion:
- ❄️ Cold air below.
- 🔥 Warm air above.
Temperature Distribution & Heat Budget
🌡️ Geographical Conditions Required for Temperature Inversion
✔️ Long Nights 🌙
- Earth loses heat at night through radiation.
- The lower layer of air becomes cold, while the upper air remains warmer.
✔️ Clear Sky ☁️❌
- A cloudless sky helps in maximum cooling by terrestrial radiation.
- Clouds act as a blanket and obstruct cooling.
✔️ Calm Air 🍃
- No air movement means no heat exchange between layers.
- This helps the lower air to remain colder → leading to inversion.
✔️ Dry Air 💨
- Dry air cannot absorb much ground radiation.
- Faster cooling at surface leads to inversion.
✔️ Snow Cover ❄️
- Snow reflects most solar radiation.
- Keeps the lower layer cold → inversion conditions are created.
📉 Normal Lapse Rate of Temperature
- With increase in altitude, temperature decreases.
- Average lapse rate = 6.5°C per 1000 m rise in height.
☀️ Earth’s Heat Budget
The atmosphere receives 100 units of solar energy (insolation).
🔆 Radiation of Insolation (100 units)
- 16% absorbed by dust & water vapor
- 3% absorbed by clouds
- 6% reflected by air
- 20% reflected by clouds
- 4% reflected by land & water
- 51% absorbed by Earth’s surface
👉 Total reflected back = 33 units
👉 Absorbed in atmosphere = 16 units
👉 Reaching Earth’s surface = 51 units
🌍 Terrestrial Radiation (from Earth’s 51 units)
- 17% → directly lost to space
- 6% → absorbed by atmosphere
- 9% → transferred via convection
- 19% → latent heat of condensation
✔️ Total outgoing = 51 units (balanced)
➡️ This balance keeps Earth’s temperature stable.
🌡️ Isotherms – January vs July
❄️ January
- Isotherms bend northward over oceans and southward over land.
- Temp ranges:
- Equatorial Oceans → 27°C+
- Tropics → 24°C+
- Mid-latitudes → 20°C to 0°C
- Eurasia interior → -18°C to -48°C
- Variation is less in Southern Hemisphere (due to more water).
☀️ July
- Isotherms form 30°C cells in tropical continents.
- 10°C isotherms seen near 40°N & 40°S.
- Southern Hemisphere → lines are straighter, parallel to latitudes.
- Isotherms bend northward in continents & southward in oceans.
📌 Max Annual Temp Range = Siberian Plains (due to extreme continental effect).
🌊 Oceans & Temperature in Southern Hemisphere
- Isotherms run parallel to latitudes.
- Temperature variation is less intense compared to Northern Hemisphere.
- Examples:
- 20°C isotherm → ~35°N
- 10°C isotherm → ~45°S
- 0°C isotherm → ~60°S
