Solar Radiation Heat Balance and Temperature Class 11 Geography Notes

This chapter explains solar radiation, atmospheric heating, heat balance and global temperature distribution. It highlights how solar energy controls atmospheric temperature, climate and weather systems across the world.

• Solar radiation
• Insolation
• Variability of insolation
• Conduction
• Convection
• Advection
• Terrestrial radiation
• Heat budget
• Temperature distribution
• Isotherms
• Annual range of temperature
• Temperature inversion

Meaning of Solar Radiation

Solar radiation is the energy received from the Sun. Earth receives most solar energy in short wavelengths.

Insolation

Insolation means incoming solar radiation received by Earth.

Average Solar Energy Received

• Average insolation at top of atmosphere is 1.94 calories per sq. cm per minute

Aphelion

• Earth farthest from Sun
• Distance – 152 million km
• Occurs on 4 July

Perihelion

• Earth nearest to Sun
• Distance – 147 million km
• Occurs on 3 January

Factors Affecting Insolation

1. Rotation of Earth
2. Angle of Sun rays
3. Length of day
4. Transparency of atmosphere
5. Aspect of land surface

Effect of Latitude

• Higher latitude receives slanting rays
• Less energy per unit area

Effect of Slanting Rays

• Cover larger area
• Pass through thicker atmosphere
• Cause more scattering and absorption

Transparency of Atmosphere

Atmosphere is mostly transparent to short wave radiation.

Absorption by Atmosphere

• Water vapour and ozone absorb near infrared radiation

Scattering of Light

• Reflection of visible light by particles
• Causes blue sky
• Causes red sunrise and sunset

Insolation in Tropics and Poles

• Tropics – Around 320 W/m²
• Poles – Around 70 W/m²

Maximum Insolation Areas

• Subtropical deserts receive maximum insolation
• Reason – Least cloud cover

Conduction

• Transfer of heat through direct contact
• Important for heating lower atmosphere

Convection

• Vertical transfer of heat
• Occurs through rising warm air currents
• Confined to troposphere

Advection

• Horizontal transfer of heat by winds
• Example – Loo winds in northern India

Meaning of Terrestrial Radiation

Earth radiates heat in long wave form.

Greenhouse Gases

• Long wave radiation absorbed by carbon dioxide and greenhouse gases

Heating of Atmosphere

Atmosphere is heated indirectly from Earth’s surface.

Heat Budget

Heat budget is the balance between incoming and outgoing heat.

Reflection of Solar Radiation

• Out of 100 units, 35 units reflected back to space
• 27 units reflected from clouds
• 2 units reflected from snow and ice

Albedo

Albedo is the reflected amount of radiation.

Absorption of Heat

• 14 units absorbed by atmosphere
• 51 units absorbed by Earth surface

Outgoing Radiation

• Earth radiates 51 units as terrestrial radiation

Heat Balance Equation

• Outgoing radiation = 17 + 48 = 65 units
• Incoming solar radiation = 65 units

Meaning of Temperature

Temperature is the measurement of hotness or coldness.

Factors Controlling Temperature Distribution

1. Latitude
2. Altitude
3. Distance from sea
4. Air masses
5. Ocean currents
6. Local aspects

Effect of Latitude

• Higher latitude receives less insolation
• Temperature decreases

Effect of Altitude

• Temperature decreases with height
• Normal lapse rate is 6.5°C per 1000 m

Distance from Sea

• Sea heats and cools slowly
• Land heats and cools quickly
• Coastal areas have moderate climate

Effect of Air Masses and Ocean Currents

• Warm currents increase temperature
• Cold currents reduce temperature

Meaning of Isotherms

Isotherms are lines joining places of equal temperature.

January Temperature Distribution

• Isotherms bend northward over oceans
• Bend southward over continents
• Caused by ocean currents and continentality

Siberian Plains Temperature

• Mean January temperature –20°C

Southern Hemisphere Pattern

• Isotherms mostly parallel to latitudes
• Gradual variation in temperature

July Temperature Distribution

• Equatorial oceans – More than 27°C
• Subtropical Asia – More than 30°C

Annual Range of Temperature

• Highest annual range – Northeastern Eurasia
• More than 60°C
• Lowest annual range – Equatorial region
• Around 3°C

Meaning of Temperature Inversion

Temperature inversion is the reversal of normal lapse rate.

Conditions for Inversion

1. Long winter nights
2. Clear skies
3. Calm air

Effects of Inversion

• Stability in lower atmosphere
• Fog formation
• Smoke accumulation

Air Drainage

• Cold air flows downward in valleys
• Protects plants from frost

Planck’s Law

• Hotter body radiates more energy
• Hotter body emits shorter wavelength radiation

Specific Heat

Specific heat is the energy needed to raise the temperature of 1 gram substance by 1°C.

Very Short Answer Questions

1. What is insolation?
2. Define albedo.
3. What is terrestrial radiation?
4. What is advection?
5. What are isotherms?

Short Answer Questions

1. Explain factors affecting insolation.
2. Differentiate conduction and convection.
3. Explain heat budget of Earth.
4. What is temperature inversion?
5. Explain the effect of ocean currents on temperature.

Long Answer Questions

1. Explain the processes of heating and cooling of atmosphere.
2. Discuss the heat balance of Earth.
3. Explain factors controlling temperature distribution.
4. Compare January and July temperature distribution.
5. Explain temperature inversion and its effects.

1. What is insolation?

Insolation is the incoming solar radiation received by Earth.

2. What is albedo?

Albedo is the reflected amount of solar radiation.

3. What is terrestrial radiation?

Heat radiated by Earth in long wave form is called terrestrial radiation.

4. What is normal lapse rate?

Normal lapse rate is the decrease in temperature with height at 6.5°C per 1000 m.

5. What is temperature inversion?

Temperature inversion is the reversal of normal lapse rate where temperature increases with height.

• Insolation:
  • Incoming solar radiation
• Average Solar Energy:
  • 1.94 cal/cm²/min
• Aphelion:
  • 4 July
  • 152 million km
• Perihelion:
  • 3 January
  • 147 million km
• Heating Processes:
  • Conduction
  • Convection
  • Advection
• Terrestrial Radiation:
  • Long wave Earth radiation
• Albedo:
  • Reflected solar radiation
• Heat Budget:
  • Incoming heat = outgoing heat
• Normal Lapse Rate:
  • 6.5°C per 1000 m
• Isotherms:
  • Equal temperature lines
• Highest Annual Range:
  • Northeastern Eurasia
• Lowest Annual Range:
  • Equatorial region
• Temperature Inversion:
  • Reverse lapse rate
• Causes of Inversion:
  • Clear skies
  • Calm winter nights