Class 11 Geography Notes Chapter 9 (Solar radiation; heat balance and temperature) – Fundamental of Physical Geography Book
Detailed Notes with MCQs of Chapter 9: 'Solar Radiation, Heat Balance and Temperature' from your Physical Geography textbook. This chapter is fundamental to understanding climate and weather patterns, making it very important for your exams. Pay close attention.
Chapter 9: Solar Radiation, Heat Balance and Temperature - Detailed Notes
1. Solar Radiation (Insolation)
- Definition: Insolation is the Incoming Solar Radiation received at the Earth's surface. It's the primary source of energy for almost all processes on Earth.
- Nature: It arrives as shortwave radiation.
- Aphelion & Perihelion:
- Perihelion: Earth is closest to the Sun (around January 3rd) - receives slightly more solar radiation.
- Aphelion: Earth is farthest from the Sun (around July 4th) - receives slightly less solar radiation.
- Key Point: Despite being closer in January, the Northern Hemisphere experiences winter due to the tilt of the Earth's axis, which is a more dominant factor than distance.
- Factors Affecting Distribution of Insolation:
- (a) Rotation of Earth on its Axis: Causes day and night, determining the duration of insolation.
- (b) Angle of Incidence of Sun's Rays:
- Higher angle (nearer to 90°, at tropics/equator) concentrates energy on a smaller area = more intense heating.
- Lower angle (slanting rays, at poles/higher latitudes) spreads energy over a larger area = less intense heating. Also passes through more atmosphere.
- (c) Duration of Daylight: Longer days (summer) allow for more total insolation compared to shorter days (winter). Varies significantly with latitude and season.
- (d) Transparency of the Atmosphere:
- Clouds, dust particles, water vapour absorb, reflect, and scatter insolation.
- Clear skies allow more insolation to reach the surface. Thick clouds significantly reduce it.
- (e) Land Configuration (Aspect): South-facing slopes in the Northern Hemisphere receive more direct sunlight than North-facing slopes (and vice-versa in the Southern Hemisphere).
2. Heating and Cooling of the Atmosphere
The atmosphere is heated indirectly by the Earth's surface, not directly by incoming shortwave solar radiation.
- Mechanisms:
- (a) Conduction: Heating through direct contact. The air layer immediately touching the warm ground gets heated. Significant only in the lowest layers.
- (b) Convection: Vertical transfer of heat. Warmed air becomes less dense, rises, and transfers heat upwards. Cooler, denser air sinks. Creates vertical currents.
- (c) Advection: Horizontal transfer of heat by wind (movement of air masses). Crucial for latitudinal heat distribution (e.g., warm winds moving poleward).
- Terrestrial Radiation:
- Earth absorbs shortwave insolation and heats up.
- It then re-radiates this energy back towards space as longwave radiation (infrared).
- Gases like Carbon Dioxide and Water Vapour in the atmosphere are largely transparent to incoming shortwave radiation but absorb outgoing longwave radiation. This traps heat and warms the lower atmosphere (the Greenhouse Effect).
3. Heat Budget of the Earth
- Concept: The Earth maintains a relatively stable average temperature because the amount of heat received (insolation) is balanced by the amount of heat lost (terrestrial radiation) over the long term.
- Process (Simplified):
- Assume 100 units of insolation reach the top of the atmosphere.
- ~35 units are reflected back to space (Planetary Albedo): ~27 by clouds, ~2 by snow/ice, ~6 scattered by atmosphere.
- ~65 units are absorbed: ~14 by the atmosphere (ozone, water vapour), ~51 by the Earth's surface (land and water).
- The Earth radiates back 51 units as terrestrial radiation.
- ~17 units go directly to space.
- ~34 units are absorbed by the atmosphere.
- The atmosphere, having absorbed 14 units directly and 34 units from terrestrial radiation (total 48 units), radiates this heat back: some towards space (~48 units) and some back towards the Earth's surface (counter-radiation).
- Net Balance: Incoming absorbed (65 units) = Outgoing reflected/radiated (35 units reflected + 17 units direct terrestrial + 48 units atmospheric radiation to space = ~100 units, maintaining balance). Note: Exact percentages can vary slightly between sources, focus on the concept of balance and major pathways.
- Albedo: The percentage of incoming solar radiation reflected by a surface. Lighter surfaces (snow, ice, clouds) have high albedo; darker surfaces (forests, asphalt, water) have low albedo.
4. Temperature
- Definition: A measure of the degree of hotness or coldness of a substance (here, air); indicates the intensity of heat energy.
- Factors Controlling Temperature Distribution:
- (a) Latitude: The most important factor. Insolation decreases poleward due to the decreasing angle of incidence and increased atmospheric path length.
- (b) Altitude: Temperature generally decreases with increasing height in the troposphere (Normal Lapse Rate: approx. 6.5°C per 1000m). Mountains are cooler than adjacent plains.
- (c) Distance from the Sea (Continentality):
- Maritime Influence: Coastal areas have moderate temperatures (cooler summers, milder winters) because water heats up and cools down slowly.
- Continental Influence: Inland areas experience extremes (hot summers, cold winters) because land heats up and cools down quickly.
- (d) Air Mass Circulation & Ocean Currents:
- Warm ocean currents (like Gulf Stream) raise temperatures of adjacent coastal areas.
- Cold ocean currents (like Labrador Current) lower temperatures.
- Movement of warm/cold air masses (advection) significantly impacts regional temperatures.
- (e) Local Aspects: Slope direction (aspect), vegetation cover (forests moderate temperature), surface type (urban areas - heat island effect) can influence local temperatures.
5. Distribution of Temperature
- Isotherms: Lines drawn on a map connecting places having the same temperature at a given time or average over a period. Temperatures are usually reduced to sea level for comparison purposes to eliminate the effect of altitude.
- General Characteristics of Isotherms:
- Generally run parallel to latitudes, indicating latitude's dominant control.
- Spacing indicates the rate of temperature change (thermal gradient) - closely spaced means rapid change, widely spaced means slow change.
- Bend significantly at land-water junctions due to differential heating. They shift north and south with the seasons.
- Seasonal Distribution (January and July):
- January: Winter in Northern Hemisphere (NH), Summer in Southern Hemisphere (SH). Isotherms bend equatorward over continents and poleward over oceans in NH (due to colder land, warmer oceans). Thermal equator shifts south of the geographical equator. Strongest thermal gradients are in NH.
- July: Summer in NH, Winter in SH. Isotherms bend poleward over continents and equatorward over oceans in NH (due to warmer land, cooler oceans). Thermal equator shifts north of the geographical equator. Thermal gradient is generally weaker than in January.
6. Inversion of Temperature
- Definition: An atmospheric condition where temperature increases with altitude instead of decreasing. This is a reversal of the normal lapse rate.
- Conditions Favoring Surface Inversion:
- Long winter nights (allows extensive cooling of the ground via radiation).
- Clear skies (allows rapid escape of terrestrial radiation).
- Calm, stable air (prevents vertical mixing).
- Dry air (less capacity to absorb heat).
- Snow cover (reflects insolation and promotes rapid radiational cooling of the surface).
- Effects: Stabilizes the atmosphere, suppresses convection, traps pollutants near the ground, often leads to fog or frost formation. Common in valleys during winter nights.
Multiple Choice Questions (MCQs)
-
The primary reason for variations in the amount of insolation received at different latitudes is:
a) Earth's rotation on its axis
b) The distance between Earth and Sun varying throughout the year
c) The angle of incidence of the sun's rays
d) The transparency of the atmosphere -
The atmosphere is mainly heated by:
a) Incoming shortwave solar radiation
b) Absorption of UV radiation by Ozone
c) Outgoing longwave terrestrial radiation
d) Conduction from the Earth's core -
Albedo refers to the:
a) Amount of solar radiation absorbed by the Earth's surface
b) Percentage of solar radiation reflected back by a surface
c) Amount of heat radiated by the Earth as longwave radiation
d) Process of heat transfer through vertical air movement -
Which of the following factors causes land surfaces to heat up and cool down more rapidly than water surfaces?
a) Water is transparent, allowing heat to penetrate deeper
b) Specific heat of water is higher than land
c) Convection currents distribute heat efficiently in water
d) All of the above -
Isotherms are lines connecting places of equal:
a) Pressure
b) Rainfall
c) Temperature
d) Humidity -
During January in the Northern Hemisphere, isotherms over continents tend to bend:
a) Poleward (towards the North Pole)
b) Equatorward (towards the Equator)
c) Parallel to the longitudes
d) They do not bend significantly -
A temperature inversion is a condition where:
a) Temperature decreases rapidly with height
b) Temperature remains constant with height
c) Temperature increases with height
d) Temperature fluctuates randomly with height -
Which mechanism of heat transfer involves the horizontal movement of air?
a) Conduction
b) Convection
c) Advection
d) Radiation -
The Earth maintains a stable average temperature primarily due to the:
a) Greenhouse effect trapping all heat
b) Balance between incoming insolation and outgoing terrestrial radiation
c) Constant distance from the Sun
d) Reflection of all solar energy by clouds -
Which condition is LEAST likely to favour the formation of a surface temperature inversion?
a) Long winter night
b) Clear sky
c) Windy conditions
d) Snow-covered ground
Answer Key:
- c
- c
- b
- d
- c
- b
- c
- c
- b
- c
Study these notes carefully. Understand the concepts of energy transfer, balance, and the factors influencing temperature distribution. These are foundational for understanding more complex geographical phenomena. Let me know if any part needs further clarification.