Solar Radiation, Heat Balance and Temperature — Study Notes

1. Introduction

The ultimate source of energy for the Earth's climate system is the Sun. The Earth receives solar radiation continuously, and the way this energy is received, reflected, absorbed, redistributed and finally re-emitted to space determines the temperature of the atmosphere, the oceans and the land. This chapter explains how the Earth and its atmosphere stay in a state of heat balance, why temperatures vary over space and time, and how the distribution of temperature over the globe is studied with the help of isotherms.

The energy that the Earth receives from the Sun is called insolation (incoming solar radiation). Not all of the Sun's energy actually reaches the Earth's surface: some is scattered, some reflected and some absorbed by the atmosphere. The Earth does not continuously heat up, because it also radiates energy back to space. The equilibrium between the energy received and the energy lost is called the heat balance of the Earth. Understanding these processes is essential for studying climate, atmospheric circulation and the distribution of temperature over the Earth.

2. Solar Radiation and Insolation

2.1 Nature of Solar Radiation

The Sun emits energy in the form of electromagnetic radiation. The Earth receives only about one two-billionth of the Sun's total output, but this is the source of all energy that drives atmospheric and oceanic processes. Insolation is the incoming solar radiation intercepted by the Earth. Its intensity varies from place to place and over time because of the angle of incidence of the sun's rays, the duration of daylight, the distance of the Earth from the Sun, and the transparency of the atmosphere.

2.2 Factors Affecting Insolation

The amount of insolation received at a place depends on: 1. Angle of incidence: The lower the angle of the sun above the horizon, the more the rays are spread out and the less heat per unit area. Higher angles give more concentrated heating. This is why the tropics are warmer than the poles. 2. Duration of daylight: Longer days mean more insolation. This varies with latitude and season. 3. Distance from the Sun: The Earth is closest to the Sun in early January (perihelion) and farthest in early July (aphelion), causing small variations in insolation. 4. Transparency of the atmosphere: Clouds, dust and aerosols reduce the amount of insolation reaching the surface.

2.3 Important Concepts

• Perihelion: The position when the Earth is closest to the Sun (about 147 million km, in early January).
• Aphelion: The position when the Earth is farthest from the Sun (about 152 million km, in early July).
• The solar constant is the amount of solar energy received per unit area per unit time at the top of the atmosphere, about 1.366 kW per square metre.

3. Heating and Cooling of the Atmosphere

3.1 The Pathways

The atmosphere is heated mainly from below, not directly by the Sun. The important processes are: 1. Conduction: The transfer of heat from the heated Earth's surface to the lowermost layer of air in contact with it. Conduction is important only in the lowest few centimetres of the atmosphere. 2. Convection: The transfer of heat by the vertical movement of heated air. Warm air rises, cools, and sinks, setting up convection currents that transfer heat through the troposphere. 3. Terrestrial radiation: The Earth's surface, heated by insolation, emits longwave infrared radiation. This is absorbed by the atmosphere, heating it.

3.2 Albedo

Albedo is the reflectivity of a surface. Fresh snow has a high albedo (reflects up to 80-95% of solar radiation), while dark surfaces like forests and oceans have a low albedo (reflect only about 5-10%). The average albedo of the Earth is about 31%. Clouds also reflect a significant portion of insolation back to space.

4. Heat Budget of the Earth

4.1 The Concept of Heat Balance

The heat budget of the Earth refers to the balance between the incoming and outgoing radiation. Out of the total 100 units of insolation received at the top of the atmosphere, about 35 units are reflected back to space (25 by clouds and atmosphere and 10 by the Earth's surface), and about 65 units are absorbed by the Earth and the atmosphere (14 by the atmosphere and 51 by the Earth's surface). The Earth then emits 51 units of longwave terrestrial radiation, of which about 34 units are absorbed by the atmosphere and re-emitted. This absorption and re-emission maintains the Earth's heat balance. In the long run, the energy received equals the energy lost, so the Earth neither heats up nor cools down overall.

4.2 Latitudinal Heat Balance

There is a surplus of insolation in the tropics and a deficit in the polar regions. The surplus heat of the tropics is transferred to the poles by atmospheric circulation (winds) and ocean currents. This transfer is responsible for the redistribution of heat over the globe and maintains the global heat balance.

5. Factors Influencing Temperature

The temperature of a place depends on: 1. Latitude: Temperature generally decreases from the equator to the poles. 2. Altitude: Temperature decreases with increasing altitude in the troposphere (about 6.5 deg C per km). 3. Distance from the sea (continentality): Coastal areas have milder, more equable climates, while interiors of continents experience extremes of temperature. 4. Ocean currents: Warm currents raise the temperature of coastal areas and cold currents lower it. 5. Prevailing winds: Winds from warm regions raise temperature; winds from cold regions lower it. 6. Relief and aspect: Mountains act as barriers; slopes facing the sun are warmer.

6. Distribution of Temperature

6.1 Isotherms

The distribution of temperature over the Earth's surface is shown on maps with the help of isotherms, which are lines joining places of equal temperature. In general, isotherms run parallel to the equator in the absence of land and ocean influences. In the Northern Hemisphere, isotherms are more distorted because of the greater amount of land, which heats and cools faster than water.

6.2 Horizontal Distribution

Temperature decreases from the equator towards the poles. The mean annual temperature at the equator is about 24-27 degrees Celsius, while at the poles it is well below zero. In January (winter in the Northern Hemisphere), the isotherms bend towards the equator over the continents because land cools faster than the ocean; in July they bend towards the poles because land heats faster. The highest temperatures are found near the equator and in hot deserts like the Sahara and the Thar.

6.3 Vertical Distribution

Temperature decreases with altitude in the troposphere at the normal lapse rate of about 6.5 degrees Celsius per kilometre. This explains why high mountain peaks are snow-covered even in the tropics.

Quick Revision Tables

Insolation and Heat Budget

Factors Controlling Temperature

Mind Map

Important Diagrams (SVG)

Diagram 1: Heat Budget of the Earth

Diagram 2: Angle of Incidence and Insolation

Common Mistakes

1. Thinking that the atmosphere is heated directly by the Sun. It is heated mainly from below by terrestrial radiation emitted by the Earth's surface.
2. Confusing conduction (contact between molecules, important only near the surface) with convection (vertical circulation of air over the whole troposphere).
3. Confusing albedo values: snow has a high albedo (reflects most radiation) while dark surfaces have a low albedo.
4. Writing that the Earth is closest to the Sun in July; it is closest in early January (perihelion) and farthest in July (aphelion).
5. Forgetting that temperature falls with altitude; the lapse rate is about 6.5 deg C per km in the troposphere.
6. Confusing isotherms (lines of equal temperature) with isobars (lines of equal pressure).
7. Saying that the poles receive no insolation at all; they receive insolation but at a very low angle of incidence, so heating is weak.
8. Thinking that inland areas have milder climates; in fact coastal areas have equable climates and continental interiors have extremes.

Exam Tips

1. Define the key terms precisely: insolation, albedo, isotherm, lapse rate, heat budget, perihelion, aphelion.
2. Memorise the heat budget numbers: 100 units in, about 35 reflected, 65 absorbed, balance maintained.
3. Explain the latitudinal heat transfer: surplus in tropics transferred to deficit poles by winds and ocean currents.
4. Practise drawing the heat budget diagram and the angle of incidence diagram for long answers.
5. In questions on factors of temperature, mention latitude, altitude, continentality, ocean currents, prevailing winds and relief.
6. Remember that isotherms bend equatorward over continents in January and poleward in July because land heats and cools faster than water.
7. Quote the normal lapse rate (6.5 deg C per km) when answering altitude questions.

Conclusion

Solar radiation is the driving force of the Earth's climate system. Insolation varies with the angle of incidence, the length of daylight, the distance of the Earth from the Sun and the transparency of the atmosphere. The atmosphere is heated largely from below through conduction, convection and terrestrial radiation, and part of the incoming energy is reflected back by the Earth's albedo. Despite the surplus of heat in the tropics and deficit at the poles, the Earth maintains a heat balance through the redistribution of energy by winds and ocean currents. The distribution of temperature, shown by isotherms, depends on latitude, altitude, continentality, ocean currents and prevailing winds. Together, these processes determine the climates we experience and provide the basis for the study of atmospheric circulation and weather systems.

Keywords: Insolation, albedo, conduction, convection, terrestrial radiation, heat budget, isotherms, lapse rate, perihelion, aphelion.