Solar Radiation, Insolation & Heat Balance

8.1 Why Solar Radiation Drives Everything

Do you feel the air around you? Do you know that we live at the bottom of a huge pile of air? We inhale and exhale every few seconds, but we feel the air only when it begins to move — when it becomes wind. Yet every wind, every cloud, every monsoon and every cyclone is ultimately powered by one source — the Sun. The earth receives almost all of its energy from the Sun, and in turn radiates it back to space. This continuous two-way exchange is the engine of the atmosphere. Because different parts of the earth's surface receive unequal amounts of this solar energy, pressure differences arise — and from pressure differences come winds, currents and the entire weather machine.

On an average, the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere — a quantity known as the solar constant^?. Expressed in modern units, this is about 1.367 kW/m². The figure is called a "constant" because it varies very little from year to year — but it is not perfectly fixed, as we shall see in the next section.

Two Forms of Insolation — Direct and Diffuse

Sunlight reaches the earth's surface in two ways. Direct insolation is the radiation that travels in a straight beam from the Sun to the surface, retaining its parallel rays — this is the bright light that casts sharp shadows. Diffuse insolation is the radiation that has been scattered by air molecules, water droplets and dust before arriving — this is the soft, shadow-less light that fills shaded verandas, cloudy days and twilight skies. The blue colour of the sky and the red glow of sunrise and sunset are both products of this scattering.

8.2 Variations in the Solar Output Reaching the Earth

The amount of solar energy received at the top of the atmosphere is not the same throughout the year, because the earth-Sun distance changes. The earth's orbit is an ellipse, with the Sun at one focus, so the planet swings closer and then farther as it revolves.

Therefore, the annual insolation received by the earth on 3 January is slightly more than the amount received on 4 July. However, this orbital variation is largely masked by other, much stronger factors — the distribution of land and sea, the tilt of the earth's axis and the circulation of the atmosphere. Hence the difference between aphelion and perihelion does not have a great effect on daily weather changes on the earth's surface; it works quietly in the background.

8.3 Variability of Insolation at the Surface of the Earth

The amount and the intensity of insolation actually delivered to the surface vary during the day, in a season and across the year. NCERT lists five factors that cause these variations: (i) the rotation of the earth on its axis; (ii) the angle of inclination of the Sun's rays; (iii) the length of the day; (iv) the transparency of the atmosphere; and (v) the configuration of the land in terms of its aspect. The last two have less influence than the first three, but all five together explain why a desert at noon and a polar slope at midnight live in such different worlds.

(i) The Earth's Axial Tilt and Rotation

The earth's axis makes an angle of 66½° with the plane of its orbit round the Sun (equivalently, it is tilted 23½° from the perpendicular). This single tilt has a greater influence on the amount of insolation received at different latitudes than any other geometric factor, because it changes which hemisphere "leans" toward the Sun across the year. As the earth also rotates once every 24 hours, every place experiences day and night — so insolation is delivered in pulses, not continuously.

(ii) Angle of Incidence of the Sun's Rays

The second factor that determines the amount of insolation received is the angle of incidence^? of the rays. This depends mainly on the latitude of a place. The higher the latitude, the smaller the angle the Sun's rays make with the surface — they fall as slant rays. Vertical rays at the equator deliver the same energy onto a smaller patch of ground; slant rays at the poles spread that same energy over a much larger area, so the energy received per unit area is far lower.

Slant rays also have to travel through a greater depth of atmosphere, which means more absorption, scattering and diffusion before reaching the surface. That is why the morning and evening Sun feel mild even on a hot day — the rays are travelling slantwise and losing energy on the way — while the noon Sun overhead is fierce.

(iii) Duration of the Day

Longer days mean more hours during which the Sun is shining on the surface, and therefore more total insolation. Day-length itself varies with latitude and season — Class 11 students will recall from earlier classes that summer days are long at high latitudes and the Sun never sets at the poles in summer. This is why, despite low Sun-angles, polar regions can briefly receive surprising amounts of insolation in mid-summer.

(iv) Transparency of the Atmosphere

Even before sunlight reaches the ground, it must thread its way through the atmosphere. Within the troposphere, water vapour, ozone and other gases absorb much of the near-infrared radiation. Very small suspended particles in the troposphere scatter the visible spectrum both upward to space and downward toward the surface — adding colour to the sky. The red colour of the rising and setting sun, and the blue colour of the daytime sky, are both produced by this scattering.

When the air is loaded with clouds, dust, smoke and pollution, transparency drops sharply, and less direct insolation reaches the surface. A dust storm or an industrial haze can cut the surface insolation of a city by a third or more. Conversely, a cloudless desert sky lets nearly all the incoming radiation reach the ground.

(v) Configuration of the Land

The shape and surface of the land also matter. Oceans absorb sunlight over a great depth and store it efficiently; land, by contrast, heats up its thin top crust quickly and reflects much of the light. Snow, ice, light-coloured deserts and concrete pavements all reflect more, while dark forests, ploughed fields and oceans absorb more. The same total insolation, falling on different surfaces, ends up producing very different temperatures.

8.4 The Passage of Solar Radiation Through the Atmosphere

The atmosphere is largely transparent to short-wave solar radiation. The incoming solar radiation passes through the atmosphere before striking the earth's surface. But the journey is not loss-free.

• Absorption: Within the troposphere, water vapour, ozone and other gases absorb much of the near-infrared radiation.
• Scattering: Very small suspended particles in the troposphere scatter the visible spectrum — both back to space and down to the earth's surface. This process adds colour to the sky.
• Reflection: Cloud tops and snow-covered surfaces reflect a large share of the incoming light directly back to space.

The combined result is that not all of the energy intercepted at the top of the atmosphere reaches the ground — a fact that becomes critically important when we balance the planet's heat budget below.

8.5 Spatial Distribution of Insolation at the Earth's Surface

The insolation received at the surface varies from about 320 W/m² in the tropics to about 70 W/m² at the poles. Maximum insolation is received over the subtropical deserts, where the cloudiness is the least. The equator receives comparatively less insolation than the tropics, mainly because of dense cloud cover. Generally, at the same latitude, the insolation is more over the continent than over the oceans, because oceanic regions are usually cloudier. In winter, the middle and higher latitudes receive much less radiation than in summer, simply because the Sun rises lower and the days are shorter.

8.6 Heating and Cooling of the Atmosphere

Once the Sun has heated the surface, the energy must be redistributed through the atmosphere — otherwise only the layer of air in immediate contact with the ground would warm up. There are four distinct mechanisms by which heat moves through the atmosphere: radiation, conduction, convection and advection. NCERT also lists terrestrial radiation as a separate but related process.

Radiation

Radiation is the transfer of heat in the form of electromagnetic waves, requiring no medium. The Sun heats the earth by radiation, and the earth in turn cools itself by radiating heat back to space. Hot bodies radiate at shorter wavelengths; the cool earth radiates in long wavelengths (infrared).

Conduction

The earth, after being heated by insolation, transmits the heat to the atmospheric layers in contact with it in long-wave form. The air in contact with the land gets heated slowly, and the upper layers in contact with the lower layers also get heated. This process is called conduction^?. Conduction takes place when two bodies of unequal temperature are in contact with one another — there is a flow of energy from the warmer to the cooler body. The transfer of heat continues until both bodies attain the same temperature, or until the contact is broken. Conduction is important in heating the lower-most layers of the atmosphere.

Convection

The air in contact with the earth, once heated, becomes lighter and rises vertically in the form of currents — and further transmits the heat of the atmosphere upward. This process of vertical heating of the atmosphere is known as convection^?. The convective transfer of energy is confined only to the troposphere, since the stratosphere above is stable and resists vertical motion.

Advection

The transfer of heat through the horizontal movement of air is called advection^?. Horizontal movement of the air is, in fact, relatively more important than vertical movement. In middle latitudes, most of the diurnal (day-and-night) variation in daily weather is caused by advection alone. In tropical regions, particularly in northern India during summer, the local hot wind called 'loo' is the outcome of the advection process — hot dry air swept across the plains from the deserts of the west.

Terrestrial Radiation — The Earth Becomes the Heater

The insolation received by the earth is in short-wave form and heats up its surface. The earth, after being heated, itself becomes a radiating body and radiates energy to the atmosphere in long-wave form. This long-wave energy heats up the atmosphere from below. This process is known as terrestrial radiation^?.

The long-wave radiation is absorbed by the atmospheric gases — particularly by carbon dioxide and the other greenhouse gases. Thus, the atmosphere is heated indirectly by the earth's radiation. The atmosphere in turn radiates and transmits heat back to space. Finally, the amount of heat received from the Sun is returned to space, thereby maintaining a constant temperature at the earth's surface and in the atmosphere.

8.7 Heat Budget of the Planet Earth

The earth as a whole does not accumulate or lose heat. Year after year, century after century, the planet maintains its temperature. This balance can happen only if the amount of heat received in the form of insolation equals the amount lost by the earth through terrestrial radiation. Geographers call this balance the heat budget^? or heat balance of the earth.

To understand it, consider that the insolation received at the top of the atmosphere is 100 units. We follow each unit on its journey.

The earth, having absorbed those 51 units, radiates them back as terrestrial (long-wave) radiation. Of these 51 outgoing units:

• 17 units are radiated directly to space.
• 34 units are absorbed by the atmosphere — split as:
  • 6 units absorbed directly by atmospheric gases,
  • 9 units transferred through convection and turbulence,
  • 19 units delivered through the latent heat of condensation (when water vapour condenses into cloud).

The atmosphere has now collected 14 + 34 = 48 units in total (14 from insolation on the way down, 34 from terrestrial radiation on the way up). All 48 are radiated back to space. So the total radiation returning from the earth and atmosphere to space is 17 + 48 = 65 units — exactly the same 65 units that were absorbed from the Sun. Net change: zero. This is termed the heat budget or heat balance of the earth — and this explains why the earth neither warms up nor cools down despite the huge transfer of heat that takes place.

Variation in the Net Heat Budget at the Earth's Surface

As explained earlier, there are variations in the amount of radiation received at the earth's surface. Some parts of the earth have a surplus radiation balance, while other parts have a deficit. The latitudinal variation in the net radiation balance of the earth-atmosphere system shows that there is a surplus between roughly 40° N and 40° S, while the regions near the poles have a deficit. The surplus heat energy from the tropics is redistributed polewards by winds and ocean currents — and as a result the tropics do not get progressively heated up due to accumulation of excess heat, nor do the high latitudes get permanently frozen due to excess deficit. The atmosphere is a great heat-redistribution machine.