Solar Radiation, Heat Balance and Temperature

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (Fundamentals of Physical Geography), Chapter 8.

The earth takes in huge energy and never overheats. How?

The earth gets almost all of its energy from the sun, and it has been doing so for billions of years. Yet the planet does not keep warming, and it does not cool down either. Over time, whatever it receives, it sends back to space. This chapter tells you how the heat arrives, where it goes, why one place is hotter than another and why the air can sometimes be warmer higher up than at the ground.

The unequal sharing of that heat matters. It creates differences in air pressure, and those differences set the winds in motion, which carry heat from one region to another. That is why this chapter leads straight into the next one.

Insolation: the sun's energy arriving at the earth

The energy the earth receives is called incoming solar radiation, or insolation. It arrives mostly in short wavelengths. The earth is a sphere-like geoid, so it intercepts only a very small portion of the sun's output. At the top of the atmosphere the earth receives, on average, 1.94 calories per square centimetre per minute.

The earth's orbit is slightly elliptical, so its distance from the sun changes.

Annual insolation on 3 January is slightly more than on 4 July. But the effect is masked by the spread of land and sea and by atmospheric circulation, so it has no great effect on the daily weather.

A common misconception. Many students think the earth is nearer the sun in summer and that this causes seasons. The table shows the opposite for the northern hemisphere: the earth is nearest the sun in January, in the northern winter. The seasons and the variation of insolation by latitude come from the tilt of the earth's axis, which makes an angle of 66½° with the plane of its orbit, not from distance.

What makes insolation vary. Insolation changes through the day, through the seasons and across the year because of five factors. The last two have less influence.

Why slanting rays are weaker. Slant rays lose out for two reasons. First, the area covered by vertical rays is always less than the area covered by slant rays, so the same energy is spread thinner. Second, slant rays pass through a greater depth of atmosphere, so more of the energy is absorbed, scattered and diffused. This is why the poles get so little and also why the sun feels weak early and late in the day.

Pause and predict: the equator is exactly where the noon sun is highest. Yet it receives less insolation than the tropics. Why might that be?

Check your answer: cloudiness. Equatorial skies are often cloudy, which reflects and absorbs sunlight. Maximum insolation is received over the subtropical deserts, where cloudiness is least.

Passage through the atmosphere. The atmosphere is largely transparent to short wave radiation. In the troposphere, water vapour, ozone and other gases absorb much of the near infrared part. Very small suspended particles scatter the visible light both to space and towards the ground, which gives colour to the sky. The blue colour of the sky and the red colour of the rising and setting sun are both results of scattering.

Where insolation is highest and lowest. The insolation at the surface ranges from about 320 watts per square metre in the tropics to about 70 watts per square metre at the poles. At the same latitude, continents receive more than oceans, and the middle and higher latitudes receive less in winter than in summer.

Heating and cooling of the atmosphere

Here is the key idea: the atmosphere is not heated mainly by sunlight directly. Sunlight passes through it largely unabsorbed and warms the earth's surface. The warmed surface then sends out heat as long waves, and the atmosphere absorbs these, mostly through carbon dioxide and other greenhouse gases. A hotter body such as the sun radiates more energy at shorter wavelengths, while the cooler earth radiates at longer wavelengths. So the air is heated from below by the earth.

Where you have felt advection. In northern India in summer, the hot local wind called the loo is the result of advection. You do not feel it because the ground right under you is heated. It blows hot air in from elsewhere.

The heat budget: where every unit goes

If the earth receives 100 units of heat at the top of the atmosphere, it must give back exactly the same amount in the long run. That is the heat budget or heat balance. The earth as a whole neither gains nor loses heat, so its temperature holds steady.

Worked example: following the 100 units.

The 65 units that returned to space balance the 65 units that were absorbed (14 + 51), because the other 35 never became heat in the first place. Remember that the 35 reflected units are separate: they went out without heating anything.

The common wrong step. Students try to add 35 to 65 and get 100 units "returned", or they treat the 51 units of terrestrial radiation as lost to space. In fact only 17 of those 51 go straight to space. The other 34 first warm the atmosphere, which is how the atmosphere gets heated from below.

Beyond the textbook. For the exam: albedo is the 35 units reflected, and the budget closes at 65 units. In reality: the exact figures are estimates, and satellite measurements give the earth's albedo as close to 30 per cent rather than 35. The principle is unchanged, but if a question asks for the number, use the textbook numbers above.

Surplus and deficit. Not every place balances its own budget. There is a surplus of net radiation between about 40° N and 40° S and a deficit near the poles. The surplus heat from the tropics is moved towards the poles. This is why the tropics do not keep heating up and the high latitudes do not freeze permanently.

What temperature is, and what controls it

Heat is the molecular movement of the particles in a substance. Temperature is a measure, in degrees, of how hot or cold something is. When insolation interacts with the atmosphere and the ground, it produces heat, and we measure that as temperature.

Worked example: temperature on a hill. A plains town at sea level records 30°C. What temperature is expected at a hill station 2,000 m above sea level, on the same day?

$$T_{\text{hill}} = T_{\text{plain}} - 6.5 \times \dfrac{h}{1000}$$

Here $h = 2000$ m, so the fall is $6.5 \times 2 = 13$°C, and $T_{\text{hill}} = 30 - 13 = 17$°C. The common wrong step is to forget to divide the height by 1,000 and subtract 6.5°C for every metre.

Why sea and land differ. Land heats up and cools down quickly. The sea heats and cools slowly, because water has a high specific heat (the energy needed to raise one gram by 1°C) and because heat is spread through a deep, moving body of water. This is why coastal cities such as Mumbai have milder temperature swings than inland cities.

World pattern of temperature

Temperature maps are drawn with isotherms, lines joining places of equal temperature. The effect of latitude is clear, because isotherms generally run parallel to the lines of latitude. The departures from this are larger in January than in July, especially in the northern hemisphere, which has far more land than the southern hemisphere.

Annual range of temperature. The range between the warmest and the coldest month is greatest, over 60°C, over north-eastern Eurasia, because of continentality, which means distance from the sea. It is least, about 3°C, between 20° S and 15° N.

Pause and predict: why would the annual range be so large in the Siberian plains and so small near the equator?

Check your answer: Siberia is deep inside a huge land mass, far from the moderating sea, so summers are warm and winters bitterly cold. Near the equator, insolation hardly changes through the year and the surrounding oceans moderate temperature, so the range is small.

Pause and predict: in India, the day temperature is highest in May, not after the summer solstice on 21 June. Why?

Check your answer: by May the sun is nearly overhead over much of India and the skies are clear and dry. With the onset of the monsoon in June, cloud and rain reduce the insolation reaching the ground, so the peak comes before the solstice rather than after it.

Inversion of temperature

Normally temperature falls with height. When this is reversed, so that temperature rises with height, it is called inversion of temperature. It is usually short-lived but quite common. The ideal setting is a long winter night with clear skies and still air. The day's heat is radiated away through the night, so by early morning the ground is cooler than the air above. Over polar areas, inversion is normal throughout the year.

Why it matters. A surface inversion makes the lower air stable, with no rising currents. Smoke and dust collect beneath the inversion layer and spread sideways, and dense morning fogs form, especially in winter. The inversion typically lasts a few hours, until the sun rises and starts warming the earth.

In hills and mountains, inversion arises from air drainage. Cold air produced at night is heavy and dense, so it flows down the slope like water and collects in pockets and valley bottoms, with warmer air above. This protects plants on the slopes from frost damage.

Beyond the textbook. For the exam: inversion makes the air stable, and smoke and dust collect beneath it, causing fog. In reality: on still winter mornings over the northern plains of India, an inversion traps smoke and dust near the ground, which is a major reason why winter air pollution episodes linger. The same law of physics that explains the textbook fog explains the smog in the news.

Exam traps

60-second revision

Where this chapter connects

• Previous: Composition and Structure of Atmosphere explains the carbon dioxide, water vapour and troposphere that this chapter relies on.
• Next: Atmospheric Circulation and Weather Systems shows how the temperature differences drawn here create pressure belts and winds.
• Related: World Climate and Climate Change revisits the greenhouse effect and global warming, and Climate applies the temperature controls to India, including the loo.