Chapter 13: Insolation
By two in the afternoon, the sand in a desert is too hot to stand on with bare feet. The sky is cloudless. There is almost no water vapour in the air, and nothing between the ground and the sun. Heat seems to rise off the surface in sheets, and the shade of a single thorn tree is worth walking a hundred metres for. Twelve hours later, on the same patch of sand, a traveller reaches for a blanket. The night is bitterly cold. Nothing has moved. The sun has not come any closer or gone any further away, the sand is the same sand, and the place is the same place. Only the time has changed. Somewhere between the afternoon and the small hours of the morning, an enormous quantity of heat arrived and then left again. Where did it come from, and where has it gone? A clear night sky is not simply darkness. It is an open door, and the earth spends every night quietly pushing the day’s heat out through it. This chapter follows that heat — in, down, and out again.
Diagram
Fig. 13.A1 Caption: The same desert surface by day and by night, receiving heat and then losing it. Brief: Two panels side by side sharing one ground line — left panel daytime with short arrows from the sun striking bare sand, right panel a clear night with arrows leaving the ground upward into space; label the incoming arrows and the outgoing arrows, show a cloudless sky in both, and mark day and night temperatures as ‘hot’ and ‘cold’ without giving figures.
13.1 Types Of Radiation
The sun pours out heat and light energy continuously, and it does so in every direction at once. This outflow of energy from the sun is called solar radiation. Only a very small share of it happens to travel towards the earth, and the earth intercepts a tiny fraction of everything the sun radiates.
The journey of that energy from the sun to the earth, and then back out into space, is a complicated one. Three different things happen to it along the way, and it helps to keep them apart from the start.
Energy is moved from the sun to the earth by transmission, and transmission takes place in the form of radiation — energy travelling in waves, needing no air or water to carry it. Once the energy arrives, it is stored and moved about as heat. That storage and transport happens in the atmosphere, the blanket of air around the earth; in the hydrosphere, all the water of the oceans, seas, ice and rivers; and in the lithosphere, the solid rocky crust beneath our feet.
Solar radiation is the only primary source of light and heat that the earth has. Every movement and every change at the earth’s surface — a wind, a wave, a growing crop, an evaporating puddle — is paid for out of energy the sun supplied.
The Three Parts Of The Sun’s Radiation
The radiation that leaves the sun is made up of three parts. (a) White light is the part our eyes can detect, and it is what makes daylight. (b) Infrared radiation is invisible to us, and it is the part of the radiation we register as warmth on the skin. (c) Ultraviolet radiation is also invisible, and it is the part that the ozone in the stratosphere takes out of the beam before it can reach the ground.
The sun sends out more than radiation. It also throws out atomic particles into space, and this stream of particles is called ‘solar wind’. Most of the particles carried in the solar wind are charged particles called electrons, and because they carry a charge, they disturb the earth’s magnetic field when they arrive.
Flowchart
Fig. 13.A2 Caption: Everything the sun sends outwards falls into one of two groups. Nodes: the sun → radiation → white light (seen) / infrared radiation (unseen) / ultraviolet radiation (unseen); the sun → atomic particles → ‘solar wind’ → mostly charged electrons → affect the earth’s magnetic field
13.1.1 Insolation
Not all of the solar radiation aimed at the earth arrives. Insolation is the amount of solar energy that is received by the earth. Along the way the sun’s energy is dissipated and scattered — spread out and turned aside in all directions — so that only one part out of two billion parts of the sun’s radiated energy reaches the earth at all.
The energy that does arrive comes as short wave rays. Short waves are the kind of radiation given out by anything as fiercely hot as the surface of the sun, and they pass through the air easily.
Even after reaching the earth, the incoming energy is split three ways. Taking the total incoming solar energy as 100%, the chapter accounts for it like this. (a) Only 51% actually reaches the earth’s surface and is available to heat the ground and the sea. (b) 35% is reflected straight back into space, so it never warms the earth at all. (c) 14% is absorbed by the layers of the atmosphere, ozone included, during insolation.
13.1.2 Terrestrial Radiation
The 51% that gets through does not stay. Insolation strikes the surface of the earth and warms it, and the warmed surface then radiates that heat back upwards into the atmosphere. Terrestrial radiation is the heat radiated by the earth in the form of long waves.
The change of wavelength is the key to the whole chapter. Energy arrives from the very hot sun as short waves, and leaves the far cooler earth as long waves. What comes in and what goes out are therefore not the same kind of radiation, and the atmosphere does not treat them the same way — a point the next section builds on.
The whole exchange, incoming and outgoing together, is set out in Fig. 13.1.
Diagram
Fig. 13.1 Caption: Heat budget. Brief: Redraw the book’s heat budget panel — incoming solar radiation 100% at the left splitting into 51% reaching the surface, 35% reflected back and 14% absorbed in the atmosphere; on the right, outgoing radiation totalling 100% as 35% reflected, 48% from the atmosphere and 17% from the earth’s surface, with 34% absorbed in the atmosphere from the earth’s surface and the working 14% + 34% = 48% shown; downward arrows for incoming, upward arrows for outgoing, atmosphere and surface of the earth labelled as bands.
13.2 How Is The Balance Achieved?
Energy keeps arriving at the earth, and energy keeps leaving it. Over the earth as a whole, the two match. This state of equilibrium between the incoming insolation from the sun and the outgoing terrestrial radiation from the earth is known as heat balance. When incoming radiation and outgoing radiation are balanced in this way, the account is called the heat budget.
The balance is a worldwide average, and averages hide differences. Low latitude areas — the belt on either side of the Equator — receive more radiation than they lose, while near the Poles the opposite happens and more is lost than is received. Neither region is in balance on its own. The earth as a whole is.
This balance matters because life depends on it. If the earth steadily gained energy it would grow hotter without limit, and if it steadily lost energy it would cool towards a temperature at which nothing could survive.
The earth rotates on its axis, so the sun’s rays never warm all parts of it equally at the same time. That uneven heating sets several processes into motion, and those processes are what move heat around and keep the distribution from becoming extreme.
Conduction And Convection
Heat travels through the earth-atmosphere system in two ways, and the difference between them is worth holding onto.
Conduction is the process by which the heat radiated by the earth’s surface warms the layers of air lying directly above it by direct contact. The air touching the ground is heated first, and it is heated by the ground, not by the sun overhead.
Convection is the transfer of heat by circulatory movement. The circulation of air, and of water in the oceans, carries heat from one medium to another and from one level to another. Where conduction needs the two bodies to be touching, convection carries the heat bodily from place to place with the moving air or water. Fig. 13.2 shows the contrast between them.
Diagram
Fig. 13.2 Caption: Difference in the heating of land (conduction) and the heating of a sea (convection). Brief: Redraw the book’s panel — slanting rays of the sun falling on a land mass on the left and an ocean on the right; label ‘Heat reaches only the surface’ over the land and ‘Heat passes to a greater depth’ in the water, with curved arrows inside the water showing circulation carrying heat downwards; label LAND and OCEAN.
Convection currents in the atmosphere are what produce the sea breeze. During the day, the air over the land becomes warmer than the air over the sea and rises, and cool air from the ocean moves in to take its place. At night the opposite happens, because the land surface cools more quickly than the ocean surface, and land breezes are caused.
The Atmosphere As A Blanket
The atmosphere absorbs 34 units of the earth’s outgoing radiation, but only 14 units of the sun’s incoming radiation. The air above us is therefore heated far more by the terrestrial radiation given out by the earth than by the insolation coming down from the sun. In effect, the atmosphere is warmed from below.
Because it holds heat in this way, the atmosphere acts like a blanket. It does two distinct jobs, one by night and one by day. (a) At night the atmosphere acts like a greenhouse, keeping the earth’s surface warm by preventing terrestrial radiation from escaping into space. (b) During the day it absorbs 14% of the insolation before that energy can reach the ground, and so keeps the daytime heating in check.
The atmosphere therefore prevents extremes of temperature at both ends of the day. Without it, the earth would become very hot during the day and very cold at night.
The whole exchange can be followed as a single loop, from the sun to the ground, into the air, and back out to space.
Flowchart
Fig. 13.A3 Caption: Heat entering the earth is passed on, moved about and finally returned to space, which is what keeps the budget balanced. Nodes: insolation as short waves → 51% absorbed at the earth’s surface → surface warms → terrestrial radiation as long waves → conduction warms the air in contact with the ground / convection carries heat upward and through the oceans → atmosphere absorbs 34 units → atmosphere radiates outward → energy leaves for space → balances the incoming total → back to insolation
13.3 Factors Affecting The Distribution Of Temperature
Temperature is not spread evenly over the earth. Two places can lie on the same parallel and still have very different air temperatures, and the reason is that several separate influences act on each place at once.
The temperature of the atmosphere at a place depends chiefly on the following factors. (a) Winds and ocean currents, which carry heat away from where it was gained and deliver it somewhere else. (b) Latitude, which fixes the angle at which the sun’s rays arrive. (c) Distance from the sea, which decides whether a place is steadied by a large body of water or not. (d) Altitude, which decides how much air lies above the place. (e) Slope of the land, which decides how much of the sun’s energy a surface catches and how quickly it loses heat again.
Each of these is taken in turn below. The broad pattern that latitude sets up is shown in Fig. 13.3, and the other four factors work on top of it.
Diagram
Fig. 13.3 Caption: Heat zones. Brief: A globe seen from the side with the Equator across the middle; shade and label the Torrid Zone between the Tropic of Cancer and the Tropic of Capricorn, the North Temperate Zone and the South Temperate Zone between the tropics and the polar circles, and the two Frigid Zones beyond the Arctic Circle and the Antarctic Circle; mark NP and SP at the poles.
13.3.1 Latitude
The temperature of a place depends on its latitude — its distance north or south of the Equator, measured in degrees. Two things follow from it. (a) Temperature decreases as latitude increases on either side of the Equator, because of the spherical shape of the earth and its annual revolution around the sun. (b) The sun’s rays strike the earth at varying angles of incidence — the angle at which a ray meets the ground — owing to the spherical shape of the earth and its inclination on its axis.
The second of these does most of the work, and it is worth slowing down over. A slanting ray is at a disadvantage twice over. It travels a longer distance through the atmosphere, so more of its energy is lost on the way, and it then spreads the energy it has left over a larger area of ground. Slanting rays therefore have less heating power, as Fig. 13.5 shows. The midday sun is almost overhead within the tropics, but stands at oblique angles outside them.
Diagram
Fig. 13.5 Caption: Vertical rays are hotter than slanting rays. Brief: Redraw the book’s panel — a bundle of vertical rays on the left striking a small patch of ground, and a bundle of slanting rays of the same width on the right striking a larger patch; label the first ‘Covers a smaller area so heat is more’ and the second ‘Covers a larger area so heat is less’, with arrows showing the direction of the rays.
The Five Temperature Zones
Taken together, these two points mean that the higher the latitude, the colder the place. On this basis the earth is divided into five temperature zones. These are the Torrid Zone, lying between the Tropic of Cancer and the Tropic of Capricorn; the North Temperate Zone and the South Temperate Zone; and the two Frigid Zones at the top and bottom of the globe.
The sun’s rays that fall over the Torrid Zone travel a shorter distance and heat up a smaller surface area, which leads to a higher temperature there. Beyond the Torrid Zone the rays travel a longer distance, because of the inclined axis of the earth and its revolution. Much of that heat is absorbed by clouds and water vapour, or reflected back by dust particles.
Beyond the Torrid Zone, then, the rays fall in a slanting position, heat up a larger area, and temperature goes on falling as you move polewards. Latitude sets the framework, but it does not finish the job. Other factors also influence climate and air temperature, and they can override the pattern completely, as the rest of this section shows.
13.3.2 Altitude
Altitude is the height of a place above the mean sea level. The temperature of the atmosphere and the altitude of a place are closely linked: the higher the altitude, the lower the temperature. Two reasons lie behind this. (a) The air is cooler at higher altitudes than at the earth’s surface, and the main reason is that the atmosphere is heated from below, more by terrestrial radiation than by incoming solar radiation. (b) Air at the surface of the earth is denser and contains more carbon dioxide, water vapour and other gases, so its heat absorption capacity is greater in the lower layers than in the thin upper layers where the density is less.
Both reasons say the same thing in different words. The warmth of the air is borrowed from the ground beneath it, and the higher you go, the less air there is to hold that borrowed heat.
Diagram
Fig. 13.4 Caption: Temperature decreases with increase in altitude. Brief: Redraw the book’s mountain profile with horizontal bands of air across it; label the upper bands ‘Cold thin layer’ and the lower bands ‘Warm thick layer’, showing the lower air denser than the upper air.
The Normal Lapse Rate
The rate of decrease of temperature with height above sea level is about 6°C per km, and this is known as the normal lapse rate. Stated the other way round, it means that for every 166 metres climbed, the temperature falls by 1°C. The rate is usually greater in summer than in winter.
The two statements are the same rate written at two different scales, and the second is the more useful one on the ground, because nobody climbs in neat kilometres. It is also worth remembering what the word ‘normal’ is doing there. This is an average rate, greater in summer than in winter, so a thermometer carried up a real hillside on a real afternoon will not obey it exactly.
This is why the top of a mountain is colder than its foot, and why hill resorts in tropical countries have a pleasant climate in summer while the plains below them are hot.
13.3.3 Distance From The Sea
Land and water do not respond to the sun in the same way, and almost everything about a coastal climate follows from that one difference.
The sun’s rays pass through water to a great depth, and water is mobile, so warm water mixes easily with the cold water below it. For both reasons, water is neither heated nor cooled quickly. Land behaves in the opposite way. The sun’s rays heat a piece of land rapidly, because the heat the area gains stays confined to that area and does not mix with the land around it.
So the land gets heated, and cooled, more quickly than the water. During the day the land is hotter than the sea. The hot air over the land becomes lighter and rises, and this creates an area of low pressure — a place where the air presses down less heavily than it does around it — over the land.
The air above the sea is cooler at this time. When that air blows towards the low pressure area over the land, it lowers the temperature of the air on land. Such a cool breeze, blowing over the earth from the surrounding seas, is known as a sea breeze, and it cools the coastal regions during the day.
At night the situation is reversed. The sea remains warmer than the land, and the breeze that blows from land to sea in the night is known as a land breeze, which makes the sea cooler. This daily interchange of breezes is what maintains the heat balance between land and sea.
Flowchart
Fig. 13.A4 Caption: The breeze turns around twice a day, because land gains and loses heat faster than the sea. Nodes: day: land heats faster than the sea → air over land warms and rises → low pressure forms over the land → cooler air moves in from the sea → sea breeze → coastal air is cooled; night: land cools faster than the sea → the sea is now the warmer surface → air moves from land towards the sea → land breeze → the sea is cooled → back to day
Diagram
Fig. 13.6 Caption: Distance from the sea. Brief: Redraw the book’s cross-section — a land mass on the left falling to an ocean on the right, with red arrows above running seawards labelled ‘warm land breeze during the night’ and green arrows running landwards labelled ‘cool sea breeze during the day’; label the interior ‘Temperature contrast and great range in continental interior’, and label LAND and OCEAN.
Areas close to the sea therefore have lower daily and annual ranges of temperature and enjoy a moderate climate, while areas lying in the interior swing much further between hot and cold.
13.3.4 Slope Of The Land
The slope of the land influences the temperature of a place, because slope decides how squarely a surface faces the sun and how quickly it sheds its heat. An area with a steep slope experiences a more rapid change in temperature than one with a gentle slope.
This is why a mountain range with an east-west alignment, like the Alps in central Europe, has a higher temperature on the south-facing sunny slope than on the north-facing sheltered slope. The two slopes lie in the same range at the same latitude, and are separated only by the ridge between them.
Inversion Of Temperature
Temperature may also be lower in a valley than higher up the slopes above it, which is the reverse of what altitude alone would predict. This happens on calm, cold winter nights, when the sky is clear and the air very dry.
Under those conditions the heat from the earth’s surface escapes rapidly back into space, and the upper slopes end up warmer than the valley floor. The cold at the bottom is deepened further by cool air sinking down from the mountain sides and collecting there. There is then a reversal in the vertical distribution of temperature, with temperature decreasing downwards into the valley, and this is known as the inversion of temperature. Fig. 13.7 shows it happening on one still night.
Diagram
Fig. 13.7 Caption: Temperature inversion at valley bottom on a calm still night. Brief: Redraw the book’s valley cross-section — a V-shaped valley with air temperatures marked on the right-hand slope as 8°C high up, 6°C lower down and 3°C at the valley bottom; arrows along both slopes pointing downwards labelled ‘cold air sinking’, the valley floor labelled ‘cold air at valley bottom’ and the air in the valley labelled ‘calm air’.
13.3.5 Winds And Ocean Currents
The last factor moves heat across the map. A landmass gets heated and cooled fast, while a water mass — the seas and the oceans — retains heat for a much longer period. Equatorial waters stay hot and give rise to warm currents, while polar waters are cold.
Winds blowing off the oceans, and the ocean currents themselves, transport heat or cold from one part of the world to another. (a) Warm currents move from equatorial latitudes towards polar latitudes, carrying warm water into cooler regions, so the coasts they pass become warmer than they would otherwise be. (b) Cold currents flow from higher polar latitudes towards lower latitudes in equatorial regions, carrying cool water into warm regions, so the coasts they pass become cooler than usual.
As you have already learnt, the warm North Atlantic Drift raises the winter temperature of North-West Europe, and especially that of the British Isles and Norway. What that does to two ports on opposite sides of the same ocean is worth following carefully.
Latitude fixed both ports at about the same distance from the Equator, and latitude alone would have predicted the same winter for both. What separated them was the water arriving at each coast. This is the clearest demonstration in the chapter that the five factors do not simply add up; one of them can cancel out another, and a place’s temperature is always the outcome of all of them acting together.
13.4 Summary
- The sun continuously radiates heat and light energy in all directions, and this outflow is called solar radiation. It is the only primary source of light and heat on the earth, and the earth intercepts only a tiny fraction of it.
- Energy is transmitted from the sun in the form of radiation.
- Energy is stored and transported as heat in the atmosphere, the hydrosphere and the lithosphere.
- The sun’s radiation is made up of three parts — white light, which we see, infrared radiation and ultraviolet radiation. The sun also emits atomic particles called ‘solar wind’, mostly charged electrons, which affect the earth’s magnetic field.
- Insolation is the amount of solar energy received by the earth. It arrives as short wave rays, and only one part out of two billion parts of the sun’s radiated energy reaches the earth at all.
- 51% of the incoming solar energy reaches the earth’s surface.
- 35% is reflected back into space.
- 14% is absorbed by the atmospheric layers, including ozone.
- Terrestrial radiation is the heat radiated by the earth in the form of long waves, after insolation has struck and warmed the surface.
- Heat balance is the state of equilibrium between the incoming insolation and the outgoing terrestrial radiation, and when the two are balanced the account is called the heat budget.
- It is a worldwide average balance, not a local one.
- Low latitude areas receive more radiation than they lose, while near the Poles the opposite happens.
- Heat is passed on in two ways once it reaches the surface.
- Conduction is the warming of the layers of air above the earth’s surface by direct contact with the heated ground.
- Convection is the transfer of heat by circulatory movement of air and of water in the oceans, from one medium or level to another, and it is what produces the sea breeze.
- The atmosphere absorbs 34 units of the earth’s outgoing radiation but only 14 units of the sun’s incoming radiation, so it is heated more from below than from above and acts like a blanket.
- At night it works like a greenhouse, stopping terrestrial radiation from escaping into space.
- By day it absorbs 14% of the insolation and so keeps the heating in check.
- Without it, the earth would be very hot by day and very cold by night.
- Temperature is not distributed uniformly over the earth. Five factors decide the temperature of a place: latitude, altitude, distance from the sea, slope of the land, and winds and ocean currents.
- Latitude works through the angle of incidence of the sun’s rays. Slanting rays travel a longer distance and spread over a larger area, so they have less heating power, and temperature falls as latitude increases.
- On this basis the earth is divided into five temperature zones — the Torrid Zone, the North and South Temperate Zones, and the two Frigid Zones.
- Altitude is the height of a place above the mean sea level, and temperature falls as altitude rises, because the atmosphere is heated from below and the thin upper air absorbs less heat.
- The normal lapse rate is a fall of about 6°C per km, which is 1°C for every 166 metres climbed, and it is usually greater in summer than in winter.
- Land heats and cools faster than water, because the sun’s rays enter water to a great depth and warm water mixes with cold, while heat gained by land stays where it falls.
- By day the land is hotter, air over it rises, low pressure forms, and the cool sea breeze blows in from the sea.
- By night the sea is warmer and the land breeze blows out to sea, so the interchange maintains the heat balance.
- Places close to the sea have lower daily and annual ranges of temperature than places in the interior.
- Slope of the land decides how much heat a surface catches and how fast it loses it. A south-facing sunny slope is warmer than a north-facing sheltered slope in an east-west range such as the Alps.
- On calm, clear, dry winter nights heat escapes rapidly and cold air sinks into the valley, so the valley floor becomes colder than the slopes above it. This reversal is called the inversion of temperature.
- Winds and ocean currents carry heat across the map. Warm currents move from equatorial to polar latitudes and warm the coasts they pass, while cold currents move from polar to lower latitudes and cool them, which is why Bergen stays open in winter and ports at the same latitude in north-east Canada freeze.
Flowchart
Fig. 13.A5 Caption: The chapter in one chain — energy arrives, is shared out unequally, and finally leaves again. Nodes: sun → short-wave insolation → 35% reflected to space / 14% absorbed by the atmosphere / 51% reaches the surface → surface warms → long-wave terrestrial radiation → conduction warms the air in contact / convection circulates air and ocean water → atmosphere absorbs 34 units and acts as a blanket → energy radiated out to space → totals match the incoming energy, giving heat balance; the 51% is never shared equally → latitude / altitude / distance from the sea / slope of the land / winds and ocean currents → the temperature of a particular place
What This Chapter Cannot Tell You
The heat budget in this chapter is a single set of figures for the whole earth, averaged over the whole year. It cannot tell you how much energy your own town received yesterday, or how the 51%, 35% and 14% shift between a cloudy monsoon week and a clear December morning.
The five factors are described one at a time, but at any real place all five act at once, and the chapter gives no way of weighing them against each other. The Bergen case shows a current defeating latitude on one coast; it does not tell you which factor will win anywhere else. For that, geographers need measured temperatures from actual stations over many years, and this chapter prints none.
13.5 Check Your Concepts
The chapter states the normal lapse rate in two ways — a fall of 1°C for every 166 metres climbed, and a fall of about 6°C per kilometre. Show the arithmetic that connects the two statements, and explain why the word ‘normal’ matters when you use the rate on a real hillside.
Look again at Fig. 13.7. Read off the three temperatures marked on the valley side, and describe the pattern they show. Then explain why the valley bottom is colder than the slope above it, and state the two conditions of sky and air that this needs.
Two slopes of the same east-west mountain range lie at the same latitude and at the same altitude, yet one is consistently warmer than the other. Say which slope is warmer, explain why, and state which of the five factors is doing the work here.
The chapter calls heat balance a worldwide average balance. Explain what is happening separately in the low latitudes and near the Poles, and say what this average hides about the earth’s actual heat distribution.
Distinguish between insolation and terrestrial radiation.
How is ‘heat balance’ achieved?
Give a reason for each of the following:
- North India has a greater range of temperature than South India.
- The temperature of Delhi is less than that of Chennai in December.
- Desert areas experience a high day temperature and a much lower night temperature.
Draw a well labelled diagram to show the heat budget of the earth.
Give a reason for each of the following:
- Distance from the sea affects the temperature of a place.
- Land is heated and cooled faster than the sea.
- The ports of the western coast of Europe remain ice-free during winter.
How would the breezes that blow during the day, and those that blow during the night, affect the temperature of a place situated in a coastal region?