Insolation, Heat Budget and Temperature: Exam Notes

Insolation is the incoming solar radiation that reaches the earth. The sun pours out energy in every direction and the earth, a small target at a great distance, intercepts only about one two-billionth of it, yet that fraction drives every wind, every current and every climate on the planet. This chapter explains how that energy arrives, how much of it is turned back, how the atmosphere is heated from below rather than from above, and why the temperature of a place depends on far more than its distance from the equator. Examiners like the chapter because its facts are numerical and exact: the units of the heat budget, the normal lapse rate, the dates of perihelion and aphelion, and the names of the four processes of heat transfer.

Insolation and what controls it

The energy the earth receives is short-wave radiation, mostly in the visible and the near infrared part of the spectrum. The amount received at any place is controlled by five things: the rotation of the earth on an axis inclined at about sixty-six and a half degrees to the plane of its orbit, the angle at which the sun's rays strike the surface, the length of the day, the transparency of the atmosphere, and the configuration of the land. Where the rays fall vertically the same beam is spread over a smaller area and passes through less atmosphere, so the heating is greater; where they fall obliquely the same beam is spread over a wider area and loses more energy on the way. Insolation therefore decreases from the equator towards the poles. At the surface, however, the maximum is not received at the equator but in the subtropical deserts, because the equatorial belt is cloudy for much of the year while the desert sky is clear. The earth is nearest the sun, at perihelion, about the third of January, and farthest, at aphelion, about the fourth of July, a difference of some five million kilometres that matters far less to our seasons than the tilt of the axis.

How the atmosphere is heated

• Radiation: the sun heats the earth by short-wave radiation, and the earth in turn radiates long-wave infrared energy, called terrestrial radiation.
• Conduction: the layer of air in contact with the warm ground takes heat from it by contact, which is why the lowest layers of the atmosphere are the warmest.
• Convection: the heated air expands, becomes lighter and rises in a column, carrying heat vertically upward; this is confined mainly to the troposphere.
• Advection: the horizontal movement of air carries heat from one place to another, and in the middle latitudes most of the daily change in temperature comes from advection, as does the loo of north India in summer.

The important point for an examiner is that the atmosphere is largely transparent to the incoming short waves but absorbs the outgoing long waves through carbon dioxide, water vapour, methane and other greenhouse gases. The air is therefore heated from below, from the earth, and not directly by the sun.

The heat budget of the earth

The earth neither heats up nor cools down over the long run, which means it must return to space as much energy as it receives. Taking the insolation at the top of the atmosphere as one hundred units, about thirty-five units are turned back before reaching the ground, twenty-seven from the tops of clouds, two from snow and ice covered surfaces and the rest scattered by the atmosphere. That reflected share is called the albedo of the earth. Of the remaining sixty-five units, fourteen are absorbed within the atmosphere and fifty-one are absorbed by the land and the oceans. The surface returns all fifty-one units: seventeen go straight out to space as terrestrial radiation, and thirty-four are passed to the atmosphere, six by direct absorption of long waves, nine by conduction and convection, and nineteen as the latent heat of condensation released when water vapour turns to cloud. The atmosphere, holding fourteen units of its own and thirty-four from the ground, radiates forty-eight units into space. Seventeen plus forty-eight is sixty-five, exactly what was absorbed, and the budget balances.

The budget balances for the earth as a whole but not for each latitude. Between about forty degrees north and forty degrees south there is a surplus of heat, and beyond those parallels a deficit. Winds and ocean currents carry the surplus polewards, and that transfer is the engine of the general circulation of the atmosphere.

What controls the temperature of a place

• Latitude: the angle of the sun's rays falls away from the equator, so mean temperature falls with latitude.
• Altitude: temperature falls with height in the troposphere at a normal lapse rate of about six and a half degrees Celsius for every thousand metres, which is why hill stations are cool at the same latitude as the plains.
• Distance from the sea: water heats and cools more slowly than land, so coasts have a small range of temperature and interiors a large one, an effect called continentality.
• Air masses and ocean currents: a warm current or a tropical air mass raises the temperature of the coast it touches, and a cold current lowers it.
• Local factors: the aspect of a slope, the cover of vegetation, the nature of the soil and the presence of a city all change the figure a thermometer shows.

Isotherms, range and inversion

Lines on a map joining places of equal temperature are called isotherms, and they run roughly east to west, bending where a continent meets an ocean. The difference between the highest and the lowest temperature of a day is the diurnal range, and between the warmest and the coldest month, the annual range; both grow as one moves inland. Normally temperature falls with height, but on a long, clear, calm winter night the ground loses heat quickly, the air just above it becomes colder than the air higher up, and the lapse rate is reversed. This is a temperature inversion. It is common in hill valleys, where cold dense air slides down the slopes at night and collects on the floor, so the valley bottom is colder than the hillside above it; orchards and settlements in such valleys are therefore placed on the slope and not on the floor. Inversions trap smoke and dust near the ground and are one reason winter mornings in north Indian cities are smoggy.