Chapter 1
The Climatic Elements
The purpose of the first chapter is to provide the reader with a general understanding of the nature of the factors which affect climatic conditions over the earth.
The "climate" of a given region is determined by the pattern of variations of several elements and their combinations. The principal climatic elements, when human comfort and building design are being considered, are solar radiation, longwave radiation to the sky, air temperature, humidity, wind and precipitation (rain, snow, etc.). The present chapter briefly describes these elements, their interrelations and their distribution over the earth. An attempt has been made to give for each factor the data related directly to human comfort and to the aspects of building design which affect this.
Standard climatological books assess the climate of a region according to the long-term averages for the levels of each of the factors, but, as conditions may vary greatly from day to day and from year to year, deviations from the average should be taken into account for a more realistic view when dealing with climatic problems. For many applications the extreme conditions and their expected frequency may be of greater importance than the average conditions.
This chapter draws for information primarily on three books by Ashbel [1.1], Miller [1.4] and Trewartha [1.5] and the author gratefully acknowledges the kind permission to quote from these works, granted by the authors and publishers.
1.1. Solar Radiation
Solar radiation is an electromagnetic radiation emitted from the sun. The different wavelengths included (solar spectrum) range, on the surface of the earth, from about 0-28 to 3-0 microns (thousandths of a millimetre). The solar spectrum is broadly divided into three
1
regions: the ultra-violet (u.v.), the visible and the infra-red (i.r.). Only the small section of the spectrum between 0-4 and 0-76 micron is light visible to the eye; waves shorter than 0-4 micron are u.v. radiations and waves longer than 0-76 micron are i.r. Although the peak intensity of solar radiation is in the visible range, over one-half of the energy is emitted as i.r. radiation.
The solar energy at the upper limit of the atmosphere varies from 1 -8 to 2-0 cal/cm2/min according to the earth's distance from the sun and the solar activity. On average it is 1-97 cal/cm2/min [1.1], and this value is known as the solar constant. As radiation penetrates the earth's atmosphere its intensity is decreased and the spectral distribution is altered by absorption, reflection and scattering.
Radiation is selectively absorbed in the atmosphere, according to wavelength. Most of the u.v. rays and all wavelengths below 0-288 micron are absorbed by ozone and an appreciable part of the i.r. rays are absorbed by water vapour and carbon dioxide. Reflection takes place mainly from water droplets and is effectively nonselective; thus the spectral distribution of reflected radiation is similar to that of the source radiation, and hence the reflected light is white. When impinging on molecules and particles of dimensions similar to or smaller than the wavelength, radiation is refracted and diffused in space. Thus light is diffused, providing illumination even in the absence of direct sunlight. This is a selective phenomenon and the amount of scattered radiation of each wavelength is proportional to the fourth power of the reciprocal of the wavelength. Thus the air molecules scatter more of the shorter wave, blue and violet light, giving the blue colour of clear sky. But when the atmospheric content of larger dust particles increases, increasing the turbidity of the air, the proportion of the longwave yellow and red light scattered is increased and the sky becomes a whiter colour.
Clouds reflect back a significant fraction of the solar radiation to outer space, but the remainderreaches the earth's surface in a diffused form.
The diurnal and annual patterns of solar energy incident on a given region of the earth's surface depend on the intensity and duration of irradiation by the sun. The potential intensity of the radiation depends on the thickness of air through which the rays must penetrate, which is determined by the earth's rotation about its axis, its revolution about the sun and the inclination of the axis to the plane of revolution, all of which can be accurately computed. However, the amount of solar energy actually reaching the earth depends also on the sky clearance with respect to cloud, and the purity of the air with respect to dust, carbon dioxide and water vapour; these are factors which have to be estimated rather than calculated exactly.
T
he
thickness of air through which the rays penetrate to reach a point
on the earth depends on the angle of the sun above the horizon, or
the altitude of the sun, and on the height of the point above
sea-level. The altitude of the sun varies with the geographical
latitude of the point, from a maximum in the tropics, decreasing
towards the north and south poles. The duration of sunlight,
however, increases in summer and is reduced in winter, with
increasing latitude. Thus in summer the greater length of the day in
high latitudes partially compensates for the low angle of the sun.
The conditions of humidity and cloudiness also vary with latitude. These factors combine to give the highest intensity of solar radiation, during summer and on a yearly average basis, not in the tropics but in the sub-tropical arid regions.
On the basis of the world-wide measurements of solar radiation during the International Geophysical Year (1957-1958), Ashbel [1.2] has prepared maps of world distribution of solar radiation. Table 1.1 is based on these maps and shows a cross-section of global solar radiation from the north to the south poles through the 40°E meridian, for January, March, June, August and for the yearly average.
The procedures for quantitative determination of solar impact on man and buildings are dealt with in Chapters 4 and 10 respectively.