Climate and Outside Design Conditions
The variations in temperature, humidity and wind occurring throughout the world are due to several factors the integration of which, for a particular locality, provides the climate experienced. There is, first, a seasonal change in climatic conditions, varying with latitude and resulting from the fact that, because the earth’s axis of rotation is tilted at about 23.5° to its axis of revolution about the sun, the amount of solar energy received at a particular place on the earth’s surface alters throughout the year. The geography of the locality provides a second factor, influential in altering climate within the confines imposed by the seasonal variation.
Figure 5.1 illustrates the geometrical considerations which show that, at a particular latitude, the earth receives less solar radiation in winter than it does in summer. The importance of this in its effect on the seasons stems from the fact that, for all practical purposes, the sun is the sole supplier of energy to the earth.
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Axis of earth’s rotation |
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Rays of the sun |
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Atmosphere |
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Relative position of sun in winter for the northern hemisphere |
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Line parallel to the earth’s axis of Path length of sun’s Rays through atmosphere In Winter |
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Relative position of sun in summer for the northern hemisphere |
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The geography of a place determines how much solar energy is absorbed by the earth, how much is stored and how readily it is released to the atmosphere. The atmosphere is comparatively transparent to the flux of radiant solar energy (termed ‘insolation’) but the land masses which receive the energy are opaque to it and are fairly good absorbers of it,
Although this depends on the reflectivity of the surface. This means that the thermal energy from the sun warms up land surfaces on which it falls. Some of this energy travels inwards and is stored in the upper layers of the earth’s crust; some is convected to the atmosphere and some is re-radiated back to space, but at a longer wavelength (about 10 micrometres), since its mean surface temperature is very much less than that of the sun. Four-fifths of the earth’s surface is water, not land, and water behaves in a different fashion as a receiver of insolation, being partially transparent to it; consequently, the energy is absorbed by the water in depth, with the result that its surface temperature does not reach such a high value during the daytime. On the other hand, at night, heat is lost from the land to the sky much more rapidly since less was stored in its shallow upper crust than was absorbed and stored in the deeper layers of water. The result is that land-surface temperatures tend to be lower at night than are water-surface temperatures. It is evident from this that places in the middle of large land masses will tend to have a more extreme annual variation of temperature than will islands in a large sea. Thus, the climate of places on the same latitude can vary enormously. To realise this we have but to compare the temperate seasons experienced in the British Isles with the extremes suffered in Central Asia and Northern Canada at about the same latitude. The exchanges of radiant energy cited above as responsible for the differences in maritime and continental climates are complicated somewhat by the amount of cloud. Cloud cover acts as an insulating barrier between the earth and its environment; not only does it reflect back to outer space a good deal of the solar energy incident upon it but it also stops the passage of the low-frequency infra-red radiation which the earth emits. In addition, the quantity of carbon dioxide in the atmosphere reduces the emission of infra-red radiation. Mountain ranges also play a part in altering the simple picture, presented above, of a radiation balance.
The effect of the unequal heating of land and sea is to produce air movement. This air movement results in adiabatic expansions and compressions taking place in the atmosphere, with consequent decreases and increases respectively in air temperature. These temperature changes, in turn, may result in cloud formation as values below the dew point are reached.
One overall aspect of the thermal radiation balance is prominent in affecting our weather and in producing permanent features of air movement such as the Trade Winds and the Doldrums. This is the fact that, for higher latitudes, the earth loses more heat to space by radiation than it receives from the sun but, for lower latitudes, the reverse is the case. The result is that the lower latitudes heat up and the higher ones cool down. This produces a thermal up-current from the equatorial regions and a corresponding down-current in the higher latitudes. While this is true for an ideal atmosphere, the fact that the earth rotates and that other complicating factors are present means that the true behaviour is quite involved and not yet fully understood.
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