CHAPTER TWO Solar Radiation and Temperature Radiation is a form of heat transfer which is completely independent of the medium through which it travels.. whatever their temperature, emi
Trang 1CHAPTER TWO Solar Radiation and Temperature
Radiation is a form of heat transfer which is completely independent of the medium through which it travels All bodies whatever their temperature, emit heat energy in the form of short electromagnetic waves which travel through space at the speed of light The actual wavelength depends on the temperarure of the radiating body The hotter the body the shorter the wave-length and the more intense is the emission At very high temperatures a body emits both heat and light e g a tire The surface temperature of the Sun is something in the nature of 6.000oC
Of the Sun's radiant energy which strikes the Earth much is absorbed, thereby increasing the temperature of the surface which emits long invisible heat waves back into space Some of the incoming short-wave radiation from the Sun is lost through absorption ret1ection and scattering by cloud A thick cloud will reflect nearly 80 per cent of the radiation which it receives Absorption is ,very little, probably about seven per cent
water vapour and cloud when present strongly absorb most of the outgoing longwave radiation some of which is re-radiated into space and some re-radiated downwards to the Earth's surface and thus compensating in some measure for loss of heat by outgoing radiation This is called the
greenhouse effect It explains why when there is a thick cloud layer at night
the fall in surface temperature during the hours of darkness is less than on
nights when there is a clear sky allowing free terrestrial radiation
DIURNAL RANGE OF SURFACE TEMPERATURE
Soon after sunrise the incoming short-wave energy begins to exceed the
outgoing long wave emission The temperature of the surface then starts to
increase anti generally reaches its maximum by about 1400 hours Local Time after which it gradually begins to cool All incoming radiation ceases when darkness falls and the surface continues cooling through the night until sunrise when the whole cycle is repeated
Bodies which are good absorbers of heat are also good radiators and the converse is true In general land may be described as a strong absorber by comparison with a water surface which is relatively very weak Thus the diurnal range in temperature of a land surface is much greater than that of the sea surface which, in ocean areas is generally less than 0.5°C (the interior of
Trang 2continents may vary by 16°C (30°F) or more) The general pattern of diurnal variation in land temperature is often modified locally by weather For instance, a change in wind direction might bring a much colder or warmer airstream into the region
FACTORS AFFECTING THE HEATING EFFECT OF SOLAR RADIATION
1 The inclination of the solar beam to the Earth's surface This depends on: (a) The latitude of the place
(b) The Sun's declination which varies with the seasons (c) The
daily change in the Sun's altitude.
In Figure 2.1 ER represents a portion of the Earth's surface X and Y are two solar beams of equal intensity and having the same cross-sectional area Beam X is directed at an oblique angle to the Earth's surface and its energy is spread over a relatively large area AB Beam Y is nearly vertical to the surface and its radiation is concentrated onto the relatively small area CD The pecked line FGH represents the upper limit of the atmosphere, from which it can be seen that the beam X has to pass through a greater thickness of atmosphere than beam Y before reaching the Earth, and so will suffer a greater loss of energy due to reflection and scattering Thus, all other things being equal the heating effect will be greatest at area CD
2 The nature of the surface Snow and ice surfaces reflect about 80 per cent
of the radiation received Dry soil, bare rock and sand, though poor conductors of heat, are very good absorbers and the heat energy received penetrates only a very shallow layer of surface amounting to a few inches
Trang 3Hence a relatively high rise in temperature for a given amount of radiation
By contrast the temperature of the sea surface changes only a very little for the same amount of heat energy The reasons for this are:
(a) The specific heat* of water is much greater than that of land
(b) The solar rays penetrate the sea surface to a depth of several metres (c) The stirring effect of the wind brings up colder water from below
(d) Much of the heat received by the sea surface becomes rapidly used
up in the process of evaporation
(e) A water surface reflects solar radiation, especially at large angles of incidence
Air temperature near the surface tends to follow that of the surface below Thus the annual range like the diurnal range, is greater over the interior of large continents than over the oceans The main factors governing air temperatures at sea are:
(a) Latitude Generally warmest within the tropics and sub-tropics
(b) Season
(c) Proximity to large land masses
(d) Prevailing winds
(e) Ocean currents
(f)Upwelling of cooler water from the depths
(g) The presence of ice or snow covering
* The specific heat of a substance is the number of joules required to raise the
any other common substance Hence the gain or loss of a given quantity of heat brings about a smaller change in temperature of sea than of land