MAUSAM. 49 , 2 (Apri l 19'18), 203·210

551.521.32 (547.1)

Effective terrestrial radiant energy at the ground at Pune

V. DESIKAN and V.R. CHIVATE

M, u orologicaI Offi cr. Pun,-411005, India

(Received 3 September 1993. Modifi ed 4 Decembe r 1996)

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AllSTRAC T. Tbe e ffect ive outgoing Ic~lnaJ f:khanl ene rgy controls the vananons in OInc:r ueteorologrca lparameters near the ground surface. In the absence of direct mca.suremerus of chis energy. the values an: derived from otberradrenon F'ncle~ .Ihat an: being measured 31 Pure. On an average day the loss of this terrestrial energy is ul lhc order of8.~ MJm"· . Tbe maximum of 12.53 MJrn-2 is reached In AJlflI and the mmrmum of .l .87 MJm"z dunng Aug~..1. The dailymaximumoccurs ar noon or juseafter that Yecrao-yccr variations bowever dependon ther.unf311 distnbunon and the moisturein the air.

Key words - Terres trial radiation. Radian t exposure. Radian! energy. Pyreoorrerer.

I. Introduction

Allhough solar irradiance is Ille primary source of allenergy transact ions in the earth-atmosphere system, a widevariety of interactions is largely controlled by the earth'sradiance.The changes in the ambie nt temperature, humidit y,evaporation etc. are affec ted by variations in this earth'sradiance. Because ofthe low temperatures (compared to thatof the sun) at which the emission lakes place. the radianceoccurs at wavelengths of terrestri al radiant energy. Theinteraction between this energy and the atmosphere, whosetemperature is not significantly different, is quite complex .The net solar irrad iance is available for heating, direc tly andindirect ly, the earth and the atmusphere. The thermal radia­tion is one of resulting transformations of the solar irradi­ancc after absorp tion by the earth's surface. The radiativeheating of each successive layer of the atmosphe re by thisthermal radiation is equally importa nt. One of the importanteffects of this thermal radiation lies on the thennodynamicalaspects leading to the various changes in the atmospheri cparameters, like, temperature , pressure, air now and cloud[ormation. The regular and co ntinuous measurement of theeffective outgoing radiant energy is. however, difficult andcarried out only at limited centres. The present paper dealswith the data deduced from other regular radiation measure-

mcnts being made at Pune and the variations that lake placeover a time.

2. Data collection

In the terrestrial region of electromagnetic spectrum,the entire environment. including the instrumen t itself. isemitting the radian t energy in the same wavelengths moni­tored. An instrument which will direc tly measure the terres­trial radiant energy must be insensitive to the wavelengthsof solar irradiance. Most of the window materials that aresuitable for such selective transmission do not. however,have sufficient hardness and are generally hygroscopic innature. resulting in abrup t changes in the transmission char­actcristics of the window materials. Added to this , the win­dow materia l itself participates in the energy transac tions .The very few materials which are impervi ous to the solarirradiance and weather resistant as wel l as are prohibitive lycostly. Thus the measureme nt of the effective outgoingterrestrial radia tion is not made on a large network scale. Themeasurements are again highly vitiated by the dust depositsand water depos ition due to dew, fog and precipitation. It ismore convenient to deduce the effective outgoing terrestrial

radiation (Hi ) from the measurement s of glooal and re­fleeted solar radiant exposures and the nCI total radian t