Use of Tephigrams in the prediction of Radiation FogK. C. CHAKR AVORTT Y

Regional Jleleurological Centre, Cutcuua

(Rccci"ed J.J April 1952)

1. Introduction

In a previous paper (Chnkmvortty (948),the author disc ussed the conditions favou ra­hle for the for mation offog at Calcutta and itsneigltbourhoO(L and suggested a method fort he prediction of fog at Calcutt.~ in th e earlymorning by a st udy of dry and wet thermo­grams of the prev ious afternoon and eve ning .The object of the present paper is to showhow the radiosonde observat ions taken at2030 1ST at Calcut ta can be used to predi ctthe occurrence of fog during the course of thenigh t or in th e following morning with agrea.ter degree of confidence. For th is purposethe 2030 1ST radiosonde dutn along with t hesurface tcmperature and fog data of Calcuttafor the periorla J anuary 1950 to February1952 nnd November 1953 to February 1954have been examined.

2. Metbod of analysll

A preliminary examination of dutu for thednys of fog has revealed the following im­portant points-

(a) Fog is generally preceded by th e occur­ren.ce of a. surface inversion in the eveningwhich can be well identified in the 2030 1STtep higmm.

(b) Before the occurrence of even theadvcc~ion type of fog (not to speak of theradl~lIon type) the rise of wet bulb during theprevlOu. n~ght aft.er 2030 1ST is very smallm comparISon with the fall of dry bulb.Hence tbe ant icipated fall of dry bulb tem­perat ure with the ad vance of night is more•mporta nt to deter mine whether satura t ionat the surface layer will ta ke place or not(Chakravortty 19/8).

(e). If Irom the 2030 1ST tephigram of theeve nmg preceding the occurrence of Cog, theportion of tbe curve indicating the surfaceinversion is wiped out. and the straight por­tion of the curve immediat ely above theinversion layer is produced to meet the sur­face isobar, the surface temperature indicat ...ed hy this new curve is very near the la teafternoon temperature in shade correspond­ing to the time from which the surface coolinghas started . (The exact time is found fromthe thermogram of the dny],

Fro m the results of the preliminary exa mi­nation ment ioned above one can, 0 11 the basisof the 20:10 1ST tephigram , picture roughlythe following ideal ised changes in th e tephi­gra m from afternoon to 2030 1ST aw l ugainfrom 20:l0 1ST tn the ti me of onset of fo"when DBC is the act ual dry bulb curve lll~iEF is th e actual wet bulb curve at 20:10 1STas shown in Fig. 1-

(a) In the afternoon, say at !GOO 1ST(the exact time to be obta ined from thethermogram) when the surface temperatu rebegan to drop cont inuously, th e drv bulbcurve was ABC (obtained by prod ucing thostraight portion of the 2030 1ST tephigramabove the inversion layer to meet 0 thesurface isobar ), -

(/) With tbe advance of t ime the dr v bu lbcurve beca me YXBC, Ray at 1900 1ST, theportion AX of the afternoon curve beingwiped out and the inversion portion YXbeing ta ken to be parallel to DB of thcactual 2030 1ST tephigram.

(c) At 2030 1ST the port ion AB of t he after­noon curve has been wiped out anti the