~ - Gaelen Le Felt . Lee lame Selentific Laboratory June 1954 is antroduction t Tua problem of longeraenge,fall-out fran very large devices (megatons) wes fiset axmined pricr to Oparstion IVY. it that time the methods of analysis were cased on a simple theoretical acdel devised to evaluate the hozards fram the Janglo shcts and on the ampirical results from lowyicld tests in Nevada, Noither source was truly aprlicable in detail to MIXE, & factwell-imow at the tine, wt the ceneral result, as we imow from waa correct; 1.6., fall-cut fromMIKE under adverse conditians couldhave been very severc at distanses of 200 to 300 miles. The actual conditicns on MIXE-Day wero, cf course, favorable tut at the same tins rather urrusual in that the location of the main fall-out waa well clear of all populated areas. The rare .courronce of ideal conditions, the length of the C’STLE oeration, aml the very evident ‘asards from the devices made necessary @ re-eximinaticn of the problem of fall-out. In the field such a ree examination could be very crude :t best, geared as it wa to the immediately practical aim °f operational forecasting. It was necessary to devise « system of forecasting simrle encugh to provide results besed on the latact possible weataer information, beth observations ani forecasts, conservative enough to guar ntese no repetition of the unfortunate results cf _ and differvnt from those know to be ideal (no serthern components frou surface to 100,070 ft and axis of fall<cut between abart 315° and 045°). The treatasn= generally used in Mevada with acderate success ws pretty clearly nct- well suited to fall<cutforecasting at the Pacific Proving Grounds axeept poesthly under the conditionof “deep easterlies", wien all ‘dnd vectors fram the surface up are cagtefty (an alternate “ideal” situatia incidentally for Bxiwetek Stoll but not for Bikini). The basic difficulty with ualag the Nevada system for analysis of the effects of larze devices in the Pacifie is that the wind structure im the Pacific is not primarily ticnal in the pertinent altitude ranges. The ueual fall-out pattern » orcadened at tines by the presence of Licht and | veritable winds and by the cecasional presence of abnormally creat directioaal shear bub still confined to a mall sector. This general sharacter is » Mere the directional shear is alncst net often found in the P. Always very sreat just above the east-northeast flow in the low altituces and again a& the tropopeuse (usually about $5,000 to 60,000 ft). ch includes all.the distant fall-out is very f s % 3 e In the frequently 180° and cecasionally evon greeter.