Editing Nuclear fallout (section)

===Meteorological===
[[Image:Fallout G&D77.JPG|right|300px|thumb|Comparison of fallout gamma dose and dose rate contours for a 1 Mt fission land surface burst, based on DELFIC calculations. Because of radioactive decay, the dose rate contours contract after fallout has arrived, but dose contours continue to grow]]

[[Meteorology|Meteorological]] conditions greatly influence fallout, particularly local fallout. Atmospheric winds are able to bring fallout over large areas.<ref>{{cite web|title= Continental US Fallout Pattern for Prevailing Winds (FEMA-196/September 1990) |url= http://ocw.nd.edu/physics/nuclear-warfare/notes/lecture-17 |website= [[University of Notre Dame]] |url-status= dead |archive-url= https://web.archive.org/web/20110315084044/http://ocw.nd.edu/physics/nuclear-warfare/notes/lecture-17 |archive-date= March 15, 2011}}</ref> For example, as a result of a ''[[Castle Bravo]]'' surface burst of a 15 Mt thermonuclear device at [[Bikini Atoll]] on 1 March 1954, a roughly cigar-shaped area of the [[Pacific]] extending over 500&nbsp;km downwind and varying in width to a maximum of 100&nbsp;km was severely contaminated. There are three very different versions of the fallout pattern from this test, because the fallout was measured only on a small number of widely spaced Pacific Atolls. The two alternative versions both ascribe the high radiation levels at north [[Rongelap]] to a downwind hot spot caused by the large amount of radioactivity carried on fallout particles of about 50–100 micrometres size.<ref>{{cite journal|publisher=General Electric Company|editor=Howard A. Hawthorne|title=COMPILATION OF LOCAL FALLOUT DATA FROM TEST DETONATIONS 1945–1962 – EXTRACTED FROM DASA 1251 – Volume II – Oceanic U.S. Tests|date=May 1979| url=http://worf.eh.doe.gov/data/ihp1d/1362e.pdf| archive-url=https://web.archive.org/web/20080410131321/http://worf.eh.doe.gov/data/ihp1d/1362e.pdf| archive-date=2008-04-10}}
</ref>

After ''Bravo'', it was discovered that fallout landing on the ocean disperses in the top water layer (above the [[thermocline]] at 100 m depth), and the land equivalent dose rate can be calculated by multiplying the ocean dose rate at two days after burst by a factor of about 530. In other 1954 tests, including ''Yankee'' and ''Nectar,'' hot spots were mapped out by ships with submersible probes, and similar hot spots occurred in 1956 tests such as ''Zuni'' and ''Tewa''.
<ref>{{cite journal|publisher=US Naval Radiological Defense Laboratory|author=Project Officer T. Triffet, P. D. LaRiviere|title=OPERATION REDWING – Project 2.63, Characterization of Fallout – Pacific Proving Grounds, May–July 1956|date=March 1961|url=http://worf.eh.doe.gov/data/ihp1c/0881_a.pdf|archive-url=https://web.archive.org/web/20080410131256/http://worf.eh.doe.gov/data/ihp1c/0881_a.pdf|archive-date=2008-04-10}}
</ref> However, the major U.S. "[[DELFIC]]" (Defence Land Fallout Interpretive Code) computer calculations use the natural size distributions of particles in soil instead of the [[afterwind]] sweep-up spectrum, and this results in more straightforward fallout patterns lacking the downwind hot spot.

[[Snow]] and [[rain]], especially if they come from considerable heights, accelerate local fallout. Under special meteorological conditions, such as a local rain shower that originates above the radioactive cloud, limited areas of heavy contamination just downwind of a nuclear blast may be formed.