Air traffic may increase cirrus cloudiness
OLIVIER BOUCHERHigh-level cirrus clouds can evolve from the condensation trails of aircraft, which form as the mixture of warm, humid exhaust gases and colder, drier air exceeds water saturation. In addition, the particles in exhaust plumes from aircraft may allow ice nucleation at lower supersaturations than those required under natural conditions. This mechanism is sensitive to environmental conditions, but may occur downstream of the exhaust aerosol source regions. Here I show that cirrus clouds increased in occurrence and coverage in the main air-traffic flight corridors between 1982 and 1991.
The Thomas Jefferson paternity case
DAVID M. ABBEYThe DNA analysis of Y-chromosome haplotypes used by Foster et al. to evaluate Thomas Jefferson's alleged paternity of Eston Hemings Jefferson, the last child of his slave Sally Hemings, is impressive. However, the authors did not consider all the data at hand in interpreting their results.
The Thomas Jefferson paternity case
GARY DAVISIf the data of Foster et al. are accurate, then any male ancestor in Thomas Jefferson's line, white or black, could have fathered Eston Hemings. Plantations were inbred communities, and the mixing of racial types was probably common. As slave families were passed as property to the owner's offspring along with land and other property, it is possible that Thomas Jefferson's father, grandfather or paternal uncles fathered a male slave whose line later impregnated another slave, in this case Sally Hemings. Sally herself was a light mulatto, known even at that time to be Thomas Jefferson's wife's half sister.
Migration of plutonium in ground water at the Nevada Test Site A. B. KERSTING, D. W. EFURD, D. L. FINNEGAN, D. J. ROKOP, D. K. SMITH & J. L. THOMPSON
Mobile colloids--suspended particles in the submicrometre size range--are known to occur naturally in ground water and have the potential to enhance transport of non-soluble contaminants through sorption. The possible implications of this transport mechanism are of particular concern in the context of radionuclide transport. Significant quantities of the element plutonium have been introduced into the environment as a result of nuclear weapons testing and production, and nuclear power-plant accidents. Moreover, many countries anticipate storing nuclear waste underground. It has been argued that plutonium introduced into the subsurface environment is relatively immobile owing to its low solubility in ground water and strong sorption onto rocks. Nonetheless, colloid-facilitated transport of radionuclides has been implicated in field observations, but unequivocal evidence of subsurface transport is lacking. Moreover, colloid filtration models predict transport over a limited distance resulting in a discrepancy between observed and modelled behaviour. Here we report that the radionuclides observed in groundwater samples from aquifers at the Nevada Test Site, where hundreds of underground nuclear tests were conducted, are associated with the colloidal fraction of the ground water. The 240Pu/239Pu isotope ratio of the samples establishes that an underground nuclear test 1.3 km north of the sample site is the origin of the plutonium. We argue that colloidal groundwater migration must have played an important role in transporting the plutonium. Models that either predict limited transport or do not allow for colloid-facilitated transport may thus significantly underestimate the extent of radionuclide migration.
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