Further Tests on the Ice Nucleation Potential of Meteoritic Material
SCHAEFER1 was unable to detect any trace of ice-crystal formation when he vaporized a sample of iron–nickel meteorite by electric spark in a supercooled cloud—even in clouds as cold as −25° C. Mason and Maybank2, who ground up and vaporized three samples of stony meteorites, and Mason3 with three further samples also reported that they were unable to detect any production of ice-crystals at temperatures above −17° C This communication records the results of tests on the nucleating properties of a further thirteen samples of meteoritic materials. These comprised iron–nickel types (siderites), stony type (aerolites), plus two samples of the intermediate (siderolite) variety (all obtained from the American Meteorite Laboratory, Denver).
The Role of Natural and Anthropogenic Pollutants in Cloud and Precipitation Formation
Since the resurgence of interest in cloud and precipitation physics, stimulated by the demonstration in 1946 of the possibility of cloud modification by artificial nucleation, an ever-increasing amount of research has been reported in the literature on the mechanisms which underlie the formation of clouds and precipitation in the atmosphere. This research has essentially been carried out along two lines of study: studies of the cloud microphysical processes including the phase change of water substance in the atmosphere and the growth of the formed cloud particles to precipitation particles, and studies of the cloud macrophysical processes including all those dynamic processes which govern the formation, growth, dimensions, shape, organization, lifetime, and vigor of the cloud as a whole. In recent years, it has been realized that the cloud dynamical and cloud microphysical processes have to be viewed in a combined manner for a complete understanding of the formation of clouds and precipitation. Thus, much work is currently being done with this aim in mind.
LAND DEGRADATION IN THE DEVELOPING WORLD: ISSUES AND POLICY OPTIONS FOR 2020
Land degradation takes a number of forms, including depletion of soil nutrients, salinization, agrochemical pollution, soil erosion, vegetative degradation as a result of overgrazing, and the cutting of forests for farmland. All of these types of degradation cause a decline in the productive capacity of the land, reducing potential yields. Farmers may need to use more inputs such as fertilizer or manure in order to maintain yields, or they may temporarily or permanently abandon some plots. Degradation may also induce farmers to convert land to lower-value uses. For example, farmers may plant cassava, which demands few nutrients, instead of maize, or convert cropland to grazing land.