absolute drought In the UK. this is a period of 15 days, on none of which more than 0.25 mm of rain falls. National definitions vary with climate; in Libya, droughts are recognized only after two years without rain. These arbitrary definitions give no indication of the impact of drought.
abscissa The horizontal, or x axis, of a graph. Where a causal factor or an 'independent variable can be clearly defined, it is recorded along the abscissa.
abrasion terrace A former 'shore platform, now above sea level because of either *tcctonic uplift of the mainland, or eustatic lowering of sea level. Abrasion terraces are thus indicative of *emergent coastlines.
Abrasion The grinding away of bedrock by fragments of rock which may be incorporated in ice (glacial abrasion), water (marine abrasion, fluvial abrasion), or wind (aeolian abrasion). In 'fluvial environments, the main agent of abrasion is the *bed load. The mass of solid material removed varies with the size, density, and velocity of the particles, and the density of the 'vector bearing these particles. Ice ceases to be an effective agent for abrasion when the weight of the ice is thick enough to bring about *plastic flow. Abrasion is an alternative term for *corrasion. See STRIATIONS.
aborigine A member of an indigenous people existing in a land before invasion or colonization by another race. This term is especially used for the original inhabitants of Australia.
At the edges of the glaciers, where ablation has occurred, large quantities of 'debris are released and accumulate to form ablation moraines (ablation till).
The ablation sub-system is the zone of a glacial system between the *firn line and the snout where there is an annual net loss of ice since annual ablation exceeds annual 'accumulation. The zone of net ice loss from a glacier is the ablation zone.
ablation Loss of snow and ice from a glacier by 'sublimation, melting, and evaporation. Ablation also results from the *calving of icebergs, and *avalanches. In temperate and sub-polar regions melting is the most important process in ablation, whereas in the Antarctic the most important ablation process is calving. The rate of loss varies with the meteorological factors of air temperature, relative humidity, wind speed, and *insolation, together with * aspect and the nature of the surface. In snowfields, ablation can also include snow removed by the wind, and is affected by aspect, depth of snow, and the nature of the underlying surface.
abiotic Not living, non-biological, usually describing factors in an ecosystem such as atmospheric gases, inorganic salts, mineral soil particles, and water. This expression is also used to describe the chemical and physical factors, such as salinity and humidity, which influence organisms. An abiotic environment is one without any life.
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