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they are not typified by a rapid onset. Instead, these hazards are relatively slow in their onset but prolonged in their impact. Droughts, famines, epidemics, and desertification are well-known long-term disasters. Drought may be defined as any unusual pro- longed dry period. Though generally associated with semiarid or desert climates, they also occur in areas that normally enjoy adequate rainfall. Droughts are usually accompanied by dry, hot winds, and they may be terminated by violent storms. The basic causes of drought are still not clear. It is generally believed that droughts are a con- sequence of changing global weather patterns triggered by such ecological events as solar radia- tion, excessive build-up of heat on the earth's surface, and increased particulate matter in the earth's atmosphere. Droughts are accompanied by reduced cloud cover, thus increasing exposure of the land to solar radiation. The result - increased transpiration and evaporation rates - tends to perpetuate the drought. Once estab- lished, these conditions are difficult to reverse. Human activities also contribute to develop- ment of drought conditions. Overgrazing, poor cropping methods, deforestation, and improper soil conservation techniques often help to create a drought. Desertification is a secondary effect of drought. Technically, desertification occurs when the soil reaches a given level of dryness. Simply stated, desertification occurs when land takes on the characteristics of a desert. It can mean the encroachment of sand dunes and the loss of most vegetation or replacement of normal vegetation with desert scrub bushes and other plants espe- cially adapted to the desert environment. In either case, the land is rendered useless without large- scale and costly reclamation measures. 51 Fighting desertification is both costly and frus- trating. Few developing countries have the re- sources necessary to stop this process once it takes hold, and reclamation successes have been rather limited. The best way to stop desertification is to prevent its initiation, and the best way to accom- plish this is through measures that address wide- spread economic and agricultural development. Reducing the hazards of drought and des- ertification requires a balanced program that develops good water resources, addresses the problems of soil erosion, and adopts realistic limits on the expansion of animal herds, or accompanies the expansion of herds with compre- hensive range management. Agricultural im- provements to prevent these hazards include modifying cropping patterns and introducing drought-resistant crop varieties; rangeland man- agement includes improvement of grazing lands and grazing patterns, introduction of feedlots, and protection of shrubs and trees. Expansive soils - soils that exhibit large poten- tial for shrinking and swelling with changes in moisture content - are another long-term hazard. Construction on these soils is extremely vulnera- ble to damage - even total destruction - as the ground surface elevation changes in response to seasonal fluctuations and rainfall. The problem is particularly acute in arid and semiarid regions. Shrinking and swelling soils are found in indus- trialized and developing nations alike. The total cost of damage associated with expansive soils is estimated at a minimum of $6 billion per year in the United States alone; it is the nation's most costly natural hazard. Extremely high costs are likely to be typical for many other nations as well. Though mitigation measures involving land use and building design are well known, they are often not applied due to ignorance, cost, and lack of enforcement.