droughts occur WHEN the rainfall in an area falls below its normal pattern for an extended period of time. If the drought occurs in a region where crops are usually not irrigated, then farmers suffer crop losses that may range from mild to severe. If the drought is extreme, the crop losses may be total, because the plants wither and die without water. The weather, which is the current atmospheric conditions locally or regionally, can develop into a pattern that seems to be stuck. In such conditions, floods or drought may occur. Meteorologists who make long-range weather predictions cannot predict droughts. However, scientists and others who study climatic conditions produced by weather patterns in an area or region over long periods of time have found that there are times in which drier weather than normal occurs and when wetter weather than normal occurs.
By studying the growth patterns of tree rings, seasons of significant growth in wet years can be seen in comparison with years in which drought occurred and as a consequence the tree grew little. When the pattern of wet and dry year growth is observed in tree ring patterns, it is not unusual to find multi-year periods when trees grew rapidly followed by little growth in multi-year times of drought. Weather over a long time period becomes patterns of the climate flowing in irregular repeated cycles. These may be cycles that occur occasionally every 50 or 100 years, or they may occur somewhat more frequently.
Droughts are caused by a lack of rainfall or snow-pack. If water vapor is not in the atmosphere, then rising temperatures causes warm air to rise and, in the case of warm moist air, to condense in the cooler upper atmosphere and to shed its moisture as rain or other forms of precipitation. If rising air or water vapor is not present in combination, then drought results.
Meteorological droughts are due to changes in climatic conditions over vast regions. Conditions that can cause a drought include shifts in the jet stream in the presence of an El Niño, which signals changes in the temperature of vast areas of ocean water that contribute moisture for rainfall or other changes.
Agricultural droughts are usually produced by meteorological droughts. They occur when the average precipitation is insufficient to support agricultural activity at its normal levels. In agricultural droughts, crops die or produce only a small portion of their normal yield. Livestock must be fed with imported food, sold, or turned loose to forage for themselves. Deforestation also affects local conditions that cause or retard rainfall. Many scientists believe that it is still too soon to determine whether the rise of average global temperatures in the last 100 years is sufficient to create global warming conditions that are causing droughts in areas where droughts have historically been rare.
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