Drought Defining Drought Drought is an extended period
- Slides: 7
Drought
Defining Drought • Drought is an extended period of dry weather leading to conditions of extreme dryness. • Absolute drought is a period of at least 15 consecutive days with less than 0. 2 mm of rainfall. • Partial drought is a period of at least 29 consecutive days during which the average daily rainfall does not exceed 0. 2 mm.
Variations In Rainfall • A large proportion of the world’s surface experiences dry conditions. • Semi-arid areas are commonly defined as having a rainfall of less than 500 mm per annum, while arid areas have less than 250 mm, and extremely arid areas less than 125 mm. • In addition to low rainfall, dry areas have variable rainfall.
Variations In Rainfall • As rainfall total decreases, variability increases. • For example, areas with a rainfall of 500 mm have an annual variability of about 33%. • This mans that in such areas rainfall could range between 330 mm and 670 mm. • This variability has important consequences for vegetation cover, farming and the risk of flooding.
Arid Conditions • The main cause is the global atmospheric circulation. • Dry, descending air associated with the Subtropical high-pressure belt is the main cause of aridity around at 20 -30 degrees North. • In addition, distance from sea, or continentality limits the amount of water carried across by winds.
Arid Conditions • In other areas, such as the Atacama and Namib deserts, cold offshore currents limit the amount of condensation into the overlying air. • Others are caused by the intense rain-shadow effects, as air passes over mountains. • This is true of the Patagonian desert. • A final cause, or range of causes, are human activities. • Many of these have given rise to the spread of desert conditions into areas previously fit for agriculture. • This is known as desertification, and is an increasing problem.
Activity Using the “Nature Caused Sahel Drought” BBC Article, create a factfile about drought… • Causes. • Effects (Human and Physical). • Management Techniques/Strategies.
