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.