Water Lesson 8: Physical Causes of Drought Flashcards
Define drought
Deficiency of water over an extended period of time
Describe meteorological drought
Defined by shortfalls in precipitation as a result of short-term variability within the longer-term average overall, as shown in many semi-arid and arid regions such as the Sahel
Describe agricultural drought
Some farming practices such as overgrazing can accelerate the onset of this type of drought. Rainfall deficiency leads to deficiency of soil moisture and water availability which has a knock-on effect on plant growth and reduces biomas
Describe hydrological drought
Occurs when there are deficiencies in stream flow and groundwater levels, which decrease because of a reduced inputs of precipitation and high rates of evaporation
Describe socio-economic drought
Occurs when the water demand for social and economic purposes exceeds water availability. This could be the result of a weather-related shortfall in water supply or the overuse of the available water supplies. It is different from the other types of drought as it is dependant upon spatial variations in supply and demand
What is the Intertropical Convergence Zone?
Belt of low atmospheric pressure located around the Equator and moves north or south of the equator seasonally
What are there high levels of at the ITCZ? Why?
Due to the intensity of the Sun’s heat, there is high rates of evaporation
What can the ITCZ cause in some regions of the world?
Causes a wet season (when it arrives) and a dry season (when it moves away)
How can the ITCZ cause drought?
Sometimes the sub-tropical high pressure zones at the descending parts of the Hadley cell block the high humidity/rain-bearing air masses, so the pattern is modified. This blockage can prevent the arrival of the wet season, causing a drought
What is formed in mid-latitudes?
Frontal precipitation is created in low-pressure systems, that form along the polar front, where tropical air rises over cold polar air
Where do depressions move in mid-latitudes? Why?
Move from west to east
Result of the Coriolis force and their track is directed by the polar front jet stream
How do mid-latitude blocking anticyclones cause drought?
Loops of the jet stream occasionally stabilise, or even break up, and this allows high-pressure areas (anticyclones) from the subtropics to move northwards. These bring stable weather with very little precipitation (often heatwaves), causing a drought in areas such as the UK
How long can anticyclones persist for?
Up to 2 weeks
Describe the process of atmospheric circulation
1) Intense solar radiation at the Equator warms the air, which rise and starts convection. Air cools and condenses to clouds - precipitation
2) Subtropical high pressure zone created - air cooled sinks to forma belt of high pressure and hot, dry conditions
3) Air returns to ground level at Equator = trade winds
4) Trade winds meet at ITCZ and warm air rises. Position of ITCZ moves with seasons - creates wet and dry seasons in tropics
5) Warm air from from sub-tropics to mid-latitudes meets polar front jet stream - warm, less dense air rises causing rainfall
6) Warmer air rises into polar jet stream and is transferred to high altitudes in poles. Cools and sinks here. Creates a movement of air at ground level back to Equator
What is El Nino?
Naturally occurring large mass of very warm seawater in the equatorial Pacific Ocean
