5.4a

• what is a drought?

  a shortage of water over an extended period of time
• • it can be difficult to predict when droughts will start and end. they tend to be slow-onset hazards
  • duration can vary from months to decades
• what happens to water stores during a drought?

  they become depleted - as they're not replenished by precipitation and people keep using them

  droughts are often accompanied by high temperatures which increase evaporation, depleting water supplies further
• which type of land is particularly vulnerable to drought?

  farmland

  if soil loses its moisture, then farmers have to irrigate their land --> this may increase the price of food in the long term, or cause food shortages
• what are meteorological droughts

  drought caused by precipitation deficit

  can be solved by water restrictions e.g. hosepipe bans
• what are hydrological droughts?

  less water in groundwater stores, rivers and streams

  • impacts urban water supplies
• what are short-term droughts caused by?

  • precipitation deficit --> lower than normal precipitation levels. usually caused by changes in the frequency of frontal precipitation
  • localised droughts can also occur when areas of cool descending air creates a high pressure weather system (called an anticyclone) this can block the movement of warm ascending air, which would otherwise create rain clouds, leading to less rainfall and drought
• • climate change causes longer-term warming and cooling of oceans, leading to changes in the frequency of droughts. this means that recovery time between droughts may be shorter
  • it also affects seasonal rains in the tropics - the rainy season is shorter and more unpredictable, leading to long term depletion of groundwater and reservoir stores
• what is a famine drought?

  food deficit due to failure of agricultural systems due to drought --> can lead to starvation and death
• what is an agricultural drought?

  rainfall deficit from meteorological drought leads to deficit in soil moisture = impacts crop yeilds
• what are jet streams?

  fast flowing air high in the atmosphere --> helps to move weather systems around the world
• how can jet streams lead to drought?

  • if the jet stream is weak, it can't move weather systems along as quickly so weather patterns can get stuck in one place for a long time
  • the same weather patterns stays in an area and there won't be any rain to break the drought
• what are the normal conditions in the Pacific ocean?

  west = low pressure so air rises, causing rain and storms

  east = high pressure so air sinks, causing dry weather

  --> air moves from high pressure to low pressure so trade winds blow from east to west
• how does El Nino cause droughts?


  pressure increases in the west and decreases in the east:

  west = high pressure so air sinks, causing dry weather and drought

  east = low pressure so air rises, causing rain and storms and flooding
• which pressure causes rain and why?

  Low pressure. Air rises, cools, and condenses into clouds, leading to precipitation.
• which pressure causes drought and why?

  high pressure. air can't rise so it can't condense into clouds
• how often do El Nino events occur + how long do they last?

  every 3 to 4 years

  lasts for 9 to 12 months
• what are La Nina events?

  the normal conditions become more extreme. trade winds increase in strength causing more cold water to rise in the east, resulting in drier conditions in the east and wetter conditions in the west