Unit 2: Water Cycle Flashcards
(14 cards)
How does precipitation form?
Precipitation forms in the atmosphere when water vapour condenses.
What conditions are needed for condensation?
1) A small particle (typically dust or sea salt) in the atmosphere around which condensation can take place. These particles are known as ‘condensation nuclei’
2) Cooling of the air below the dew point. The temperature at which condensation occurs is called the dew point and is dependent upon the humidity of the air
What is the Lapse Rate?
As altitude (height) increases temperature decreases.
Temperature typically drops between 6.5 and 9.8 degrees for every 1,000m increase in
altitude.
What is Orographic/relief rainfall?
Moist air is forced to rise over high land, cools and condenses, causing rain on the windward side and dry conditions (rain shadow) on the leeward side.
What is Convectional Rainfall?
The sun heats the ground, causing warm air to rise rapidly, cool and condense, forming clouds and rain.
—common in summer.
What is Frontal rainfall?
Warm air is forced to rise over colder air at a weather front, causing widespread cloud formation and heavy rain.
What is the Collision Mechanism?
• This theory explains the formation of rain in the warm tropics where cloud temperatures are too warm for ice crystals to form
• ‘Super-sized’ condensation nuclei, e.g. large sea salt particles, provide ‘seeds’ around which very large water droplets form
• The larger ‘super’ droplets fall and collide with smaller droplets, absorbing them
• This mechanism is often associated with convectional rainfall resulting heavy downpours and flash flooding from infiltration-excess overland flow
What is Excess run-off?
If the rate at which the rain falls is greater than the rate at which it can infiltrate into the ground, then excess runoff will occur.
What are some Natural and Human factors that cause Excess run-off?
• Natural Physical Factors:
- Prolonged Rainfall
- Intense Storms
- Snowmelt
- Monsoon Rains
• Human Factors:
- Deforestation
- Urbanisation
What happened at Boscastle in August 2004?
Flood (intense storm event)
Was produced as a result warm
moist air from an ex-category 2 hurricane which moved eastwards over the Atlantic Ocean
Orographic rainfall played a huge part
24.1mm of rain was measured in 15 minutes
What factors may have played a role in the Boscastle flood?
• Geology and Topography
- The underlying geology of the
catchment area is
impermeable granite and
slate.
- Boscastle sits in the bottom of
steep sided valley.
• Land Use
- Deforestation has occurred over much of the catchment which is now used for permanent grazing land
- Some urban development
How does deforestation cause excess runoff?
• Rainfall impact: Strikes soil directly or grass near the surface, leading to soil compaction.
• Deforestation: Clear cutting of forests removes vegetation that helps absorb water.
• Grazing animals: Their movement compacts the soil further.
• Surface run-off: Rainwater does not soak into the ground, increasing erosion.
• Flooding: Water moves quickly over the surface to rivers, causing floods.
• Reduced groundwater recharge:
Less water infiltrates the soil, lowering groundwater levels.
• Dry rivers: Rivers quickly run dry after rain stops due to lack of groundwater replenishment.
How does deforestation cause excess runoff?
• Rainfall impact: Strikes soil directly or grass near the surface, leading to soil compaction.
• Deforestation: Clear cutting of forests removes vegetation that helps absorb water.
• Grazing animals: Their movement compacts the soil further.
• Surface run-off: Rainwater does not soak into the ground, increasing erosion.
• Flooding: Water moves quickly over the surface to rivers, causing floods.
• Reduced groundwater recharge:
Less water infiltrates the soil, lowering groundwater levels.
• Dry rivers: Rivers quickly run dry after rain stops due to lack of groundwater replenishment.
How does urbanisation lead to more excess runoff?
Impermeable surfaces such as concrete, roofs, etc will mean it doesn’t reach the soil and flows off the surface
