Unit 2 And Unit 3: The Water Cycle Flashcards
How does Precipitation Form?
In the atmosphere when water vapour condenses. Condensation requires:
- a small particle, dust or sea salt, in the atmosphere around which condensation can take place. These particles are known as condensation nuclei
- 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.
Why does air cool in the atmosphere?
As altitude increases temperature decreases. Lapse rate - temperature typically drops between 6.5 and 9.8 degrees for every 1000m increase in altitude. Therefore, if air rises or is uplifted in the atmosphere it will cool and subsequently may reach the dew point resulting in condensation and water vapour changing into a liquid or solid.
What is uplift?
Is the process of air rising; as air rises it cools and condenses forming clouds and precipitation.
What are the 3 main mechanisms of air uplift that produce rainfall?
- orographic or relief rainfall
- convection rainfall
- frontal rainfall
What is orographic or relief rainfall?
- Warm, moist air is forced to rise over high areas (mountain ranges)
- Air cools and condenses, forming clouds
- It rains
- Air descends, warms and becomes drier
> occurs in high altitudes
What is convectional rainfall?
- Sun heats the land and the air above
- Warm air rises, cools and condenses, forming clouds
- Rain can then occur
> occurs over land that is subjected to the suns heat - insulation
evaporation
intense
brief thunderstorms
What is frontal rainfall?
> occurs at the boundaries of two air masses of varing temperatures and densities
1. Warmer air gets forced upwards because the old air is denser
2. condensation forms clouds
3. It rains heavily along the front
What are the 2 key mechanisms of precipitation formation with clouds?
The Bergeron-Findeisen theory of ice-crystal growth and the Collison mechanism
What is The Bergeron-Findeisen theory of ice-crystal growth?
- it occurs in clouds where temps are just below 0 degrees; s, operates in high latitude and higher altitudes
1. water droplets can become supercooled and will stay as a liquid below the normal freezing point of 0, yet will only freeze upon contact with a condensation nuclei
2. dust particles act as condensation nuclei allowing droplets to freeze, upon contact with other supercooled droplets these will also freeze, thus enabling hexagonal ice crystals - snowflakes - to start forming.
3. Eventually the large snowflake will become too heavy to be kept aloft by updrafts (rising air) and will begin to fall. As they fall they will pass through warmer air and melt to produce rain
Where does The Bergeron-Findeisen theory of ice-crystal growth commonly occur?
Over the UK where cloud temperatures are often below -5 degrees. If temps close to the ground remain below 2 degrees snow will reach the ground, however if temperatures are above 2 degrees, which they commonly are the snowflakes melt and rain occurs.
Where does the collision mechanism occur?
It explains the formation of rain in the warm tropics where cloud temperatures are too warm for ice crystals to form
What is the collision mechanism?
- supersized 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 in heavy downpours and flash flooding from infiltration-excess overland flow
What is excess runoff?
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. It can be caused by both natural physical factors and human factors
What are natural physical factors which affect excess runoff?
- prolonged rainfall
- intense storms
- snowmelt
- monsoon rains
What are human factors which affect excess runoff?
- deforestation
- urbanisation
Whats an example of a Prolonged Rainfall event?
February 2020 Floods UK
- wettest February on record for the UK
- 5th wettest winter on record since 1862
- three named storms crossed the UK during February, Ciara, Dennis and Jorge.
Why did it rain so much during February 2020?
The type of rain during this period was mainly frontal rainfall. The meeting point between the two air masses of warm tropical air and cold polar air is known as the polar front. Along this front frontal rain will form. The position of this front is controlled by the Jet Stream, stayed over the UK for much of the winter of 2020, hence prolonged periods of rain.
What were some effects of the flooding along River Severn?
- flooding of businesses
- evacuation
- mental + physical illness’ are higher in regularly flooded areas
- in Shrewsbury 70,000 property were at risk of flooding and 1600 property’s were flooded
What is a location example of an intense storm?
Boscastle Flood 16th August 2004
What do intense storms do?
Produce high levels of precipitation and result in flash flooding from infiltration-excess overland flow
What were the key impacts of Boscastle?
- 5 buildings were demolished and 60 more were damaged
- 30 vehicles were washed into harbour
- over 100 people were rescued no-one died
Why did this intense storm at Boscastle happen?
- around 75mm of rain fell in two hours which is the same amount that normally falls in the whole of august
- two types of precipitation occurred which were frontal and orographic
- the torrential rain led to a 2m rise in river levels in one our
- was an intense storm event and was produced as a result of warm moist air from an ex-category 2 hurricane which moved eastwards over the Atlantic ocean
How does geology and topography increase levels of run-off?
- the underlying geology of the catchment is impermeable granite and slate
- Boscastle sits in the bottom of a steep sided valley
How does land use increase levels of run-off?
- deforestation has occurred over much of the catchment which is now used for permanent grazing land which means less vegetation so less interception
- some urban development, impermeable concrete, increased soil erosion
What is the location of the snowmelt case study?
Red River Flooding Of Fargo, 2009 , North of the United States
What are the key points of the Red River Flooding Of Fargo, 2009 case study?
- in march 2009 the Red River flooded in North Dakota and Minnesota in the USA, bringing record flood levels to Fargo at 12.45m on March 28th
- the winter 2008/9 had seen high levels of snow fall and wide spread frozen ground. This was followed by unusually high temperatures which resulted in rapid snow melt
- ice jams - ice carried along by runoff blocks water
- the community helped stop the impacts by volunteers from 100 miles used sandbags to soak up the water
How does snowmelt lead to excess runoff and flooding?
- frozen ground acts as impermeable rock so water cannot percolate/infiltrate
- high temps melt snow
- rapid change in river level
- river bursts its banks
- flashy hydrograph
How does deforestation cause excess runoff?
- grazing animals compact soil, less infiltration and percolation due to less pore spaces
- less vegetation so less water intercepted and less evapotranspiration
- more flashy
What is the deforestation case study?
Haiti, the Caribbean Sea.
It is the poorest country in the western hemisphere
What are the key facts about Haiti?
- in 2008, 4 successive tropical storms and hurricanes hit Haiti , destroying homes, buildings, infrastructure and agricultural crops affecting 800,000 people.
- 793 dead
- 548 injured
- 1923 60% of Haiti was covered by forest, trees protect from impact of storms, prevent soil erosion, and regulate the water cycle.
- less fertile soil for farmers
- now 4% forest cover in Haiti
- hurricane Matthew 2016 cost $2.8 billion in damages
How does urbanisation lead to excess runoff?
Urbanisation renders previously permeable ground surfaces impermeable. Surfaces such as concrete and tarmac (roads + pavements) increase surface runoff/overland flow and decrease rates of infiltration, throughflow and soil storage.
The majority of surface runoff will flow into drains and sewers which will result in a rapid transfer of water into river systems.
What is a water deficit?
A water deficit is experienced when an area lacks sufficient water to meet its needs.
What can water deficits be caused by?
• natural (meteorological)
• human
What is drought?
Drought is an extended period of low or absent rainfall relative to the expected average for a region. Therefore it means different amounts of rainfall in different regions.
What’s a drought in the UK defined as?
A period of at least 15 consecutive days on none of which is there more than 0.2mm of rainfall.
What’s a drought in Libya defined as?
Droughts are recognised only after two years without rain!
What are meteorological droughts caused by?
Variations in the normal atmospheric pressure systems.
What is atmospheric pressure?
Refers to whether air is rising or sinking in the atmosphere.
What’s the pressure when warm air rises?
Low pressure
What’s the pressure when cold air descends?
High pressure
Why is drought linked to extended periods of high pressure over a region?
Where cold air sinks through the atmosphere, it warms, no condensation occurs and consequently no precipitation.
What is the position of the Polar Front controlled by?
The Jet Stream
